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Rework mkdocs based documentation 

Lots of restructoring and extra information.
Antonio SJ Musumeci 2 weeks ago
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  1. 11
      mkdocs/README.md
  2. 2
      mkdocs/docs/benchmarking.md
  3. 54
      mkdocs/docs/config/branches.md
  4. 187
      mkdocs/docs/config/cache.md
  5. 21
      mkdocs/docs/config/deprecated_options.md
  6. 14
      mkdocs/docs/config/export-support.md
  7. 21
      mkdocs/docs/config/flush-on-close.md
  8. 23
      mkdocs/docs/config/follow-symlinks.md
  9. 17
      mkdocs/docs/config/func_readdir.md
  10. 152
      mkdocs/docs/config/functions_categories_and_policies.md
  11. 29
      mkdocs/docs/config/fuse_msg_size.md
  12. 75
      mkdocs/docs/config/inodecalc.md
  13. 17
      mkdocs/docs/config/link-exdev.md
  14. 13
      mkdocs/docs/config/link_cow.md
  15. 26
      mkdocs/docs/config/nfsopenhack.md
  16. 20
      mkdocs/docs/config/nullrw.md
  17. 91
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  18. 23
      mkdocs/docs/config/pin-threads.md
  19. 38
      mkdocs/docs/config/readahead.md
  20. 21
      mkdocs/docs/config/rename-exdev.md
  21. 59
      mkdocs/docs/config/rename_and_link.md
  22. 26
      mkdocs/docs/config/statfs.md
  23. 23
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  24. 12
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  25. 53
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  26. 27
      mkdocs/docs/config/xattr.md
  27. 2
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  28. 33
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  29. 31
      mkdocs/docs/faq/configuration_and_policies.md
  30. 5
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  31. 23
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  32. 31
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  33. 47
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  34. 35
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  36. 271
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  37. 2
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  38. 54
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  47. 64
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  48. 148
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  62. 62
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  67. 42
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  69. 2
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11
mkdocs/README.md
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@ -3,13 +3,14 @@
## Getting started
```bash
python3 -m venv env
source env/bin/activate
pip3 install -r requirements.txt
python3 -m venv venv
source venv/bin/activate
pip3 install --no-cache-dir mkdocs mkdocs-material pymdown-extensions
mkdocs serve
```
## References
- https://squidfunk.github.io/mkdocs-material/
- https://www.mkdocs.org/
* https://www.mkdocs.org/
* https://squidfunk.github.io/mkdocs-material/

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# BENCHMARKING
# Benchmarking
Filesystems are complicated. They do many things and many of those are
interconnected. Additionally, the OS, drivers, hardware, etc. can all

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# branches
The 'branches' argument is a colon (':') delimited list of paths to be
pooled together. It does not matter if the paths are on the same or
different filesystems nor does it matter the filesystem type (within
reason). Used and available space will not be duplicated for paths on
the same filesystem and any features which aren't supported by the
underlying filesystem (such as file attributes or extended attributes)
will return the appropriate errors.
Branches currently have two options which can be set. A type which
impacts whether or not the branch is included in a policy calculation
and a individual minfreespace value. The values are set by prepending
an `=` at the end of a branch designation and using commas as
delimiters. Example: `/mnt/drive=RW,1234`
### branch mode
- RW: (read/write) - Default behavior. Will be eligible in all policy
categories.
- RO: (read-only) - Will be excluded from `create` and `action`
policies. Same as a read-only mounted filesystem would be (though
faster to process).
- NC: (no-create) - Will be excluded from `create` policies. You can't
create on that branch but you can change or delete.
### minfreespace
Same purpose and syntax as the global option but specific to the
branch. If not set the global value is used.
### globbing
To make it easier to include multiple branches mergerfs supports
[globbing](http://linux.die.net/man/7/glob). **The globbing tokens
MUST be escaped when using via the shell else the shell itself will
apply the glob itself.**
```
# mergerfs /mnt/hdd\*:/mnt/ssd /media
```
The above line will use all mount points in /mnt prefixed with **hdd**
and **ssd**.
To have the pool mounted at boot or otherwise accessible from related
tools use `/etc/fstab`.
```
# <file system> <mount point> <type> <options> <dump> <pass>
/mnt/hdd*:/mnt/ssd /media mergerfs minfreespace=16G 0 0
```
**NOTE:** The globbing is done at mount or when updated using the runtime API. If a new directory is added matching the glob after the fact it will not be automatically included.

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# caching
## cache.files
Controls how [page caching](https://en.wikipedia.org/wiki/Page_cache)
works for mergerfs itself. Not the underlying filesystems.
* `cache.files=off`: Disables page caching for mergerfs.
* `cache.files=partial`: Enables page caching. Files are cached
while open.
* `cache.files=full`: Enables page caching. Files are cached across
opens.
* `cache.files=auto-full`: Enables page caching. Files are cached
across opens if mtime and size are unchanged since previous open.
* `cache.files=per-process`: Enable page caching (equivalent to
`cache.files=partial`) only for processes whose 'comm' name matches
one of the values defined in cache.files.process-names. If the name
does not match the file open is equivalent to `cache.files=off`.
Generally, enabling the page cache actually *harms*
performance[^1]. In part because it can lead to buffer bloat due to
the kernel caching both the underlying filesystem's file content as
well as the file through mergerfs. However, if you want to confirm
performance differences it is recommended that you perform some
benchmark to confirm which option works best for your setup.
Why then would you want to enable page caching if it consumes ~2x the
RAM as normal and is on average slower? Because it is the only way to
support
[mmap](https://man7.org/linux/man-pages/man2/mmap.2.html). `mmap` is a
way for programs to treat a file as if it is a contiguous RAM buffer
which is regularly used by a number of programs such as those that
leverage **sqlite3**. Despite `mmap` not being supported by all
filesystems it is unfortunately common for software to not have an
option to use regular file IO instead of `mmap`.
The good thing is that in Linux v6.6[^2] and above FUSE can now
transparently enable page caching when mmap is requested. This means
it should be safe to set `cache.files=off`. However, on Linux v6.5 and
below you will need to configure `cache.files` as you need.
[^1]: This is not unique to mergerfs and affects all FUSE
filesystems. It is something that the FUSE community hopes to
investigate at some point but as of early 2025 there are a number
of major reworking going on with FUSE which needs to be finished
first.
[^2]: [https://kernelnewbies.org/Linux_6.6#FUSE](https://kernelnewbies.org/Linux_6.6#FUSE)
## cache.entry
* `cache.entry=UINT`: Sets the number of seconds to cache
entry queries. Defaults to `1`.
The kernel must ask mergerfs about the existence of files. The entry
cache caches that those details which limits the number of requests
sent to mergerfs.
The risk of setting this value, as with most any cache, is related to
[out-of-band](https://en.wikipedia.org/wiki/Out-of-band) changes. If
the filesystems are changed outside mergerfs there is a risk of files
which have been removed continuing to show as available. It will fail
gracefully if a phantom file is actioned on in some way so there is
little risk in setting the value much higher. Especially if there are
no out-of-band changes.
## cache.negative_entry
* `cache.negative_entry=UINT`: Sets the number of seconds to cache
negative entry queries. Defaults to `1`.
This is a cache for negative entry query responses. Such as when a
file which does not exist is referenced.
The risk of setting this value, as with most any cache, is related to
[out-of-band](https://en.wikipedia.org/wiki/Out-of-band) changes. If
the filesystems are changed outside mergerfs there is a risk of files
which have been added outside mergerfs not appearing correctly till
the cache entry times out if there had been a request for the same
name within mergerfs which didn't exist. This is mostly an
inconvenience.
## cache.attr
* `cache.attr=UINT`: Sets the number of seconds to cache file
attributes. Defaults to `1`.
This is a cache for file attributes and metadata such as that which is
collected by the
[stat](https://man7.org/linux/man-pages/man2/stat.2.html) system call
which is used when you run commands such as `find` or `ls -lh`.
As with other caches the risk of enabling the attribute cache is if
changes are made to the file out-of-band there could be
inconsistencies between the actual file and the cached details which
could result in different issues depending on how the data is used. If
the simultaneous writing of a file from inside and outside is unlikely
then you should be safe. That said any simultaneous, uncoordinated
manipulation of a file can lead to unexpected results.
## cache.statfs
* `cache.statfs=UINT`: Sets the number of seconds to cache `statfs`
calls used by policies. Defaults to `0`.
A number of policies require looking up the available space of the
branches being considered. This is accomplished by calling
[statfs](https://man7.org/linux/man-pages/man2/statfs.2.html). This
call however is a bit expensive so this cache reduces the overhead by
limiting how often the calls are actually made.
This will mean that if the available space of branches changed
somewhat rapidly there is a risk of `create` or `mkdir` calls made
within the timeout period ending up on the same branch. This however
should even itself out over time.
## cache.symlinks
* `cache.symlinks=BOOL`: Enable kernel caching of symlink
values. Defaults to `false`.
As of Linux v4.20 there is an ability to cache the value of symlinks
so that the kernel does not need to make a request to mergerfs every
single time a
[readlink](https://man7.org/linux/man-pages/man2/readlink.2.html)
request is made. While not a common usage pattern, if software very
regularly queries symlink values, the use of this cache could
significantly improve performance.
mergerfs will not error if the kernel used does not support symlink
caching.
As with other caches the main risk in enabling it is if you are
manipulating symlinks from both within and without the mergerfs
mount. Should the value be changed outside of mergerfs then it will
not be reflected in the mergerfs mount till the cached value is
invalidated.
## cache.readdir
* `cache.readdir=BOOL`: Enable kernel caching of readdir
results. Defaults to `false`.
As of Linux v4.20 it supports readdir caching. This can have a
significant impact on directory traversal. Especially when combined
with entry (`cache.entry`) and attribute (`cache.attr`) caching. If
the kernel doesn't support readdir caching setting the option to true
has no effect. This option is configurable at runtime via xattr
user.mergerfs.cache.readdir.
## cache.writeback
* `cache.writeback=BOOL`: Enable writeback cache. Defaults to `false`.
When `cache.files` is enabled the default is for it to perform
writethrough caching. This behavior won't help improve performance as
each write still goes one for one through the filesystem. By enabling
the FUSE writeback cache small writes *may* be aggregated by the
kernel and then sent to mergerfs as one larger request. This can
greatly improve the throughput for apps which write to files
inefficiently. The amount the kernel can aggregate is limited by the
size of a FUSE message. Read the fuse_msg_size section for more
details.
There is a side effect as a result of enabling writeback
caching. Underlying files won't ever be opened with O_APPEND or
O_WRONLY. The former because the kernel then manages append mode and
the latter because the kernel may request file data from mergerfs to
populate the write cache. The O_APPEND change means that if a file is
changed outside of mergerfs it could lead to corruption as the kernel
won't know the end of the file has changed. That said any time you use
caching you should keep from writing to the same file outside of
mergerfs at the same time.
Note that if an application is properly sizing writes then writeback
caching will have little or no effect. It will only help with writes
of sizes below the FUSE message size (128K on older kernels, 1M on
newer). Even then its effectiveness might not be great. Given the side
effects of enabling this feature it is recommended that its benefits
be proved out with benchmarks.

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# Deprecated Options
These are old, deprecated options which may no longer have any
function or have been replaced.
* **direct_io**: Bypass page cache. Use `cache.files=off`
instead.
* **kernel_cache**: Do not invalidate data cache on file open. Use
`cache.files=full` instead.
* **auto_cache**: Invalidate data cache if file mtime or
size change. Use `cache.files=auto-full` instead. (default: false)
* **async_read**: Perform reads asynchronously. Use
`async_read=true` instead.
* **sync_read**: Perform reads synchronously. Use
`async_read=false` instead.
* **splice_read**: Does nothing.
* **splice_write**: Does nothing.
* **splice_move**: Does nothing.
* **allow_other**: mergerfs v2.35.0 and above sets this FUSE option
automatically if running as root.
* **use_ino**: Effectively replaced with `inodecalc`.

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# export-support
* `export-support=true|false`
* Defaults to `true`.
In theory, this flag should not be exposed to the end user. It is a
low-level FUSE flag which indicates whether or not the kernel can send
certain kinds of messages to it for the purposes of using it with
NFS. mergerfs does support these messages but due to bugs and quirks
found in the kernel and mergerfs this option is provided just in case
it is needed for debugging.
Given that this flag is set when the FUSE connection is first
initiated it is not possible to change during run time.

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# flush-on-close
By default, FUSE would issue a flush before the release of a file
descriptor. This was considered a bit aggressive and a feature added
to give the FUSE server the ability to choose when that happens.
* `flush-on-close=always`
* `flush-on-close=never`
* `flush-on-close=opened-for-write`
* Defaults to `opened-for-write`.
For now it defaults to `opened-for-write` which is less aggressive
than the behavior before this feature was added. It should not be a
problem because the flush is really only relevant when a file is
written to. Given flush is irrelevant for many filesystems in the
future a branch specific flag may be added so only files opened on a
specific branch would be flushed on close.
## References
* [https://lkml.kernel.org/linux-fsdevel/20211024132607.1636952-1-amir73il@gmail.com/T/](https://lkml.kernel.org/linux-fsdevel/20211024132607.1636952-1-amir73il@gmail.com/T/)

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# follow-symlinks
This feature, when enabled, will cause symlinks to be interpreted by
mergerfs as their target.
When there is a getattr/stat request for a file mergerfs will check if
the file is a symlink and depending on the `follow-symlinks` setting
will replace the information about the symlink with that of that which
it points to.
When unlink'ing or rmdir'ing the followed symlink it will remove the
symlink itself and not that which it points to.
* `follow-symlinks=never`: Behave as normal. Symlinks are treated as such.
* `follow-symlinks=directory`: Resolve symlinks only which point to directories.
* `follow-symlinks=regular`: Resolve symlinks only which point to regular files.
* `follow-symlinks=all`: Resolve all symlinks to that which they point
to. Symlinks which do not point to anything are left as is.
* Defaults to `never`.
**WARNING:** This feature should be considered experimental. There
might be edge cases yet found. If you find any odd behaviors please
file a ticket on [github](https://github.com/trapexit/mergerfs/issues/new?assignees=&labels=bug%2C+investigating&projects=&template=bug_report.md&title=).

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# func.readdir
examples: `func.readdir=seq`, `func.readdir=cor:4`
`readdir` has policies to control how it reads directory content.
| Policy | Description |
| ------ | ----------- |
| seq | "sequential" : Iterate sequentially over branches in the order defined in `branches`. This is the default and traditional behavior found prior to the readdir policy introduction. This will be increasingly slower as more branches are added to the pool. Especially if needing to wait for drives to spin up or network filesystems to respond. |
| cosr | "concurrent open, sequential read" : Concurrently open branch directories using a thread pool and process them in the order defined in `branches`. This keeps memory and CPU usage low while also reducing the time spent waiting on branches to respond. Number of threads defaults to the number of logical cores. Can be overwritten via the syntax `func.readdir=cosr:N` where `N` is the number of threads. |
| cor | "concurrent open and read" : Concurrently open branch directories and immediately start reading their contents using a thread pool. This will result in slightly higher memory and CPU usage but reduced latency. Particularly when using higher latency / slower speed network filesystem branches. Unlike `seq` and `cosr` the order of files could change due the async nature of the thread pool. This should not be a problem since the order of files listed in not guaranteed. Number of threads defaults to the number of logical cores. Can be overwritten via the syntax `func.readdir=cor:N` where `N` is the number of threads. |
Keep in mind that `readdir` mostly just provides a list of file names
in a directory and possibly some basic metadata about said files. To
know details about the files, as one would see from commands like
`find` or `ls`, it is required to call `stat` on the file which is
controlled by `fuse.getattr`.

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@ -1,4 +1,4 @@
# FUNCTIONS, CATEGORIES and POLICIES
# functions, categories and policies
The POSIX filesystem API is made up of a number of
functions. **creat**, **stat**, **chown**, etc. For ease of
@ -8,13 +8,8 @@ and categories can be assigned a policy which dictates which branch is
chosen when performing that function.
Some functions, listed in the category `N/A` below, can not be
assigned the normal policies. These functions work with file handles,
rather than file paths, which were created by `open` or `create`. That
said many times the current FUSE kernel driver will not always provide
the file handle when a client calls `fgetattr`, `fchown`, `fchmod`,
`futimens`, `ftruncate`, etc. This means it will call the regular,
path based, versions. `statfs`'s behavior can be modified via other
options.
assigned the normal policies because they are directly related to a
file which has already been opened.
When using policies which are based on a branch's available space the
base path provided is used. Not the full path to the file in
@ -28,19 +23,19 @@ some may not be very useful in practice. For instance: **rand**
very odd behavior if used for `chmod` if there were more than one copy
of the file.
### Functions and their Category classifications
## Functions and their Category classifications
| Category | FUSE Functions |
| -------- | -------------------------------------------------------------------------------------------------------------------------------------- |
| action | chmod, chown, link, removexattr, rename, rmdir, setxattr, truncate, unlink, utimens |
| create | create, mkdir, mknod, symlink |
| search | access, getattr, getxattr, ioctl (directories), listxattr, open, readlink |
| Category | Functions |
| -------- | --------- |
| action | chmod, chown, link, removexattr, rename, rmdir, setxattr, truncate, unlink, utimens |
| create | create, mkdir, mknod, symlink |
| search | access, getattr, getxattr, ioctl (directories), listxattr, open, readlink |
| N/A | fchmod, fchown, futimens, ftruncate, fallocate, fgetattr, fsync, ioctl (files), read, readdir, release, statfs, write, copy_file_range |
In cases where something may be searched for (such as a path to clone)
**getattr** will usually be used.
### Policies
## Policies
A policy is the algorithm used to choose a branch or branches for a
function to work on or generally how the function behaves.
@ -49,7 +44,7 @@ Any function in the `create` category will clone the relative path if
needed. Some other functions (`rename`,`link`,`ioctl`) have special
requirements or behaviors which you can read more about below.
#### Filtering
## Filtering
Most policies basically search branches and create a list of files / paths
for functions to work on. The policy is responsible for filtering and
@ -78,7 +73,7 @@ to be read-only as such (IE will set the mode `RO`) and will rerun the
policy and try again. This is mostly for `ext4` filesystems that can
suddenly become read-only when it encounters an error.
#### Path Preservation
## Path Preservation
Policies, as described below, are of two basic classifications. `path
preserving` and `non-path preserving`.
@ -98,7 +93,7 @@ With the `msp` or `most shared path` policies they are defined as
behaviors since `ignorepponrename` is available to disable that
behavior.
#### Policy descriptions
## Policy descriptions
A policy's behavior differs, as mentioned above, based on the function
it is used with. Sometimes it really might not make sense to even
@ -135,129 +130,10 @@ policy calculations. **df** does NOT use `f_bavail`, it uses
`f_bfree`, so direct comparisons between **df** output and mergerfs'
policies is not appropriate.
#### Defaults
## Defaults
| Category | Policy |
| -------- | ------ |
| action | epall |
| create | epmfs |
| search | ff |
#### func.readdir
examples: `func.readdir=seq`, `func.readdir=cor:4`
`readdir` has policies to control how it manages reading directory
content.
| Policy | Description |
| ------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| seq | "sequential" : Iterate over branches in the order defined. This is the default and traditional behavior found prior to the readdir policy introduction. |
| cosr | "concurrent open, sequential read" : Concurrently open branch directories using a thread pool and process them in order of definition. This keeps memory and CPU usage low while also reducing the time spent waiting on branches to respond. Number of threads defaults to the number of logical cores. Can be overwritten via the syntax `func.readdir=cosr:N` where `N` is the number of threads. |
| cor | "concurrent open and read" : Concurrently open branch directories and immediately start reading their contents using a thread pool. This will result in slightly higher memory and CPU usage but reduced latency. Particularly when using higher latency / slower speed network filesystem branches. Unlike `seq` and `cosr` the order of files could change due the async nature of the thread pool. Number of threads defaults to the number of logical cores. Can be overwritten via the syntax `func.readdir=cor:N` where `N` is the number of threads. |
Keep in mind that `readdir` mostly just provides a list of file names
in a directory and possibly some basic metadata about said files. To
know details about the files, as one would see from commands like
`find` or `ls`, it is required to call `stat` on the file which is
controlled by `fuse.getattr`.
#### ioctl
When `ioctl` is used with an open file then it will use the file
handle which was created at the original `open` call. However, when
using `ioctl` with a directory mergerfs will use the `open` policy to
find the directory to act on.
#### rename and link
**NOTE:** If you're receiving errors from software when files are
moved / renamed / linked then you should consider changing the create
policy to one which is **not** path preserving, enabling
`ignorepponrename`, or contacting the author of the offending software
and requesting that `EXDEV` (cross device / improper link) be properly
handled.
`rename` and `link` are tricky functions in a union
filesystem. `rename` only works within a single filesystem or
device. If a rename can't be done atomically due to the source and
destination paths existing on different mount points it will return
**-1** with **errno = EXDEV** (cross device / improper link). So if a
`rename`'s source and target are on different filesystems within the pool
it creates an issue.
Originally mergerfs would return EXDEV whenever a rename was requested
which was cross directory in any way. This made the code simple and
was technically compliant with POSIX requirements. However, many
applications fail to handle EXDEV at all and treat it as a normal
error or otherwise handle it poorly. Such apps include: gvfsd-fuse
v1.20.3 and prior, Finder / CIFS/SMB client in Apple OSX 10.9+,
NZBGet, Samba's recycling bin feature.
As a result a compromise was made in order to get most software to
work while still obeying mergerfs' policies. Below is the basic logic.
- If using a **create** policy which tries to preserve directory paths (epff,eplfs,eplus,epmfs)
- Using the **rename** policy get the list of files to rename
- For each file attempt rename:
- If failure with ENOENT (no such file or directory) run **create** policy
- If create policy returns the same branch as currently evaluating then clone the path
- Re-attempt rename
- If **any** of the renames succeed the higher level rename is considered a success
- If **no** renames succeed the first error encountered will be returned
- On success:
- Remove the target from all branches with no source file
- Remove the source from all branches which failed to rename
- If using a **create** policy which does **not** try to preserve directory paths
- Using the **rename** policy get the list of files to rename
- Using the **getattr** policy get the target path
- For each file attempt rename:
- If the source branch != target branch:
- Clone target path from target branch to source branch
- Rename
- If **any** of the renames succeed the higher level rename is considered a success
- If **no** renames succeed the first error encountered will be returned
- On success:
- Remove the target from all branches with no source file
- Remove the source from all branches which failed to rename
The removals are subject to normal entitlement checks.
The above behavior will help minimize the likelihood of EXDEV being
returned but it will still be possible.
**link** uses the same strategy but without the removals.
#### statfs / statvfs
[statvfs](http://linux.die.net/man/2/statvfs) normalizes the source
filesystems based on the fragment size and sums the number of adjusted
blocks and inodes. This means you will see the combined space of all
sources. Total, used, and free. The sources however are dedupped based
on the filesystem so multiple sources on the same drive will not result in
double counting its space. Other filesystems mounted further down the tree
of the branch will not be included when checking the mount's stats.
The options `statfs` and `statfs_ignore` can be used to modify
`statfs` behavior.
#### flush-on-close
https://lkml.kernel.org/linux-fsdevel/20211024132607.1636952-1-amir73il@gmail.com/T/
By default, FUSE would issue a flush before the release of a file
descriptor. This was considered a bit aggressive and a feature added
to give the FUSE server the ability to choose when that happens.
Options:
- always
- never
- opened-for-write
For now it defaults to "opened-for-write" which is less aggressive
than the behavior before this feature was added. It should not be a
problem because the flush is really only relevant when a file is
written to. Given flush is irrelevant for many filesystems in the
future a branch specific flag may be added so only files opened on a
specific branch would be flushed on close.

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# fuse_msg_size
* `fuse_msg_size=UINT`
* Defaults to `256`
FUSE applications communicate with the kernel over a special character
device: `/dev/fuse`. A large portion of the overhead associated with
FUSE is the cost of going back and forth between user space and kernel
space over that device. Generally speaking, the fewer trips needed the
better the performance will be. Reducing the number of trips can be
done a number of ways. Kernel level caching and increasing message
sizes being two significant ones. When it comes to reads and writes if
the message size is doubled the number of trips are approximately
halved.
In Linux v4.20 a new feature was added allowing the negotiation of the
max message size. Since the size is in multiples of
[pages](https://en.wikipedia.org/wiki/Page_(computer_memory)) the
feature is called `max_pages`. There is a maximum `max_pages` value of
256 (1MiB) and minimum of 1 (4KiB). The default used by Linux >=4.20,
and hardcoded value used before 4.20, is 32 (128KiB). In mergerfs it's
referred to as fuse_msg_size to make it clear what it impacts and
provide some abstraction.
Since there should be no downsides to increasing `fuse_msg_size`,
outside a minor increase in RAM usage due to larger message buffers,
mergerfs defaults the value to 256. On kernels before v4.20 the value
has no effect. The reason the value is configurable is to enable
experimentation and benchmarking.

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# inodecalc
Inodes (`st_ino`) are unique identifiers within a filesystem. Each
mounted filesystem has device ID (st_dev) as well and together they
can uniquely identify a file on the whole of the system. Entries on
the same device with the same inode are in fact references to the same
underlying file. It is a many to one relationship between names and an
inode. Directories, however, do not have multiple links on most
systems due to the complexity they add.
FUSE allows the server (mergerfs) to set inode values but not device
IDs. Creating an inode value is somewhat complex in mergerfs' case as
files aren't really in its control. If a policy changes what directory
or file is to be selected or something changes out of band it becomes
unclear what value should be used. Most software does not to care what
the values are but those that do often break if a value changes
unexpectedly. The tool find will abort a directory walk if it sees a
directory inode change. NFS can return stale handle errors if the
inode changes out of band. File dedup tools will usually leverage
device ids and inodes as a shortcut in searching for duplicate files
and would resort to full file comparisons should it find different
inode values.
mergerfs offers multiple ways to calculate the inode in hopes of
covering different usecases.
* `passthrough`: Passes through the underlying inode value. Mostly
intended for testing as using this does not address any of the
problems mentioned above and could confuse file deduplication
software as inodes from different filesystems can be the same.
* `path-hash`: Hashes the relative path of the entry in question. The
underlying file's values are completely ignored. This means the
inode value will always be the same for that file path. This is
useful when using NFS and you make changes out of band such as copy
data between branches. This also means that entries that do point to
the same file will not be recognizable via inodes. That does not
mean hard links don't work. They will.
* `path-hash32`: 32bit version of path-hash.
* `devino-hash`: Hashes the device id and inode of the underlying
entry. This won't prevent issues with NFS should the policy pick a
different file or files move out of band but will present the same
inode for underlying files that do too.
* `devino-hash32`: 32bit version of devino-hash.
* `hybrid-hash`: Performs path-hash on directories and devino-hash on
other file types. Since directories can't have hard links the static
value won't make a difference and the files will get values useful
for finding duplicates. Probably the best to use if not using
NFS. As such it is the default.
* `hybrid-hash32`: 32bit version of hybrid-hash.
32bit versions are provided as there is some software which does not
handle 64bit inodes well.
While there is a risk of hash collision in tests of a couple of
million entries there were zero collisions. Unlike a typical
filesystem FUSE filesystems can reuse inodes and not refer to the same
entry. The internal identifier used to reference a file in FUSE is
different from the inode value presented. The former is the nodeid and
is actually a tuple of 2 64bit values: nodeid and generation. This
tuple is not client facing. The inode that is presented to the client
is passed through the kernel uninterpreted.
From FUSE docs for `use_ino`:
> Honor the st_ino field in the functions getattr() and
> fill_dir(). This value is used to fill in the st_ino field
> in the stat(2), lstat(2), fstat(2) functions and the d_ino
> field in the readdir(2) function. The filesystem does not
> have to guarantee uniqueness, however some applications
> rely on this value being unique for the whole filesystem.
> Note that this does *not* affect the inode that libfuse
> and the kernel use internally (also called the "nodeid").
**NOTE:** As of version 2.35.0 the use_ino option has been
removed. mergerfs should always be managing inode values.

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# link-exdev
If using path preservation and a `link` fails with `EXDEV` make a call
to `symlink` where the target is the `oldlink` and the `linkpath` is
the newpath. The target value is determined by the value of
`link-exdev`.
* `link-exdev=passthrough`: Return EXDEV as normal.
* `link-exdev=rel-symlink`: A relative path from the newpath.
* `link-exdev=abs-base-symlink`: An absolute value using the
underlying branch.
* `link-exdev=abs-pool-symlink`: An absolute value using the mergerfs
mount point.
* Defaults to `passthrough`.
**NOTE:** It is possible that some applications check the file they
link. In those cases, it is possible it will error or complain.

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# link_cow
* `link_cow=true|false`
* Defaults to `false`
This feature offers similar functionality to what
[cow-shell](https://manpages.ubuntu.com/manpages/noble/man1/cow-shell.1.html)
offers.
When enabled if mergerfs is asked to open a file to write and the link
count on the file is greater than 1 it will copy the file to a
temporary new file and then rename it over the original. This will
atomically "break" the link. After that it will open the new file.

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# nfsopenhack
* `nfsopenhack=off`: No hack applied.
* `nfsopenhack=git`: Apply hack if path includes `/.git/`.
* `nfsopenhack=all`: Apply hack on all empty read-only files opened
for writing.
* Defaults to `off`.
NFS is not fully POSIX compliant and historically certain behaviors,
such as opening files with `O_EXCL`, are not or not well
supported. When mergerfs (or any FUSE filesystem) is exported over NFS
some of these issues come up due to how NFS and FUSE interact.
This hack addresses the issue where the creation of a file with a
read-only mode but with a read/write or write only flag. Normally this
is perfectly valid but NFS chops the one open call into multiple
calls. Exactly how it is translated depends on the configuration and
versions of the NFS server and clients but it results in a permission
error because a normal user is not allowed to open a read-only file as
writable.
Even though it's a more niche situation this hack breaks normal
security and behavior and as such is `off` by default. If set to `git`
it will only perform the hack when the path in question includes
`/.git/`. `all` will result in it applying anytime a read-only file
which is empty is opened for writing.

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# nullrw
* `nullrw=true|false`
* Defaults to `false`.
Due to how FUSE works there is an overhead to all requests made to a
FUSE filesystem that wouldn't exist for an in kernel one. Meaning that
even a simple passthrough will have some slowdown. However, generally
the overhead is minimal in comparison to the cost of the underlying
I/O. By disabling the underlying I/O we can test the theoretical
performance boundaries.
By enabling `nullrw` mergerfs will work as it always does **except**
that all reads and writes will be no-ops. A write will succeed (the
size of the write will be returned as if it were successful) but
mergerfs does nothing with the data it was given. Similarly a read
will return the size requested but won't touch the buffer.
See the [benchmarking](../benchmarking.md) section for suggestions on
how to test.

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# OPTIONS
# Options
These options are the same regardless of whether you use them with the
`mergerfs` commandline program, in fstab, or in a config file.
### types
- BOOL = 'true' | 'false'
- INT = [MIN_INT,MAX_INT]
- UINT = [0,MAX_INT]
- SIZE = 'NNM'; NN = INT, M = 'K' | 'M' | 'G' | 'T'
- STR = string (may refer to an enumerated value, see details of
argument)
- FUNC = filesystem function
- CATEGORY = function category
- POLICY = mergerfs function policy
### mount options
- **config**: Path to a config file. Same arguments as below in
@ -139,8 +152,6 @@ These options are the same regardless of whether you use them with the
create category. (default: epmfs)
- **category.search=POLICY**: Sets policy of all FUSE functions in the
search category. (default: ff)
- **cache.open=UINT**: 'open' policy cache timeout in
seconds. (default: 0)
- **cache.statfs=UINT**: 'statfs' cache timeout in seconds. (default: 0)
- **cache.attr=UINT**: File attribute cache timeout in
seconds. (default: 1)
@ -164,82 +175,10 @@ These options are the same regardless of whether you use them with the
multiple, parallel (non-extending) write requests for files opened
with `cache.files=per-process` (if the process is not in `process-names`)
or `cache.files=off`. (This requires kernel support, and was added in v6.2)
- **direct_io**: deprecated - Bypass page cache. Use `cache.files=off`
instead. (default: false)
- **kernel_cache**: deprecated - Do not invalidate data cache on file
open. Use `cache.files=full` instead. (default: false)
- **auto_cache**: deprecated - Invalidate data cache if file mtime or
size change. Use `cache.files=auto-full` instead. (default: false)
- **async_read**: deprecated - Perform reads asynchronously. Use
`async_read=true` instead.
- **sync_read**: deprecated - Perform reads synchronously. Use
`async_read=false` instead.
- **splice_read**: deprecated - Does nothing.
- **splice_write**: deprecated - Does nothing.
- **splice_move**: deprecated - Does nothing.
- **allow_other**: deprecated - mergerfs v2.35.0 and newer sets this FUSE option
automatically if running as root.
- **use_ino**: deprecated - mergerfs should always control inode
calculation so this is enabled all the time.
**NOTE:** Options are evaluated in the order listed so if the options
are **func.rmdir=rand,category.action=ff** the **action** category
setting will override the **rmdir** setting.
**NOTE:** Always look at the documentation for the version of mergerfs
you're using. Not all features are available in older releases. Use
`man mergerfs` or find the docs as linked in the release.
#### Value Types
- BOOL = 'true' | 'false'
- INT = [MIN_INT,MAX_INT]
- UINT = [0,MAX_INT]
- SIZE = 'NNM'; NN = INT, M = 'K' | 'M' | 'G' | 'T'
- STR = string (may refer to an enumerated value, see details of
argument)
- FUNC = filesystem function
- CATEGORY = function category
- POLICY = mergerfs function policy
### branches
The 'branches' argument is a colon (':') delimited list of paths to be
pooled together. It does not matter if the paths are on the same or
different filesystems nor does it matter the filesystem type (within
reason). Used and available space will not be duplicated for paths on
the same filesystem and any features which aren't supported by the
underlying filesystem (such as file attributes or extended attributes)
will return the appropriate errors.
Branches currently have two options which can be set. A type which
impacts whether or not the branch is included in a policy calculation
and a individual minfreespace value. The values are set by prepending
an `=` at the end of a branch designation and using commas as
delimiters. Example: `/mnt/drive=RW,1234`
#### branch mode
- RW: (read/write) - Default behavior. Will be eligible in all policy
categories.
- RO: (read-only) - Will be excluded from `create` and `action`
policies. Same as a read-only mounted filesystem would be (though
faster to process).
- NC: (no-create) - Will be excluded from `create` policies. You can't
create on that branch but you can change or delete.
#### minfreespace
Same purpose and syntax as the global option but specific to the
branch. If not set the global value is used.
#### globbing
To make it easier to include multiple branches mergerfs supports
[globbing](http://linux.die.net/man/7/glob). **The globbing tokens
MUST be escaped when using via the shell else the shell itself will
apply the glob itself.**
```
```
you're using. Not all features are available in older releases.

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# pin-threads
Simple strategies for pinning read and/or process threads. If process
threads are not enabled then the strategy simply works on the read
threads. Invalid values are ignored.
* `pin-threads=R1L`: All read threads pinned to a single logical CPU.
* `pin-threads=R1P`: All read threads pinned to a single physical CPU.
* `pin-threads=RP1L`: All read and process threads pinned to a single logical CPU.
* `pin-threads=RP1P`: All read and process threads pinned to a single physical CPU.
* `pin-threads=R1LP1L`: All read threads pinned to a single logical
CPU, all process threads pinned to a (if possible) different logical
CPU.
* `pin-threads=R1PP1P`: All read threads pinned to a single physical
CPU, all process threads pinned to a (if possible) different logical
CPU.
* `pin-threads=RPSL`: All read and process threads are spread across
all logical CPUs.
* `pin-threads=RPSP`: All read and process threads are spread across
all physical CPUs.
* `pin-threads=R1PPSP`: All read threads are pinned to a single
physical CPU while process threads are spread across all other
physical CPUs.

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# readahead
Sets the mergerfs and underlying filesystem `readahead` values. The
value unit is in kibibytes.
* `readahead=1024`
While the max size of messages sent between the kernel and mergerfs is
configurable via the [fuse_msg_size](fuse_msg_size.md) option that
doesn't mean that is the size used by the kernel for read and
writes.
Linux has a max read/write size of 2GB. Since the max FUSE message
size is just over 1MB the kernel will break up read and write requests
with buffers larger than that 1MB.
When page caching is disabled (`cache.files=off`), besides the kernel
breaking up requests with larger buffers, requests are effectively one
for one to mergerfs. A read or write request for X bytes is made to
the kernel and a request for X bytes is made to mergerfs. No
[readahead](https://en.wikipedia.org/wiki/Readahead) behavior will
occur because there is no page cache available for it to store that
data. In FUSE this is referred to as "direct IO". Note that "direct
IO" is not the same as `O_DIRECT`.
When page caching is enabled the kernel can and will utilize
`readahead`. However, there are two values which impact the size of
the `readahead` requests. The filesystem's `readahead` value and the
FUSE `max_readahead` value. Whichever is lowest is used. The default
`max_readahead` in mergerfs is maxed out meaning only the filesystem
`readahead` value is relevant.
Preferably this value would be set by the user externally since it is
a generic feature but there is no standard way to do so mergerfs added
this feature to make it easier to set.
There is currently no way to set separate values for different
branches through mergerfs.

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# rename-exdev
If using path preservation and a `rename` fails with `EXDEV`:
1. Move file from `/branch/a/b/c` to `/branch/.mergerfs_rename_exdev/a/b/c`.
2. symlink the rename's `newpath` to the moved file.
The `target` value is determined by the value of `rename-exdev`.
* `rename-exdev=passthrough`: Return `EXDEV` as normal.
* `rename-exdev=rel-symlink`: A relative path from the `newpath`.
* `rename-exdev=abs-symlink`: An absolute value using the mergerfs
mount point.
* Defaults to `passthrough`.
**NOTE:** It is possible that some applications check the file they
rename. In those cases it is possible it will error or complain.
**NOTE:** The reason `abs-symlink` is not split into two like
`link-exdev` is due to the complexities in managing absolute base
symlinks when multiple `oldpaths` exist.

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# rename and link
**NOTE:** If you're receiving errors from software when files are
moved / renamed / linked then you should consider changing the create
policy to one which is **not** path preserving, enabling
`ignorepponrename`, or contacting the author of the offending software
and requesting that `EXDEV` (cross device / improper link) be properly
handled.
`rename` and `link` are arguably the most complicated functions to
create in a union filesystem. `rename` only works within a single
filesystem or device. If a rename can't be done due to the source and
destination paths existing on different mount points it will return
**-1** with **errno = EXDEV** (cross device / improper link). So if a
`rename`'s source and target are on different filesystems within the
pool it creates an issue.
Originally mergerfs would return EXDEV whenever a rename was requested
which was cross directory in any way. This made the code simple and
was technically compliant with POSIX requirements. However, many
applications fail to handle EXDEV at all and treat it as a normal
error or otherwise handle it poorly. Such apps include: gvfsd-fuse
v1.20.3 and prior, Finder / CIFS/SMB client in Apple OSX 10.9+,
NZBGet, Samba's recycling bin feature.
As a result a compromise was made in order to get most software to
work while still obeying mergerfs' policies. Below is the basic logic.
* If using a **create** policy which tries to preserve directory paths (epff,eplfs,eplus,epmfs)
* Using the **rename** policy get the list of files to rename
* For each file attempt rename:
* If failure with ENOENT (no such file or directory) run **create** policy
* If create policy returns the same branch as currently evaluating then clone the path
* Re-attempt rename
* If **any** of the renames succeed the higher level rename is considered a success
* If **no** renames succeed the first error encountered will be returned
* On success:
* Remove the target from all branches with no source file
* Remove the source from all branches which failed to rename
* If using a **create** policy which does **not** try to preserve directory paths
* Using the **rename** policy get the list of files to rename
* Using the **getattr** policy get the target path
* For each file attempt rename:
* If the source branch != target branch:
* Clone target path from target branch to source branch
* Rename
* If **any** of the renames succeed the higher level rename is considered a success
* If **no** renames succeed the first error encountered will be returned
* On success:
* Remove the target from all branches with no source file
* Remove the source from all branches which failed to rename
The removals are subject to normal entitlement checks. If the unlink
fails it will fail silently.
The above behavior will help minimize the likelihood of EXDEV being
returned but it will still be possible.
**link** uses the same strategy but without the removals.

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# statfs / statvfs
* `statfs=base`: Aggregate details from all branches using their base directory.
* `statfs=full`: Aggregate details using the full path of the file
requested. Limiting it to only branches where the file exists.
* Defaults to `base`.
[statvfs](http://linux.die.net/man/2/statvfs) normalizes the source
filesystems based on the fragment size and sums the number of adjusted
blocks and inodes. This means you will see the combined space of all
sources. Total, used, and free. The sources however are dedupped based
on the filesystem so multiple sources on the same drive will not result in
double counting its space. Other filesystems mounted further down the tree
of the branch will not be included when checking the mount's stats.
## statfs_ignore
Modifies how `statfs` works. Will cause it to ignore branches of a
certain mode.
* `statfs_ignore=none`: Include all branches.
* `statfs_ignore=ro`: Ignore available space for branches mounted as
read-only or have a mode `RO` or `NC`.
* `statfs_ignore=nc`: Ignore available space for branches with a mode
of `NC`.
* Defaults to `none`.

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# symlinkify
* `symlinkify=true|false`
* Defaults to `false`.
Due to the levels of indirection introduced by mergerfs and the
underlying technology FUSE there can be varying levels of performance
degradation. This feature will turn non-directories which are not
writable into symlinks to the original file found by the `readlink`
policy after the mtime and ctime are older than the timeout.
**WARNING:** The current implementation has a known issue in which if
the file is open and being used when the file is converted to a
symlink then the application which has that file open will receive an
error when using it. This is unlikely to occur in practice but is
something to keep in mind.
**WARNING:** Some backup solutions, such as CrashPlan, do not backup
the target of a symlink. If using this feature it will be necessary to
point any backup software to the original filesystems or configure the
software to follow symlinks if such an option is
available. Alternatively, create two mounts. One for backup and one
for general consumption.

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# Terminology
- `branch`: A base path used in the pool. Keep in mind that mergerfs
does not work on devices or even filesystems but on paths. It can
accomidate for multiple paths pointing to the same filesystem.
- `pool`: The mergerfs mount. The union of the branches. The instance
of mergerfs. You can have as many pools as you wish.
- `relative path`: The path in the pool relative to the branch and mount.
- `function`: A filesystem call (open, unlink, create, getattr, rmdir, etc.)
- `category`: A collection of functions based on basic behavior (action, create, search).
- `policy`: The algorithm used to select a file or files when performing a function.
- `path preservation`: Aspect of some policies which includes checking the path for which a file would be created.

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# threads
There are multiple thread pools used in mergerfs to provide
parallel behaviors.
## read-thread-count
The number of threads used to read (and possibly process) messages
from the kernel.
* `read-thread-count=0`: Create a thread pool sized to the number of
logical CPUs.
* `read-thread-count=N` where `N>0`: Create a thread pool of `N` threads.
* `read-thread-count=N` where `N<0`: Create a thread pool of `CPUCount /
-N` threads.
* `read-thread-count=-1` where `process-thread-count=-1`: Creates `2`
read threads and `max(2,CPUCount-2)` process threads.
* Defaults to `0`.
When `process-thread-count=-1` (the default) this option sets the
number of threads which read and then process requests from the
kernel.
When `process-thread-count` is set to anything else mergerfs will
create two thread pools. A "read" thread pool which just reads from
the kernel and hands off requests to the "process" thread pool.
Generally, only 1 or 2 "read" threads are necessary.
## process-thread-count
When enabled this sets the number of threads in the message processing pool.
* `process-thread-count=-1`: Process thread pool is disabled.
* `process-thread-count=0`: Create a thread pool sized to the number
of logical CPUs.
* `process-thread-count=N` where `N>0`: Create a thread pool of `N` threads.
* `process-thread-count=N` where `N<-1`: Create a thread pool of `CPUCount /
-N` threads.
* Defaults to `-1`.
## process-thread-queue-depth
* `process-thread-queue-depth=N` where `N>0`: Sets the number of outstanding
requests that a process thread can have to N. If requests come in
faster than can be processed and the max queue depth hit then
queuing the request will block in order to limit memory growth.
* `process-thread-queue-depth=0`: Sets the queue depth to the thread
pool count.
* Defaults to `0`.

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@ -0,0 +1,27 @@
# xattr
* `xattr=passthrough`: Passes through all requests to underlying file.
* `xattr=noattr`: mergerfs receives the request but returns `NOATTR`.
* `xattr=nosys`: Tells the kernel to reject all `xattr` requests.
* Defaults to `passthrough`.
Runtime extended attribute support can be managed via the `xattr`
option. By default it will passthrough any xattr calls. Given xattr
support is rarely used and can have significant performance
implications mergerfs allows it to be disabled at runtime. The
performance problems mostly comes when file caching is enabled. The
kernel will send a `getxattr` for `security.capability` *before every
single write*. It doesn't cache the responses to any `getxattr`. This
might be addressed in the future but for now mergerfs can really only
offer the following workarounds.
`noattr` will cause mergerfs to short circuit all xattr calls and
return ENOATTR where appropriate. mergerfs still gets all the requests
but they will not be forwarded on to the underlying filesystems. The
runtime control will still function in this mode.
`nosys` will cause mergerfs to return `ENOSYS` for any xattr call. The
difference with `noattr` is that the kernel will cache this fact and
itself short circuit future calls. This is more efficient than
`noattr` but will cause mergerfs' runtime control via the hidden file
to stop working.

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@ -1,4 +1,4 @@
# ERROR HANDLING
# Error Handling
POSIX filesystem functions offer a single return code meaning that
there is some complication regarding the handling of multiple branches

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@ -2,7 +2,24 @@
## Can I use mergerfs without SnapRAID? SnapRAID without mergerfs?
Yes. They are completely unrelated pieces of software.
Yes. They are completely unrelated pieces of software that just happen
to work well together.
## Does mergerfs support CoW / copy-on-write / writes to read-only filesystems?
Not in the sense of a filesystem like BTRFS or ZFS nor in the
overlayfs or aufs sense. It does offer a
[cow-shell](http://manpages.ubuntu.com/manpages/bionic/man1/cow-shell.1.html)
like hard link breaking (copy to temp file then rename over original)
which can be useful when wanting to save space by hardlinking
duplicate files but wish to treat each name as if it were a unique and
separate file.
If you want to write to a read-only filesystem you should look at
overlayfs. You can always include the overlayfs mount into a mergerfs
pool.
## Can mergerfs run via Docker, Podman, Kubernetes, etc.
@ -23,17 +40,3 @@ managing shared files.
Also, as mentioned by [hotio](https://hotio.dev/containers/mergerfs),
with Docker you should probably be mounting with `bind-propagation`
set to `slave`.
## Does mergerfs support CoW / copy-on-write / writes to read-only filesystems?
Not in the sense of a filesystem like BTRFS or ZFS nor in the
overlayfs or aufs sense. It does offer a
[cow-shell](http://manpages.ubuntu.com/manpages/bionic/man1/cow-shell.1.html)
like hard link breaking (copy to temp file then rename over original)
which can be useful when wanting to save space by hardlinking
duplicate files but wish to treat each name as if it were a unique and
separate file.
If you want to write to a read-only filesystem you should look at
overlayfs. You can always include the overlayfs mount into a mergerfs
pool.

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@ -38,17 +38,18 @@ branch. So it keeps the system in a known state. It's much easier to
stop using `epmfs` or redistribute files around the filesystem than it
is to consolidate them back.
## What settings should I use?
Depends on what features you want. Generally speaking, there are no
"wrong" settings. All settings are performance or feature related. The
best bet is to read over the available options and choose what fits
your situation. If something isn't clear from the documentation please
"wrong" settings. All settings are feature related. The best bet is to
read over the available options and choose what fits your
situation. If something isn't clear from the documentation please
reach out and the documentation will be improved.
That said, for the average person, the following should be fine:
For the average person the settings described in the [Quick
Start](../quickstart.md) are sufficient.
`cache.files=off,dropcacheonclose=true,category.create=mfs`
## Why are all my files ending up on 1 filesystem?!
@ -63,14 +64,12 @@ directory is created. Anything, files or directories, created in that
first directory will be placed on the same branch because it is
preserving paths.
This catches a lot of new users off guard but changing the default
would break the setup for many existing users and this policy is the
safest policy as it will not change the general layout of the existing
filesystems. If you do not care about path preservation and wish your
files to be spread across all your filesystems change to `mfs` or
similar policy as described above. If you do want path preservation
you'll need to perform the manual act of creating paths on the
filesystems you want the data to land on before transferring your
data. Setting `func.mkdir=epall` can simplify managing path
preservation for `create`. Or use `func.mkdir=rand` if you're
interested in just grouping directory content by filesystem.
This may catch new users off guard but this policy is the safest
policy to start with as it will not change the general layout of the
underlying filesystems. If you do not care about path preservation
(most shouldn't) and wish your files to be spread across all your
filesystems change to `mfs` or similar
[policy](../config/functions_categories_and_policies.md). If you do
want path preservation you'll need to perform the manual act of
creating paths on the filesystems you want the data to land on before
transferring your data.

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@ -1,5 +1,10 @@
# Limiting drive spinup
## How can I setup my system to limit drive spinup?
TL;DR: You really can't. Not through mergerfs alone.
mergerfs is a proxy. Not a cache. It proxies calls between client software and underlying filesystems. If a client does an `open`, `readdir`, `stat`, etc. it must translate that into something that makes sense across N filesystems. For `readdir` that means running the call against all branches and aggregating the output. For `open` that means finding the file to open and doing so. The only way to find the file to open is to scan across all branches and sort the results and pick one. There is no practical way to do otherwise. Especially given so many mergerfs users expect out of band changes to "just work."
The best way to limit spinup of drives is to limit their usage at the client level. Meaning keeping software from interacting with the filesystem all together.

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@ -4,21 +4,22 @@
- databases: Even if the database stored data in separate files
(mergerfs wouldn't offer much otherwise) the higher latency of the
indirection will kill performance. If it is a lightly used SQLITE
database then it may be fine but you'll need to test.
indirection will really harm performance. If it is a lightly used
sqlite3 database then it should be fine.
- VM images: For the same reasons as databases. VM images are accessed
very aggressively and mergerfs will introduce too much latency (if
it works at all).
very aggressively and mergerfs will introduce a lot of extra latency.
- As replacement for RAID: mergerfs is just for pooling branches. If
you need that kind of device performance aggregation or high
availability you should stick with RAID.
availability you should stick with RAID. However, it is fine to put
a filesystem which is on a RAID setup in mergerfs.
## It's mentioned that there are some security issues with mhddfs. What are they? How does mergerfs address them?
[mhddfs](https://github.com/trapexit/mhddfs) manages running as
**root** by calling
`root` by calling
[getuid()](https://github.com/trapexit/mhddfs/blob/cae96e6251dd91e2bdc24800b4a18a74044f6672/src/main.c#L319)
and if it returns **0** then it will
and if it returns `0` then it will
[chown](http://linux.die.net/man/1/chown) the file. Not only is that a
race condition but it doesn't handle other situations. Rather than
attempting to simulate POSIX ACL behavior the proper way to manage
@ -27,11 +28,11 @@ this is to use [seteuid](http://linux.die.net/man/2/seteuid) and
user making the original call, and perform the action as them. This is
what mergerfs does and why mergerfs should always run as root.
In Linux setreuid syscalls apply only to the thread. GLIBC hides this
In Linux setreuid syscalls apply only to the thread. glibc hides this
away by using realtime signals to inform all threads to change
credentials. Taking after **Samba**, mergerfs uses
**syscall(SYS_setreuid,...)** to set the callers credentials for that
thread only. Jumping back to **root** as necessary should escalated
credentials. Taking after Samba, mergerfs uses
`syscall(SYS_setreuid,...)` to set the callers credentials for that
thread only. Jumping back to `root` as necessary should escalated
privileges be needed (for instance: to clone paths between
filesystems).

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# Reliability and Scalability
## Is mergerfs "production ready?"
Yes.
mergerfs has been around for over a decade and used by many users on
their systems. Typically running 24/7 with constant load.
At least a few companies are believed to use mergerfs in production
environments. A number of [NAS focused operating
systems](../related_projects.md) includes mergerfs as a solution for
pooling filesystems.
Most serious issues (crashes or data corruption) have been due to
[kernel bugs](../known_issues_bugs.md#fuse-and-linux-kernel). All of
which are fixed in stable releases.
## How well does mergerfs scale?
Users have reported running mergerfs on everything from a Raspberry Pi
to multi-socket Xeon enterprise servers.
Users have pooled everything from USB thumb drives to enterprise NVME
SSDs to remote filesystems and rclone mounts.
The cost of many calls can be `O(n)` meaning adding more branches to
the pool will increase the cost of certain functions but there are a
number of caches and stategies in place to limit overhead where
possible.

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@ -166,3 +166,50 @@ pay close attention to anything regarding permissioning and
users. Root squashing and user translation for instance has bitten a
few mergerfs users. Some of these also affect the use of mergerfs from
container platforms such as Docker.
## Why use FUSE? Why not a kernel based solution?
As with any solution to a problem, there are advantages and
disadvantages to each one.
A FUSE based solution has all the downsides of FUSE:
- Higher IO latency due to the trips in and out of kernel space
- Higher general overhead due to trips in and out of kernel space
- Double caching when using page caching
- Misc limitations due to FUSE's design
But FUSE also has a lot of upsides:
- Easier to offer a cross platform solution
- Easier forward and backward compatibility
- Easier updates for users
- Easier and faster release cadence
- Allows more flexibility in design and features
- Overall easier to write, secure, and maintain
- Much lower barrier to entry (getting code into the kernel takes a
lot of time and effort initially)
## Is my OS's libfuse needed for mergerfs to work?
No. Normally `mount.fuse` is needed to get mergerfs (or any FUSE
filesystem to mount using the `mount` command but in vendoring the
libfuse library the `mount.fuse` app has been renamed to
`mount.mergerfs` meaning the filesystem type in `fstab` can simply be
`mergerfs`. That said there should be no harm in having it installed
and continuing to using `fuse.mergerfs` as the type in `/etc/fstab`.
If `mergerfs` doesn't work as a type it could be due to how the
`mount.mergerfs` tool was installed. Must be in `/sbin/` with proper
permissions.
## Why was splice support removed?
After a lot of testing over the years, splicing always appeared to
at best, provide equivalent performance, and in some cases, worse
performance. Splice is not supported on other platforms forcing a
traditional read/write fallback to be provided. The splice code was
removed to simplify the codebase.

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@ -3,8 +3,8 @@
## Can mergerfs be used with filesystems which already have data / are in use?
Yes. mergerfs is really just a proxy and does **NOT** interfere with
the normal form or function of the filesystems / mounts / paths it
manages. It is just another userland application that is acting as a
the normal form or function of the filesystems, mounts, paths it
manages. A userland application that is acting as a
man-in-the-middle. It can't do anything that any other random piece of
software can't do.
@ -14,34 +14,45 @@ manipulate the data that passes through it. It does **not** shard data
across filesystems. It merely shards some **behavior** and aggregates
others.
## Can drives/filesystems be removed from the pool at will?
## Can filesystems be removed from the pool without affecting them?
Yes. See previous question's answer.
## Can mergerfs be removed without affecting the data?
Yes. See the previous question's answer.
## Can drives/filesystems be moved to another pool?
## Can filesystems be moved to another pool?
Yes. See the previous question's answer.
## How do I migrate data into or out of the pool when adding/removing drives/filesystems?
## Can filesystems be part of multiple pools?
Yes.
## How do I migrate data into or out of the pool when adding/removing filesystems?
You don't need to. See the previous question's answer.
## How do I remove a drive/filesystem but keep the data in the pool?
## How do I remove a filesystem but keep the data in the pool?
Nothing special needs to be done. Remove the branch from mergerfs'
config and copy (rsync) the data from the removed filesystem into the
pool. Effectively the same as if it were you transfering data from one
filesystem to another.
pool. The same as if it were you transfering data from one filesystem
to another.
If you wish to continue using the pool while performing the transfer
simply create another, temporary pool without the filesystem in
question and then copy the data. It would probably be a good idea to
set the branch to `RO` prior to doing this to ensure no new content is
written to the filesystem while performing the copy.
simply create a temporary pool without the filesystem in question and
then copy the data. It would probably be a good idea to set the branch
to `RO` prior to doing this to ensure no new content is written to the
filesystem while performing the copy.
## Can filesystems be written to directly? Outside of mergerfs while pooled?

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@ -1,35 +1,77 @@
# mergerfs - a featureful union filesystem
## DESCRIPTION
**mergerfs** is a
[FUSE](https://en.wikipedia.org/wiki/Filesystem_in_Userspace) based
[union filesystem](https://en.wikipedia.org/wiki/Union_mount) geared
towards simplifying storage and management of files across numerous
commodity storage devices. It is similar to **mhddfs**, **unionfs**,
and **aufs**.
**mergerfs** is a union filesystem geared towards simplifying storage
and management of files across numerous commodity storage devices. It
is similar to **mhddfs**, **unionfs**, and **aufs**.
## Features
## FEATURES
* Logicially combine numerous filesystems/paths into a single
mount point
* Combine paths of the same or different filesystems
* Ability to add or remove filesystems/paths without impacting the
rest of the data
* Unaffected by individual filesystem failure
* Configurable file selection and creation placement
* Works with filesystems of any size
* Works with filesystems of almost any type
* Ignore read-only filesystems when creating files
* Hard link copy-on-write / CoW
* Runtime configurable
* Support for extended attributes (xattrs)
* Support for file attributes (chattr)
* Support for POSIX ACLs
- Configurable behaviors / file placement
- Ability to add or remove filesystems at will
- Resistance to individual filesystem failure
- Support for extended attributes (xattrs)
- Support for file attributes (chattr)
- Runtime configurable (via xattrs)
- Works with heterogeneous filesystem types
- Moving of file when filesystem runs out of space while writing
- Ignore read-only filesystems when creating files
- Turn read-only files into symlinks to underlying file
- Hard link copy-on-write / CoW
- Support for POSIX ACLs
- Misc other things
## SYNOPSIS
## Non-features
mergerfs -o&lt;options&gt; &lt;branches&gt; &lt;mountpoint&gt;
* Read/write overlay on top of readonly filesystem like OverlayFS
* File whiteout
* RAID like parity calculation
* Redundency
* Splitting of files across branches
## DOCUMENTATION
- [https://trapexit.github.io/mergerfs/](https://trapexit.github.io/mergerfs/)
## How it works
## TOOLS
mergerfs logically merges multiple filesystem paths together. It acts
as a proxy to the underlying filesystem paths. Combining the
behaviors of some functions and being a selector for others.
- [mergerfs tools](https://github.com/trapexit/mergerfs-tools)
When the contents of a directory are requested mergerfs combines the
list of files from each directory, deduplicating entries, and returns
that list.
When a file or directory is created a policy is first run to determine
which branch will be selected for the creation.
For functions which chant attributes or remove the file the behavior
may be applied to all instances found.
Read more about [policies here](config/functions_categories_and_policies.md).
### Visualization
```
A + B = C
/disk1 /disk2 /merged
| | |
+-- /dir1 +-- /dir1 +-- /dir1
| | | | | |
| +-- file1 | +-- file2 | +-- file1
| | +-- file3 | +-- file2
+-- /dir2 | | +-- file3
| | +-- /dir3 |
| +-- file4 | +-- /dir2
| +-- file5 | |
+-- file6 | +-- file4
|
+-- /dir3
| |
| +-- file5
|
+-- file6
```

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@ -0,0 +1,271 @@
# Known Issues and Bugs
## mergerfs
### Supplemental user groups
Due to the overhead of
[getgroups/setgroups](http://linux.die.net/man/2/setgroups) mergerfs
utilizes a cache. This cache is opportunistic and per thread. Each
thread will query the supplemental groups for a user when that
particular thread needs to change credentials and will keep that data
for the lifetime of the thread. This means that if a user is added to
a group it may not be picked up without the restart of
mergerfs. In the future this may be improved to allow a periodic or
manual clearing of the cache.
While not a bug some users have found when using containers that
supplemental groups defined inside the container don't work as
expected. Since mergerfs lives outside the container it is querying
the host's group database. Effectively containers have their own user
and group definitions unless setup otherwise just as different systems
would.
Users should mount in the host group file into the containers or use a
standard shared user & groups technology like NIS or LDAP.
### directory mtime is not being updated
Remember that the default policy for `getattr` is `ff`. The
information for the first directory found will be returned. If it
wasn't the directory which had been updated then it will appear
outdated.
The reason this is the default is because any other policy would be
more expensive and for many applications it is unnecessary. To always
return the directory with the most recent mtime or a faked value based
on all found would require a scan of all filesystems.
If you always want the directory information from the one with the
most recent mtime then use the `newest` policy for `getattr`.
### 'mv /mnt/pool/foo /mnt/disk1/foo' removes 'foo'
This is not a bug.
Run in verbose mode to better understand what's happening:
```
$ mv -v /mnt/pool/foo /mnt/disk1/foo
copied '/mnt/pool/foo' -> '/mnt/disk1/foo'
removed '/mnt/pool/foo'
$ ls /mnt/pool/foo
ls: cannot access '/mnt/pool/foo': No such file or directory
```
`mv`, when working across devices, is copying the source to target and
then removing the source. Since the source **is** the target in this
case, depending on the unlink policy, it will remove the just copied
file and other files across the branches.
If you want to move files to one filesystem just copy them there and
use mergerfs.dedup to clean up the old paths or manually remove them
from the branches directly.
### cached memory appears greater than it should be
Use `cache.files=off` and/or `dropcacheonclose=true`. See the section
on [page caching](config/cache.md).
### NFS clients returning ESTALE / Stale file handle
NFS generally does not like out of band changes. Take a look at the
section on NFS in the [remote-filesystems](remote_filesystems.md) for
more details.
### rtorrent fails with ENODEV (No such device)
Be sure to set
[cache.files=partial|full|auto-full|per-process](config/cache.md)
or use Linux kernel v6.6 or above. rtorrent and some other
applications use [mmap](http://linux.die.net/man/2/mmap) to read and
write to files and offer no fallback to traditional methods.
### Plex / Jellyfin doesn't work with mergerfs
It does. If you're trying to put the software's config / metadata /
database on mergerfs you can't set
[cache.files=off](config/cache.md) (unless you use Linux v6.6 or
above) because Plex is using **sqlite3** with **mmap** enabled.
That said it is recommended that config and runtime files be stored on
SSDs on a regular filesystem for performance reasons and if you are
using HDDs in your pool to help limit spinup.
Other software that leverages **sqlite3** which require **mmap**
includes Radarr, Sonarr, Lidarr.
It is recommended that you reach out to the developers of the software
you're having troubles with and asking them to add a fallback to
regular file IO when **mmap** is unavailable. It is not only more
compatible and resilient but also can be more performant in certain
situations.
If the issue is that quick scanning doesn't seem to pick up media then
be sure to set `func.getattr=newest`, though generally, a full scan
will pick up all media anyway.
### When a program tries to move or rename a file it fails
Please read the docs regarding [rename and
link](config/functions_categories_and_policies.md#rename-and-link).
The problem is that many applications do not properly handle `EXDEV`
errors which `rename` and `link` may return even though they are
perfectly valid situations which do not indicate actual device,
filesystem, or OS errors. The error will only be returned by mergerfs
if using a path preserving policy as described in the policy section
above. If you do not care about path preservation simply change the
mergerfs policy to the non-path preserving version. For example: `-o
category.create=mfs` Ideally the offending software would be fixed and
it is recommended that if you run into this problem you contact the
software's author and request proper handling of `EXDEV` errors.
### my 32bit software has problems
Some software have problems with 64bit inode values. The symptoms can
include EOVERFLOW errors when trying to list files. You can address
this by setting `inodecalc` to one of the 32bit based algos as
described in the relevant section.
### Moving files and directories fails with Samba
Workaround: Copy the file/directory and then remove the original
rather than move.
This isn't an issue with Samba but some SMB clients. GVFS-fuse v1.20.3
and prior (found in Ubuntu 14.04 among others) failed to handle
certain error codes correctly. Particularly `STATUS_NOT_SAME_DEVICE`
which comes from the `EXDEV` that is returned by `rename` when the
call is crossing mount points. When a program gets an `EXDEV` it needs
to explicitly take an alternate action to accomplish its goal. In the
case of `mv` or similar it tries `rename` and on `EXDEV` falls back to
a copying the file to the destination and deleting the source. In
these older versions of GVFS-fuse if it received `EXDEV` it would
translate that into `EIO`. This would cause `mv` or most any
application attempting to move files around on that SMB share to fail
with a generic IO error.
[GVFS-fuse v1.22.0](https://bugzilla.gnome.org/show_bug.cgi?id=734568)
and above fixed this issue but a large number of systems use the older
release. On Ubuntu, the version can be checked by issuing `apt-cache
showpkg gvfs-fuse`. Most distros released in 2015 seem to have the
updated release and will work fine but older systems may
not. Upgrading gvfs-fuse or the distro in general will address the
problem.
In Apple's MacOSX 10.9 they replaced Samba (client and server) with
their own product. It appears their new client does not handle
`EXDEV` either and responds similarly to older releases of gvfs on
Linux.
### Trashing files occasionally fails
This is the same issue as with Samba. `rename` returns `EXDEV` (in our
case that will really only happen with path preserving policies like
`epmfs`) and the software doesn't handle the situation well. This is
unfortunately a common failure of software which moves files
around. The standard indicates that an implementation **MAY** choose
to support non-user home directory trashing of files (which is a
**MUST**). The implementation **MAY** also support "top directory
trashes" which many probably do.
To create a `$topdir/.Trash` directory as defined in the standard use
the [mergerfs-tools](https://github.com/trapexit/mergerfs-tools) tool
`mergerfs.mktrash`.
## FUSE and Linux kernel
There have been a number of kernel issues / bugs over the years which
mergerfs has run into. Here is a list of them for reference and
posterity.
### NFS and EIO errors
[https://lore.kernel.org/linux-fsdevel/20240228160213.1988854-1-mszeredi@redhat.com/T/](https://lore.kernel.org/linux-fsdevel/20240228160213.1988854-1-mszeredi@redhat.com/T/)
Over the years some users have reported that while exporting mergerfs
via NFS, after significant filesystem activity, not only will the NFS
client start returning ESTALE and EIO errors but mergerfs itself would
start returning EIO errors. The problem was that no one could
reliability reproduce the issue. After a string of reports in late
2023 and early 2024 more investigation was done.
In Linux 5.14 new validation was put into FUSE which caught a few
invalid situations and would tag a FUSE node as invalid if a check
failed. Such checks include invalid file type, changing of type from
one request to another, a size greater than 63bit, and the generation
of a inode changing while in use.
What happened was that mergerfs was using a different fixed, non-zero
value for the generation of all nodes as it was suggested that unique
inode + generation pairs are needed for proper integration with
NFS. That non-zero value was being sent back to the kernel when a
lookup request was made for root. The reason this was hard to track
down was because NFS almost uniquely uses an API which can lead to a
lookup of the root node that simply won't happen under normal
workloads and usage. And that lookup will only happen if child nodes
of the root were forgotten but NFS still had a handle to that node and
later asked for details about it. It would trigger a set of requests
to lookup info on those nodes.
This wasn't a bug in FUSE but mergerfs. However, the incorrect
behavior of mergerfs lead to FUSE behave in an unexpected and
incorrect manner. It would issue a lookup of the "parent of a child of
the root" and mergerfs would send the invalid generation value. As a
result the kernel would mark the root node as "bad" which would then
trigger the kernel to issue a "forget root" message. In between those
it would issue a request for the parent of the root... which doesn't
exist.
So the kernel was doing two invalid things. Requesting the parent of
the root and then when that failed issuing a forget for the
root. These led to chasing after the wrong possible causes.
The change was for FUSE to revert the marking of root node bad if the
generation is non-zero and warn about it. It will mark the node bad
but not unhash/forget/remove it.
mergerfs in v2.40.1 ensures that generation for root is always 0 on
lookup which should work across any kernel version.
### Truncated files
This was a bug with `mmap` and `FUSE` on 32bit platforms. Should be fixed in all LTS releases.
* [https://marc.info/?l=linux-fsdevel&m=155550785230874&w=2](https://marc.info/?l=linux-fsdevel&m=155550785230874&w=2)
### Crashing on OpenVZ
There was a bug in the OpenVZ kernel with regard to how it handles `ioctl` calls. It was making invalid requests which would lead to crashes due to mergerfs not expecting them.
* [https://bugs.openvz.org/browse/OVZ-7145](https://bugs.openvz.org/browse/OVZ-7145)
* [https://www.mail-archive.com/devel@openvz.org/msg37096.html](https://www.mail-archive.com/devel@openvz.org/msg37096.html)
### Really bad mmap performance
There was a bug in caching which affects overall performance of `mmap` through `FUSE` in Linux 4.x kernels. It is fixed in 4.4.10 and 4.5.4.
* [https://lkml.org/lkml/2016/3/16/260](https://lkml.org/lkml/2016/3/16/260)
* [https://lkml.org/lkml/2016/5/11/59](https://lkml.org/lkml/2016/5/11/59)
### Heavy load and memory pressure leads to kernel panic
* [https://lkml.org/lkml/2016/9/14/527](https://lkml.org/lkml/2016/9/14/527)
* [https://lkml.org/lkml/2016/10/4/1](https://lkml.org/lkml/2016/10/4/1)
* [https://www.theregister.com/2016/10/05/linus_torvalds_admits_buggy_crap_made_it_into_linux_48/](https://www.theregister.com/2016/10/05/linus_torvalds_admits_buggy_crap_made_it_into_linux_48/)

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# Featured Media and Publicity
# Media and Publicity
## Tutorials / Articles

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# BASIC SETUP
If you don't already know that you have a special use case then just
start with one of the following option sets.
#### You need `mmap` (used by rtorrent and many sqlite3 base software)
`cache.files=auto-full,dropcacheonclose=true,category.create=mfs`
or if you are on a Linux kernel >= 6.6.x mergerfs will enable a mode
that allows shared mmap when `cache.files=off`. To be sure of the best
performance between `cache.files=off` and `cache.files=auto-full`
you'll need to do your own benchmarking but often `off` is faster.
#### You don't need `mmap`
`cache.files=off,dropcacheonclose=true,category.create=mfs`
### Command Line
`mergerfs -o cache.files=auto-full,dropcacheonclose=true,category.create=mfs /mnt/hdd0:/mnt/hdd1 /media`
### /etc/fstab
`/mnt/hdd0:/mnt/hdd1 /media mergerfs cache.files=auto-full,dropcacheonclose=true,category.create=mfs 0 0`
### systemd mount
https://github.com/trapexit/mergerfs/wiki/systemd
```
[Unit]
Description=mergerfs service
[Service]
Type=simple
KillMode=none
ExecStart=/usr/bin/mergerfs \
-f \
-o cache.files=auto-full \
-o dropcacheonclose=true \
-o category.create=mfs \
/mnt/hdd0:/mnt/hdd1 \
/media
ExecStop=/bin/fusermount -uz /media
Restart=on-failure
[Install]
WantedBy=default.target
```
See the mergerfs [wiki for real world
deployments](https://github.com/trapexit/mergerfs/wiki/Real-World-Deployments)
for comparisons / ideas.

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# CACHING
#### page caching
https://en.wikipedia.org/wiki/Page_cache
- cache.files=off: Disables page caching. Underlying files cached,
mergerfs files are not.
- cache.files=partial: Enables page caching. Underlying files cached,
mergerfs files cached while open.
- cache.files=full: Enables page caching. Underlying files cached,
mergerfs files cached across opens.
- cache.files=auto-full: Enables page caching. Underlying files
cached, mergerfs files cached across opens if mtime and size are
unchanged since previous open.
- cache.files=libfuse: follow traditional libfuse `direct_io`,
`kernel_cache`, and `auto_cache` arguments.
- cache.files=per-process: Enable page caching (equivalent to
`cache.files=partial`) only for processes whose 'comm' name matches
one of the values defined in `cache.files.process-names`. If the
name does not match the file open is equivalent to
`cache.files=off`.
FUSE, which mergerfs uses, offers a number of page caching modes. mergerfs tries to simplify their use via the `cache.files`
option. It can and should replace usage of `direct_io`,
`kernel_cache`, and `auto_cache`.
Due to mergerfs using FUSE and therefore being a userland process
proxying existing filesystems the kernel will double cache the content
being read and written through mergerfs. Once from the underlying
filesystem and once from mergerfs (it sees them as two separate
entities). Using `cache.files=off` will keep the double caching from
happening by disabling caching of mergerfs but this has the side
effect that _all_ read and write calls will be passed to mergerfs
which may be slower than enabling caching, you lose shared `mmap`
support which can affect apps such as rtorrent, and no read-ahead will
take place. The kernel will still cache the underlying filesystem data
but that only helps so much given mergerfs will still process all
requests.
If you do enable file page caching,
`cache.files=partial|full|auto-full`, you should also enable
`dropcacheonclose` which will cause mergerfs to instruct the kernel to
flush the underlying file's page cache when the file is closed. This
behavior is the same as the rsync fadvise / drop cache patch and Feh's
nocache project.
If most files are read once through and closed (like media) it is best
to enable `dropcacheonclose` regardless of caching mode in order to
minimize buffer bloat.
It is difficult to balance memory usage, cache bloat & duplication,
and performance. Ideally, mergerfs would be able to disable caching for
the files it reads/writes but allow page caching for itself. That
would limit the FUSE overhead. However, there isn't a good way to
achieve this. It would need to open all files with O_DIRECT which
places limitations on what the underlying filesystems would be
supported and complicates the code.
kernel documentation: https://www.kernel.org/doc/Documentation/filesystems/fuse-io.txt
#### entry & attribute caching
Given the relatively high cost of FUSE due to the kernel <-> userspace
round trips there are kernel side caches for file entries and
attributes. The entry cache limits the `lookup` calls to mergerfs
which ask if a file exists. The attribute cache limits the need to
make `getattr` calls to mergerfs which provide file attributes (mode,
size, type, etc.). As with the page cache these should not be used if
the underlying filesystems are being manipulated at the same time as
it could lead to odd behavior or data corruption. The options for
setting these are `cache.entry` and `cache.negative_entry` for the
entry cache and `cache.attr` for the attributes
cache. `cache.negative_entry` refers to the timeout for negative
responses to lookups (non-existent files).
#### writeback caching
When `cache.files` is enabled the default is for it to perform
writethrough caching. This behavior won't help improve performance as
each write still goes one for one through the filesystem. By enabling
the FUSE writeback cache small writes may be aggregated by the kernel
and then sent to mergerfs as one larger request. This can greatly
improve the throughput for apps which write to files
inefficiently. The amount the kernel can aggregate is limited by the
size of a FUSE message. Read the `fuse_msg_size` section for more
details.
There is a small side effect as a result of enabling writeback
caching. Underlying files won't ever be opened with O_APPEND or
O_WRONLY. The former because the kernel then manages append mode and
the latter because the kernel may request file data from mergerfs to
populate the write cache. The O_APPEND change means that if a file is
changed outside of mergerfs it could lead to corruption as the kernel
won't know the end of the file has changed. That said any time you use
caching you should keep from using the same file outside of mergerfs
at the same time.
Note that if an application is properly sizing writes then writeback
caching will have little or no effect. It will only help with writes
of sizes below the FUSE message size (128K on older kernels, 1M on
newer).
#### statfs caching
Of the syscalls used by mergerfs in policies the `statfs` / `statvfs`
call is perhaps the most expensive. It's used to find out the
available space of a filesystem and whether it is mounted
read-only. Depending on the setup and usage pattern these queries can
be relatively costly. When `cache.statfs` is enabled all calls to
`statfs` by a policy will be cached for the number of seconds its set
to.
Example: If the create policy is `mfs` and the timeout is 60 then for
that 60 seconds the same filesystem will be returned as the target for
creates because the available space won't be updated for that time.
#### symlink caching
As of version 4.20 Linux supports symlink caching. Significant
performance increases can be had in workloads which use a lot of
symlinks. Setting `cache.symlinks=true` will result in requesting
symlink caching from the kernel only if supported. As a result it's
safe to enable it on systems prior to 4.20. That said it is disabled
by default for now. You can see if caching is enabled by querying the
xattr `user.mergerfs.cache.symlinks` but given it must be requested at
startup you can not change it at runtime.
#### readdir caching
As of version 4.20 Linux supports readdir caching. This can have a
significant impact on directory traversal. Especially when combined
with entry (`cache.entry`) and attribute (`cache.attr`)
caching. Setting `cache.readdir=true` will result in requesting
readdir caching from the kernel on each `opendir`. If the kernel
doesn't support readdir caching setting the option to `true` has no
effect. This option is configurable at runtime via xattr
`user.mergerfs.cache.readdir`.
#### tiered caching
Some storage technologies support what some call "tiered" caching. The
placing of usually smaller, faster storage as a transparent cache to
larger, slower storage. NVMe, SSD, Optane in front of traditional HDDs
for instance.
mergerfs does not natively support any sort of tiered caching. Most
users have no use for such a feature and its inclusion would
complicate the code. However, there are a few situations where a cache
filesystem could help with a typical mergerfs setup.
1. Fast network, slow filesystems, many readers: You've a 10+Gbps network
with many readers and your regular filesystems can't keep up.
2. Fast network, slow filesystems, small'ish bursty writes: You have a
10+Gbps network and wish to transfer amounts of data less than your
cache filesystem but wish to do so quickly.
With #1 it's arguable if you should be using mergerfs at all. RAID
would probably be the better solution. If you're going to use mergerfs
there are other tactics that may help: spreading the data across
filesystems (see the mergerfs.dup tool) and setting `func.open=rand`,
using `symlinkify`, or using dm-cache or a similar technology to add
tiered cache to the underlying device.
With #2 one could use dm-cache as well but there is another solution
which requires only mergerfs and a cronjob.
1. Create 2 mergerfs pools. One which includes just the slow devices
and one which has both the fast devices (SSD,NVME,etc.) and slow
devices.
2. The 'cache' pool should have the cache filesystems listed first.
3. The best `create` policies to use for the 'cache' pool would
probably be `ff`, `epff`, `lfs`, or `eplfs`. The latter two under
the assumption that the cache filesystem(s) are far smaller than the
backing filesystems. If using path preserving policies remember that
you'll need to manually create the core directories of those paths
you wish to be cached. Be sure the permissions are in sync. Use
`mergerfs.fsck` to check / correct them. You could also set the
slow filesystems mode to `NC` though that'd mean if the cache
filesystems fill you'd get "out of space" errors.
4. Enable `moveonenospc` and set `minfreespace` appropriately. To make
sure there is enough room on the "slow" pool you might want to set
`minfreespace` to at least as large as the size of the largest
cache filesystem if not larger. This way in the worst case the
whole of the cache filesystem(s) can be moved to the other drives.
5. Set your programs to use the cache pool.
6. Save one of the below scripts or create you're own.
7. Use `cron` (as root) to schedule the command at whatever frequency
is appropriate for your workflow.
##### time based expiring
Move files from cache to backing pool based only on the last time the
file was accessed. Replace `-atime` with `-amin` if you want minutes
rather than days. May want to use the `fadvise` / `--drop-cache`
version of rsync or run rsync with the tool "nocache".
_NOTE:_ The arguments to these scripts include the cache
**filesystem** itself. Not the pool with the cache filesystem. You
could have data loss if the source is the cache pool.
[mergerfs.time-based-mover](https://raw.githubusercontent.com/trapexit/mergerfs/refs/heads/latest-release/tools/mergerfs.time-based-mover)
##### percentage full expiring
Move the oldest file from the cache to the backing pool. Continue till
below percentage threshold.
_NOTE:_ The arguments to these scripts include the cache
**filesystem** itself. Not the pool with the cache filesystem. You
could have data loss if the source is the cache pool.
[mergerfs.percent-full-mover](https://raw.githubusercontent.com/trapexit/mergerfs/refs/heads/latest-release/tools/mergerfs.percent-full-mover)

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# HOW IT WORKS
mergerfs logically merges multiple paths together. Think a union of
sets. The file/s or directory/s acted on or presented through mergerfs
are based on the policy chosen for that particular action. Read more
about policies below.
```
A + B = C
/disk1 /disk2 /merged
| | |
+-- /dir1 +-- /dir1 +-- /dir1
| | | | | |
| +-- file1 | +-- file2 | +-- file1
| | +-- file3 | +-- file2
+-- /dir2 | | +-- file3
| | +-- /dir3 |
| +-- file4 | +-- /dir2
| +-- file5 | |
+-- file6 | +-- file4
|
+-- /dir3
| |
| +-- file5
|
+-- file6
```
mergerfs does **not** support the copy-on-write (CoW) or whiteout
behaviors found in **aufs** and **overlayfs**. You can **not** mount a
read-only filesystem and write to it. However, mergerfs will ignore
read-only filesystems when creating new files so you can mix
read-write and read-only filesystems. It also does **not** split data
across filesystems. It is not RAID0 / striping. It is simply a union of
other filesystems.

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# KNOWN ISSUES / BUGS
#### kernel issues & bugs
[https://github.com/trapexit/mergerfs/wiki/Kernel-Issues-&-Bugs](https://github.com/trapexit/mergerfs/wiki/Kernel-Issues-&-Bugs)
#### directory mtime is not being updated
Remember that the default policy for `getattr` is `ff`. The
information for the first directory found will be returned. If it
wasn't the directory which had been updated then it will appear
outdated.
The reason this is the default is because any other policy would be
more expensive and for many applications it is unnecessary. To always
return the directory with the most recent mtime or a faked value based
on all found would require a scan of all filesystems.
If you always want the directory information from the one with the
most recent mtime then use the `newest` policy for `getattr`.
#### 'mv /mnt/pool/foo /mnt/disk1/foo' removes 'foo'
This is not a bug.
Run in verbose mode to better understand what's happening:
```
$ mv -v /mnt/pool/foo /mnt/disk1/foo
copied '/mnt/pool/foo' -> '/mnt/disk1/foo'
removed '/mnt/pool/foo'
$ ls /mnt/pool/foo
ls: cannot access '/mnt/pool/foo': No such file or directory
```
`mv`, when working across devices, is copying the source to target and
then removing the source. Since the source **is** the target in this
case, depending on the unlink policy, it will remove the just copied
file and other files across the branches.
If you want to move files to one filesystem just copy them there and
use mergerfs.dedup to clean up the old paths or manually remove them
from the branches directly.
#### cached memory appears greater than it should be
Use `cache.files=off` and/or `dropcacheonclose=true`. See the section
on page caching.
#### NFS clients returning ESTALE / Stale file handle
NFS generally does not like out of band changes. Take a look at the
section on NFS in the [remote-filesystems](remote_filesystems.md) for
more details.
#### rtorrent fails with ENODEV (No such device)
Be sure to set
`cache.files=partial|full|auto-full|per-processe`. rtorrent and some
other applications use [mmap](http://linux.die.net/man/2/mmap) to read
and write to files and offer no fallback to traditional methods. FUSE
does not currently support mmap while using `direct_io`. There may be
a performance penalty on writes with `direct_io` off as well as the
problem of double caching but it's the only way to get such
applications to work. If the performance loss is too high for other
apps you can mount mergerfs twice. Once with `direct_io` enabled and
one without it. Be sure to set `dropcacheonclose=true` if not using
`direct_io`.
#### Plex doesn't work with mergerfs
It does. If you're trying to put Plex's config / metadata / database
on mergerfs you can't set `cache.files=off` because Plex is using
sqlite3 with mmap enabled. Shared mmap is not supported by Linux's
FUSE implementation when page caching is disabled. To fix this place
the data elsewhere (preferable) or enable `cache.files` (with
`dropcacheonclose=true`). Sqlite3 does not need mmap but the developer
needs to fall back to standard IO if mmap fails.
This applies to other software: Radarr, Sonarr, Lidarr, Jellyfin, etc.
I would recommend reaching out to the developers of the software
you're having troubles with and asking them to add a fallback to
regular file IO when mmap is unavailable.
If the issue is that scanning doesn't seem to pick up media then be
sure to set `func.getattr=newest`, though generally, a full scan will
pick up all media anyway.
#### When a program tries to move or rename a file it fails
Please read the section above regarding [rename and link](functions_categories_and_policies.md#rename-and-link).
The problem is that many applications do not properly handle `EXDEV`
errors which `rename` and `link` may return even though they are
perfectly valid situations which do not indicate actual device,
filesystem, or OS errors. The error will only be returned by mergerfs
if using a path preserving policy as described in the policy section
above. If you do not care about path preservation simply change the
mergerfs policy to the non-path preserving version. For example: `-o
category.create=mfs` Ideally the offending software would be fixed and
it is recommended that if you run into this problem you contact the
software's author and request proper handling of `EXDEV` errors.
#### my 32bit software has problems
Some software have problems with 64bit inode values. The symptoms can
include EOVERFLOW errors when trying to list files. You can address
this by setting `inodecalc` to one of the 32bit based algos as
described in the relevant section.
#### Samba: Moving files / directories fails
Workaround: Copy the file/directory and then remove the original
rather than move.
This isn't an issue with Samba but some SMB clients. GVFS-fuse v1.20.3
and prior (found in Ubuntu 14.04 among others) failed to handle
certain error codes correctly. Particularly **STATUS_NOT_SAME_DEVICE**
which comes from the **EXDEV** which is returned by **rename** when
the call is crossing mount points. When a program gets an **EXDEV** it
needs to explicitly take an alternate action to accomplish its
goal. In the case of **mv** or similar it tries **rename** and on
**EXDEV** falls back to a manual copying of data between the two
locations and unlinking the source. In these older versions of
GVFS-fuse if it received **EXDEV** it would translate that into
**EIO**. This would cause **mv** or most any application attempting to
move files around on that SMB share to fail with a IO error.
[GVFS-fuse v1.22.0](https://bugzilla.gnome.org/show_bug.cgi?id=734568)
and above fixed this issue but a large number of systems use the older
release. On Ubuntu, the version can be checked by issuing `apt-cache
showpkg gvfs-fuse`. Most distros released in 2015 seem to have the
updated release and will work fine but older systems may
not. Upgrading gvfs-fuse or the distro in general will address the
problem.
In Apple's MacOSX 10.9 they replaced Samba (client and server) with
their own product. It appears their new client does not handle
**EXDEV** either and responds similarly to older releases of gvfs on
Linux.
#### Trashing files occasionally fails
This is the same issue as with Samba. `rename` returns `EXDEV` (in our
case that will really only happen with path preserving policies like
`epmfs`) and the software doesn't handle the situation well. This is
unfortunately a common failure of software which moves files
around. The standard indicates that an implementation `MAY` choose to
support non-user home directory trashing of files (which is a
`MUST`). The implementation `MAY` also support "top directory trashes"
which many probably do.
To create a `$topdir/.Trash` directory as defined in the standard use
the [mergerfs-tools](https://github.com/trapexit/mergerfs-tools) tool
`mergerfs.mktrash`.
#### Supplemental user groups
Due to the overhead of
[getgroups/setgroups](http://linux.die.net/man/2/setgroups) mergerfs
utilizes a cache. This cache is opportunistic and per thread. Each
thread will query the supplemental groups for a user when that
particular thread needs to change credentials and will keep that data
for the lifetime of the thread. This means that if a user is added to
a group it may not be picked up without the restart of
mergerfs. However, since the high level FUSE API's (at least the
standard version) thread pool dynamically grows and shrinks it's
possible that over time a thread will be killed and later a new thread
with no cache will start and query the new data.
The gid cache uses fixed storage to simplify the design and be
compatible with older systems which may not have C++11
compilers. There is enough storage for 256 users' supplemental
groups. Each user is allowed up to 32 supplemental groups. Linux >=
2.6.3 allows up to 65535 groups per user but most other \*nixs allow
far less. NFS allows only 16. The system does handle overflow
gracefully. If the user has more than 32 supplemental groups only the
first 32 will be used. If more than 256 users are using the system
when an uncached user is found it will evict an existing user's cache
at random. So long as there aren't more than 256 active users this
should be fine. If either value is too low for your needs you will
have to modify `gidcache.hpp` to increase the values. Note that doing
so will increase the memory needed by each thread.
While not a bug some users have found when using containers that
supplemental groups defined inside the container don't work properly
with regard to permissions. This is expected as mergerfs lives outside
the container and therefore is querying the host's group
database. There might be a hack to work around this (make mergerfs
read the /etc/group file in the container) but it is not yet
implemented and would be limited to Linux and the /etc/group
DB. Preferably users would mount in the host group file into the
containers or use a standard shared user & groups technology like NIS
or LDAP.

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# LINKS
- https://spawn.link
- https://github.com/trapexit/mergerfs
- https://github.com/trapexit/mergerfs/wiki
- https://github.com/trapexit/mergerfs-tools
- https://github.com/trapexit/scorch
- https://github.com/trapexit/bbf

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# PERFORMANCE
mergerfs is at its core just a proxy and therefore its theoretical max
performance is that of the underlying devices. However, given it is a
FUSE filesystem working from userspace there is an increase in
overhead relative to kernel based solutions. That said the performance
can match the theoretical max but it depends greatly on the system's
configuration. Especially when adding network filesystems into the mix
there are many variables which can impact performance. Device speeds
and latency, network speeds and latency, general concurrency,
read/write sizes, etc. Unfortunately, given the number of variables it
has been difficult to find a single set of settings which provide
optimal performance. If you're having performance issues please look
over the suggestions below (including the benchmarking section.)
NOTE: be sure to read about these features before changing them to
understand what behaviors it may impact
- disable `security_capability` and/or `xattr`
- increase cache timeouts `cache.attr`, `cache.entry`, `cache.negative_entry`
- enable (or disable) page caching (`cache.files`)
- enable `parallel-direct-writes`
- enable `cache.writeback`
- enable `cache.statfs`
- enable `cache.symlinks`
- enable `cache.readdir`
- change the number of worker threads
- disable `posix_acl`
- disable `async_read`
- test theoretical performance using `nullrw` or mounting a ram disk
- use `symlinkify` if your data is largely static and read-only
- use tiered cache devices
- use LVM and LVM cache to place a SSD in front of your HDDs
- increase readahead: `readahead=1024`
If you come across a setting that significantly impacts performance
please contact trapexit so he may investigate further. Please test
both against your normal setup, a singular branch, and with
`nullrw=true`

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# SUPPORT
Filesystems are complex and difficult to debug. mergerfs, while being
just a proxy of sorts, can be difficult to debug given the large
number of possible settings it can have itself and the number of
environments it can run in. When reporting on a suspected issue
**please** include as much of the below information as possible
otherwise it will be difficult or impossible to diagnose. Also please
read the above documentation as it provides details on many previously
encountered questions/issues.
**Please make sure you are using the [latest
release](https://github.com/trapexit/mergerfs/releases) or have tried
it in comparison. Old versions, which are often included in distros
like Debian and Ubuntu, are not ever going to be updated and the issue
you are encountering may have been addressed already.**
**For commercial support or feature requests please [contact me
directly.](mailto:support@spawn.link)**
#### Information to include in bug reports
- [Information about the broader problem along with any attempted
solutions.](https://xyproblem.info)
- Solution already ruled out and why.
- Version of mergerfs: `mergerfs --version`
- mergerfs settings / arguments: from fstab, systemd unit, command
line, OMV plugin, etc.
- Version of the OS: `uname -a` and `lsb_release -a`
- List of branches, their filesystem types, sizes (before and after issue): `df -h`
- **All** information about the relevant paths and files: permissions, ownership, etc.
- **All** information about the client app making the requests: version, uid/gid
- Runtime environment:
- Is mergerfs running within a container?
- Are the client apps using mergerfs running in a container?
- A `strace` of the app having problems:
- `strace -fvTtt -s 256 -o /tmp/app.strace.txt <cmd>`
- A `strace` of mergerfs while the program is trying to do whatever it is failing to do:
- `strace -fvTtt -s 256 -p <mergerfsPID> -o /tmp/mergerfs.strace.txt`
- **Precise** directions on replicating the issue. Do not leave **anything** out.
- Try to recreate the problem in the simplest way using standard programs: `ln`, `mv`, `cp`, `ls`, `dd`, etc.
#### Contact / Issue submission
- github.com: https://github.com/trapexit/mergerfs/issues
- discord: https://discord.gg/MpAr69V
- reddit: https://www.reddit.com/r/mergerfs
#### Donations
https://github.com/trapexit/support
Development and support of a project like mergerfs requires a
significant amount of time and effort. The software is released under
the very liberal ISC license and is therefore free to use for personal
or commercial uses.
If you are a personal user and find mergerfs and its support valuable
and would like to support the project financially it would be very
much appreciated.
If you are using mergerfs commercially please consider sponsoring the
project to ensure it continues to be maintained and receive
updates. If custom features are needed feel free to [contact me
directly](mailto:support@spawn.link).

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# TERMINOLOGY
- branch: A base path used in the pool.
- pool: The mergerfs mount. The union of the branches.
- relative path: The path in the pool relative to the branch and mount.
- function: A filesystem call (open, unlink, create, getattr, rmdir, etc.)
- category: A collection of functions based on basic behavior (action, create, search).
- policy: The algorithm used to select a file when performing a function.
- path preservation: Aspect of some policies which includes checking the path for which a file would be created.

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# TIPS / NOTES
- This document is literal and thorough. If a suspected feature isn't
mentioned it doesn't exist. If certain libfuse arguments aren't
listed they probably shouldn't be used.
- Ensure you're using the latest version.
- Run mergerfs as `root`. mergerfs is designed and intended to be run
as `root` and may exibit incorrect behavior if run otherwise..
- If you don't see some directories and files you expect, policies
seem to skip branches, you get strange permission errors, etc. be
sure the underlying filesystems' permissions are all the same. Use
`mergerfs.fsck` to audit the filesystem for out of sync permissions.
- If you still have permission issues be sure you are using POSIX ACL
compliant filesystems. mergerfs doesn't generally make exceptions
for FAT, NTFS, or other non-POSIX filesystem.
- Do **not** use `cache.files=off` if you expect applications (such as
rtorrent) to use [mmap](http://linux.die.net/man/2/mmap)
files. Shared mmap is not currently supported in FUSE w/ page
caching disabled. Enabling `dropcacheonclose` is recommended when
`cache.files=partial|full|auto-full`.
- [Kodi](http://kodi.tv), [Plex](http://plex.tv),
[Subsonic](http://subsonic.org), etc. can use directory
[mtime](http://linux.die.net/man/2/stat) to more efficiently
determine whether to scan for new content rather than simply
performing a full scan. If using the default **getattr** policy of
**ff** it's possible those programs will miss an update on account
of it returning the first directory found's **stat** info and it's a
later directory on another mount which had the **mtime** recently
updated. To fix this you will want to set
**func.getattr=newest**. Remember though that this is just
**stat**. If the file is later **open**'ed or **unlink**'ed and the
policy is different for those then a completely different file or
directory could be acted on.
- Some policies mixed with some functions may result in strange
behaviors. Not that some of these behaviors and race conditions
couldn't happen outside **mergerfs** but that they are far more
likely to occur on account of the attempt to merge multiple sources
of data which could be out of sync due to the different policies.
- For consistency it's generally best to set **category** wide policies
rather than individual **func**'s. This will help limit the
confusion of tools such as
[rsync](http://linux.die.net/man/1/rsync). However, the flexibility
is there if needed.

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# General Information and Overview
## How well does mergerfs scale? Is it "production ready?"
Users have reported running mergerfs on everything from a Raspberry Pi
to dual socket Xeon systems with >20 cores. I'm aware of at least a
few companies which use mergerfs in production. [Open Media
Vault](https://www.openmediavault.org) includes mergerfs as its sole
solution for pooling filesystems. The author of mergerfs had it
running for over 300 days managing 16+ devices with reasonably heavy
24/7 read and write usage. Stopping only after the machine's power
supply died.
Most serious issues (crashes or data corruption) have been due to
[kernel
bugs](https://github.com/trapexit/mergerfs/wiki/Kernel-Issues-&-Bugs). All
of which are fixed in stable releases.
## Why use FUSE? Why not a kernel based solution?
As with any solution to a problem, there are advantages and
disadvantages to each one.
A FUSE based solution has all the downsides of FUSE:
- Higher IO latency due to the trips in and out of kernel space
- Higher general overhead due to trips in and out of kernel space
- Double caching when using page caching
- Misc limitations due to FUSE's design
But FUSE also has a lot of upsides:
- Easier to offer a cross platform solution
- Easier forward and backward compatibility
- Easier updates for users
- Easier and faster release cadence
- Allows more flexibility in design and features
- Overall easier to write, secure, and maintain
- Much lower barrier to entry (getting code into the kernel takes a
lot of time and effort initially)
FUSE was chosen because of all the advantages listed above. The
negatives of FUSE do not outweigh the positives.
## Is my OS's libfuse needed for mergerfs to work?
No. Normally `mount.fuse` is needed to get mergerfs (or any FUSE
filesystem to mount using the `mount` command but in vendoring the
libfuse library the `mount.fuse` app has been renamed to
`mount.mergerfs` meaning the filesystem type in `fstab` can simply be
`mergerfs`. That said there should be no harm in having it installed
and continuing to using `fuse.mergerfs` as the type in `/etc/fstab`.
If `mergerfs` doesn't work as a type it could be due to how the
`mount.mergerfs` tool was installed. Must be in `/sbin/` with proper
permissions.
## Why was splice support removed?
After a lot of testing over the years, splicing always appeared to
at best, provide equivalent performance, and in some cases, worse
performance. Splice is not supported on other platforms forcing a
traditional read/write fallback to be provided. The splice code was
removed to simplify the codebase.

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Originally from [Reddit](https://www.reddit.com/r/synology/comments/etz32q/instructions_on_how_to_install_mergerfs_on_a/). Copied and edited with permission.
A different version to overcome some problems with the method below, can be [found here](https://web.archive.org/web/20221205205446/https://daniellemarco.nl/wp/2022/01/01/adding-mergerfs-to-your-synology/)
Install Entware
1. SSH into your NAS and switch to the root user:
```
sudo su
```
2. Create a folder on your hdd (outside rootfs):
```
mkdir -p /volume1/@Entware/opt
```
2. Remove `/opt` and mount optware folder.
Make sure that `/opt` folder is empty (Optware is not installed), we will remove the `/opt` folder with its contents at this step.
```
rm -rf /opt
mkdir /opt
mount -o bind "/volume1/@Entware/opt" /opt
```
3. Run install script depending on the processor. Use command `uname -m` to find out. Then run the corresponding command.
#### armv8 (aarch64) - Realtek RTD129x
```
wget -O - http://bin.entware.net/aarch64-k3.10/installer/generic.sh | /bin/sh
```
#### armv5
```
wget -O - http://bin.entware.net/armv5sf-k3.2/installer/generic.sh | /bin/sh
```
#### armv7
```
wget -O - http://bin.entware.net/armv7sf-k3.2/installer/generic.sh | /bin/sh
```
#### x64
```
wget -O - http://bin.entware.net/x64-k3.2/installer/generic.sh | /bin/sh
```
4. Create an Autostart Task On Synology
Create a triggered user-defined task in Task Scheduler.
- Go to: DSM > Control Panel > Task Scheduler
- Create > Triggered Task > User Defined Script
- General
- Task: Entware
- User: root
- Event: Boot-up
- Pretask: none
- Task Settings
- Run Command: Paste the below script in.
```
#!/bin/sh
# Mount/Start Entware
mkdir -p /opt
mount -o bind "/volume1/@Entware/opt" /opt
/opt/etc/init.d/rc.unslung start
# Add Entware Profile in Global Profile
if grep -qF '/opt/etc/profile' /etc/profile; then
echo "Confirmed: Entware Profile in Global Profile"
else
echo "Adding: Entware Profile in Global Profile"
cat >> /etc/profile <<"EOF"
# Load Entware Profile
[ -r "/opt/etc/profile" ] && . /opt/etc/profile
EOF
fi
# Update Entware List
/opt/bin/opkg update
```
5. Reboot your NAS.
6. SSH back into your NAS
7. Install mergerfs by the following command.
```
sudo opkg install mergerfs
```
9. Make sure it's installed by running the following command. Mergerfs binary is expected to be listed there.
```
sudo ls /volume1/@Entware/opt/bin
```
This should print the usage helper of mergerfs.
```
mergerfs --help
```
10. If you want the latest build of mergerfs, you can download the `mergerfs-static-linux_$ARCH.tar.gz` from [Github releases page](https://github.com/trapexit/mergerfs/releases/latest), remember to replace `$ARCH` with your architecture, e.g. what `uname -m` tells you.
Extract the `.tar.gz` archive and use its content to update the `mergerfs` and `mergerfs-fusermount` binaries in `/opt/bin/`
11. Configure mergerfs. Note: Change the file paths to your setup.
_MY CONFIG IS (Don't know if it is the perfect setting, but works in my testing) _
```
mergerfs -o rw,use_ino,allow_other,func.getattr=newest,category.action=all,category.create=ff,dropcacheonclose=true /volume1/Media/TempMedia:/volume1/Media/GMedia /volume1/Media/FinalMedia
```
If mergerfs complains about existing files because the destination already has the Synology `@eaDir` directory, you can use the option `nonempty`.
12. Create an Autostart Task On Synology for Mergerfs
- Go to: DSM > Control Panel > Task Scheduler
- Create > Triggered Task > User Defined Script
- General
- Task: Mergerfs
- User: root
- Event: Boot-up
- Pretask: Entware
- Task Settings
- Run Command: Paste the below script in.
```
#!/bin/sh
/opt/bin/mergerfs -o rw,use_ino,allow_other,func.getattr=newest,category.action=all,category.create=ff,dropcacheonclose=true /volume1/Media/TempMedia:/volume1/Media/GMedia /volume1/Media/FinalMedia
```
13. Profit

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There have been a number of kernel issues / bugs over the years which mergerfs has run into. Here is a list of them for reference and posterity.
## NFS and EIO errors
https://lore.kernel.org/linux-fsdevel/20240228160213.1988854-1-mszeredi@redhat.com/T/
Over the years some users have reported that while exporting mergerfs via NFS, after significant filesystem activity, not only will the NFS client start returning ESTALE and EIO errors but mergerfs itself would start returning EIO errors. The problem was that no one could reliability reproduce the issue. After a string of reports in late 2023 and early 2024 more investigation was done.
In Linux 5.14 new validation was put into FUSE which caught a few invalid situations and would tag a FUSE node as invalid if a check failed. Such checks include invalid file type, changing of type from one request to another, a size greater than 63bit, and the generation of a inode changing while in use.
What happened was that mergerfs was using a different fixed, non-zero value for the generation of all nodes as it was suggested that unique inode + generation pairs are needed for proper integration with NFS. That non-zero value was being sent back to the kernel when a lookup request was made for root. The reason this was hard to track down was because NFS almost uniquely uses an API which can lead to a lookup of the root node that simply won't happen under normal workloads and usage. And that lookup will only happen if child nodes of the root were forgotten but NFS still had a handle to that node and later asked for details about it. It would trigger a set of requests to lookup info on those nodes.
This wasn't a bug in FUSE but mergerfs. However, the incorrect behavior of mergerfs lead to FUSE behave in an unexpected and incorrect manner. It would issue a lookup of the "parent of a child of the root" and mergerfs would send the invalid generation value. As a result the kernel would mark the root node as "bad" which would then trigger the kernel to issue a "forget root" message. In between those it would issue a request for the parent of the root... which doesn't exist.
So the kernel was doing two invalid things. Requesting the parent of the root and then when that failed issuing a forget for the root. These led to chasing after the wrong possible causes.
The proposed change is for FUSE to revert the marking of root node bad if the generation is non-zero and warn about it. It will mark the node bad but not unhash/forget/remove it.
mergerfs in v2.40.1 ensures that generation for root is always 0 on lookup which should work across any kernel version.
## Truncated files
This was a bug with mmap and FUSE on 32bit platforms. Should be fixed in all LTS releases.
- https://marc.info/?l=linux-fsdevel&m=155550785230874&w=2
## Crashing on OpenVZ
There is/was a bug in the OpenVZ kernel with regard to how it handles ioctl calls. It was making invalid requests which would lead to crashes due to mergerfs not expecting them.
- https://bugs.openvz.org/browse/OVZ-7145
- https://www.mail-archive.com/devel@openvz.org/msg37096.html
## Really bad mmap performance
There is/was a bug in caching which affects overall performance of mmap through FUSE in Linux 4.x kernels. It is fixed in 4.4.10 and 4.5.4.
- https://lkml.org/lkml/2016/3/16/260
- https://lkml.org/lkml/2016/5/11/59
## Heavy load and memory pressure leads to kernel panic
- https://lkml.org/lkml/2016/9/14/527
- https://lkml.org/lkml/2016/10/4/1
- https://www.theregister.com/2016/10/05/linus_torvalds_admits_buggy_crap_made_it_into_linux_48/

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# Links
- [Another way installing MergerFS on Synology and overcoming problems](https://mjanssen.nl/2022/01/01/adding-mergerfs-to-your-synology/)
- [fstab](<https://github.com/trapexit/backup-and-recovery-howtos/blob/master/docs/setup_(fstab).md>)

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# Projects incorporating mergerfs directly in some way
- [Lakka.tv](https://lakka.tv/): A turnkey software emulation Linux distribution. Used to pool user and local storage. Also includes my other project [Opera](https://retroarch.com/). A 3DO emulator.
- [OpenMediaVault](https://www.openmediavault.org): A network attached storage (NAS) solution based on Debian Linux. They provide plugins to manage mergerfs.
- [CasaOS](https://casaos.io): "A simple, easy to use, elegant open source home cloud system." Has added initial integration with mergerfs to create pools from existing filesystems.
- [ZimaOS](https://github.com/IceWhaleTech/zimaos-rauc): A more commercially focused NAS OS by the authors of CasaOS at [Ice Whale](https://www.zimaboard.com/).
# Software and services commonly used with mergerfs
- [snapraid](https://www.snapraid.it/)
- [rclone](https://rclone.org/)
- rclone's [union](https://rclone.org/union/) feature is based on mergerfs policies
- [ZFS](https://openzfs.org/): Common to use ZFS w/ mergerfs
- [UnRAID](https://unraid.net): While UnRAID has its own union filesystem it isn't uncommon to see UnRAID users leverage mergerfs given the differences in the technologies.
- For a time there were a number of Chia miners recommending mergerfs
- [cloudboxes.io](https://cloudboxes.io/wiki/how-to/apps/set-up-mergerfs-using-ssh)
# Distributions including mergerfs
mergerfs can be found in the [repositories](https://pkgs.org/download/mergerfs) of [many Linux](https://repology.org/project/mergerfs/versions) (and maybe FreeBSD) distributions.
Note: Any non-rolling release based distro is likely to have out-of-date versions.
- [Debian](https://packages.debian.org/bullseye/mergerfs)
- [Ubuntu](https://launchpad.net/ubuntu/+source/mergerfs)
- [Fedora](https://rpmsphere.github.io/)
- [T2](https://t2sde.org/packages/mergerfs)
- [Alpine](https://pkgs.alpinelinux.org/packages?name=mergerfs&branch=edge&repo=&arch=&maintainer=)
- [Gentoo](https://packages.gentoo.org/packages/sys-fs/mergerfs)
- [Arch (AUR)](https://aur.archlinux.org/packages/mergerfs)
- [Void](https://voidlinux.org/packages/?arch=x86_64&q=mergerfs)
- [NixOS](https://search.nixos.org/packages?channel=22.11&show=mergerfs&from=0&size=50&sort=relevance&type=packages&query=mergerfs)
- [Guix]()
- [Slackware](https://slackbuilds.org/repository/15.0/system/mergerfs/?search=mergerfs)

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# trapexit's (mergerfs' author)
## Current setup
- SilverStone Technology CS380B-X V2.0 case
- Intel Core i7-4790S
- 32GB DDR3 RAM
- LSI SAS 9201-16e
- 8 SATA connections to the CS380B backplane
- 8 SATA connections to a generic 8-bay enclosure (similar to a Sans Digital 8-bay enclosure)
- Connections via SAS to SATA breakout cables fished through the back. Not elegant but cost effective. SAS SCSI cutout boards are difficult to find and would add $50 to $100 to the cost.
- Marvell 88SE9230 PCIe SATA 6Gb/s Controller on motherboard
- 4 SATA connections to a [StarTech SATSASBP425](https://www.amazon.com/StarTech-com-4-Bay-Mobile-Backplane-Drives/dp/B00X7B3CUE)
- ASMedia ASM1062 SATA Controller on motherboard
- 4 SATA connections to a second [StarTech SATSASBP425](https://www.amazon.com/StarTech-com-4-Bay-Mobile-Backplane-Drives/dp/B00X7B3CUE)
- 1 MSATA connection on the motherboard
- NVidia Quadro P2000 (for hardware transcoding in Plex, Jellyfin, etc.)
- Mix of 3.5" SATA HDD: 8TB - 14TB
- Mix of 2.5" SATA HDD: 2TB - 5TB
- Mix of 2.5" SATA SSD:
- primary boot drive, backup boot drive, application specific caches
- Some of the SSDs are used enterprise drives which can often be found for a reasonable price on eBay
- Mix of 2.5" U.2 NVME: 3x 2TB Intel P4510, 1x 3.84TB Dell P5500
- Connected via a [Ceacent ANU28PE16 NVMe SSD Riser SFF8643 to SFF8639](https://www.aliexpress.us/item/2255800570129198.html)
- Have a [IcyDock MB931U-1VB](https://www.icydock.com/goods.php?id=363) for using U.2 NVME drives externally
- All drives formatted with EXT4 to make recovery easier in case of failure
- `mkfs.ext4 -L DRIVE_SERIAL_NUMBER -m 0 /dev/DEV`
- HDDs, some SSDs, some NVMEs merged together in a single mergerfs mount
- branches-mount-timeout=300
- cache.attr=120
- cache.entry=120
- cache.files=per-process
- cache.readdir=true
- cache.statfs=10
- category.create=pfrd
- dropcacheonclose=true
- fsname=media
- lazy-umount-mountpoint=true
- link_cow=true
- readahead=2048
- some SSDs/NVMes used for bespoke purposes such as main storage for Docker/container config storage and caching (Plex transcoding, etc.)
- Filesystem labels are set to the serial number of the drive for easy identification
- Drives mounted to:
- /mnt/hdd/SIZE-LABEL
- /mnt/sdd/SIZE-LABEL
- /mnt/nvme/SIZE-LABEL
- ex: /mnt/hdd/8TB-ABCDEF
- Total drives in main mergerfs pool: 24
- Total storage combined in main mergerfs pool: 155TB
- RAM usage by mergerfs under load: 512MB - 1GB of resident memory
## Old setup
- Core i7 3770s
- 16GB RAM
- 4 Port ASMedia Technology 106x eSATA PCIE 4x card
- 4x [ICYCube MB561U3S-4SB R1 Quad Bay enclosure](https://www.icydock.com/goods.php?id=219)
NOTES: The eSATA enclosure setup was easier to manage physically as the enclosures are smaller but the LSI SAS HBA & generic enclosure setup is more reliable/stable, more performant, and actually cost less. Port multipliers tend to behave poorly with different brand controllers (if they work at all). They can perform poorly if a drive is bad leading to the other drives acting as if they have issues leading to a full hard reset of the computer and enclosure to 'fix'. Port multiplier enclosures, over USB, tend not to support hot swapping and the drives will all be reset if a drive is swapped.
---
# (´･ω･`)
- 2x Intel(R) Xeon(R) CPU X5690 @ 3.47GHz
- 64G RAM
- Chassis: 847E16-R1K28LPB
- 36 bays + 2 sytem bays
- SAS2008
- X8DTH
- Drives
- 26x 8TB Data
- luks
- btrfs: `mount -ospace_cache=v2,noatime,rw`
- 6x 8TB Parity
- luks
- ext4: `mkfs.ext4 -J size=4 -m 0 -i 67108864 -L <LABEL> <DEVICE>`
- Software
- `mergerfs` all data disks to a single root: `-o defaults,allow_other,category.create=msplfs,minfreespace=100G,use_ino,dropcacheonclose=true,ignorepponrename=true,moveonenospc=mspmfs`
- `snapraid` all drives and 6 parity
- `snapraid-btrfs` btrfs snapshots+snapraid
- Misc
- 1.2GB/s max with 2x SAS2 connections
- ~56-65 hours for full sync
- 1-4 hours on "small" changes
- a fuckton of reads against the hdds if you change too much
Note: Buy good chassis with even better backplanes. The backplane 847E16-R1K28LPB uses, can cascade 24 drives and allow access over a single SAS2 connection.
You can interconnect the front- and backplane and it will cascade 36 drives over a single SAS2 connection. But most HBAs have 2 connections, so you can do 1.2GB/s.
The backplanes actually have SAS3, but the cards are.... well... expensive. But if you got one, ayyy 2x 12Gbit connection to the backplane. Or maybe 4x 12Gbit if you are up to it.
Tweak your snapraid autosave to ~4-6TB, otherwise it will take a long ass time.
Oh, yeah. If you got all the drives and the right connection to your backpanel get ready for cpu carnage.
![rape](https://i.imgur.com/ClJAbmF.png "the rape")

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# systemd
## Start mergerfs after some arbitrary script
## prep script
`/usr/local/bin/prepare-for-mergerfs`
```shell
#!/usr/bin/env sh
# Setup things
# Wait for things
/bin/sleep 10
# Report back to systemd that things are ready
/bin/systemd-notify --ready
```
## prep script system service
`/etc/systemd/system/prepare-for-mergerfs.service`
```
[Unit]
Description=Dummy mount service
[Service]
Type=notify
RemainAfterExit=yes
ExecStart=/usr/local/bin/prepare-for-mergerfs
[Install]
WantedBy=default.target
```
## mergerfs systemd service
`/etc/systemd/system/mergerfs.service`
```
[Unit]
Description=Dummy mergerfs service
Requires=prepare-for-mergerfs.service
After=prepare-for-mergerfs.service
[Service]
Type=simple
KillMode=none
ExecStart=/usr/bin/mergerfs \
-f \
-o OPTIONS \
/mnt/filesystem0:/mnt/filesystem1 \
/mnt/mergerfs
ExecStop=/bin/fusermount -uz /mnt/mergerfs
Restart=on-failure
[Install]
WantedBy=default.target
```

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- mergerfs rocks! And you rock too! Keep going! Plus, I just found your posting about using mergerfs on Synology and that made me love it even more! Thank you! - Vladimir
- mergerfs is pretty awesome - [RUakij](https://www.reddit.com/r/homelab/comments/109rka2/comment/j401lyc/?utm_source=reddit&utm_medium=web2x&context=3)
- mergerfs is surprisingly pleasant - [@blankxk](https://twitter.com/blankxk/status/1619385944578347008)
- mergerfs good - SE

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# Tweaking Performance
mergerfs is at its is a proxy and therefore its theoretical max
performance is that of the underlying devices. However, given it is a
FUSE based filesystem working from userspace there is an increase in
overhead relative to kernel based solutions. That said the performance
can match the theoretical max but it depends greatly on the system's
configuration. Especially when adding network filesystems into the mix
there are many variables which can impact performance. Device speeds
and latency, network speeds and latency, concurrency and parallel
limits of the hardware, read/write sizes, etc.
While some settings can impact performance they are all **functional**
in nature. Meaning they change mergerfs' behavior in some way. As a
result there is no such thing as a "performance mode".
If you're having performance issues please look over the suggestions
below and the [benchmarking section.](benchmarking.md)
NOTE: Be sure to [read about these features](config/options.md) before
changing them to understand how functionality will change.
* test theoretical performance using `nullrw` or mounting a ram disk
* increase readahead: `readahead=1024`
* disable `security_capability` and/or `xattr`
* increase cache timeouts `cache.attr`, `cache.entry`, `cache.negative_entry`
* enable (or disable) page caching (`cache.files`)
* enable `parallel-direct-writes`
* enable `cache.writeback`
* enable `cache.statfs`
* enable `cache.symlinks`
* enable `cache.readdir`
* change the number of threads available
* disable `posix_acl`
* disable `async_read`
* use `symlinkify` if your data is largely static and read-only
* use tiered cache devices
* use LVM and LVM cache to place a SSD in front of your HDDs
If you come across a setting that significantly impacts performance
please [contact trapexit](support.md) so he may investigate further. Please test
both against your normal setup, a singular branch, and with
`nullrw=true`

100
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@ -1,10 +1,10 @@
# mergerfs versus X
# Project Comparisons
#### mhddfs
## mhddfs
mhddfs had not been maintained for some time and has some known
stability and security issues. mergerfs provides a superset of mhddfs'
features and should offer the same or better performance.
mhddfs had not been updated in over a decade and has known stability
and security issues. mergerfs provides a superset of mhddfs' features
and offers better performance.
Below is an example of mhddfs and mergerfs setup to work similarly.
@ -12,30 +12,45 @@ Below is an example of mhddfs and mergerfs setup to work similarly.
`mergerfs -o minfreespace=4G,category.create=ff /mnt/drive1:/mnt/drive2 /mnt/pool`
#### aufs
aufs is mostly abandoned and no longer available in most Linux distros.
## aufs
aufs is abandoned and no longer available in most Linux distros.
While aufs can offer better peak performance mergerfs provides more
configurability and is generally easier to use. mergerfs however does
not offer the overlay / copy-on-write (CoW) features which aufs has.
#### unionfs-fuse
unionfs-fuse is more like aufs than mergerfs in that it offers overlay /
copy-on-write (CoW) features. If you're just looking to create a union
of filesystems and want flexibility in file/directory placement then
mergerfs offers that whereas unionfs is more for overlaying read/write
filesystems over read-only ones.
## Linux unionfs
FILL IN
## unionfs-fuse
#### overlayfs
unionfs-fuse is more like aufs than mergerfs in that it offers overlay
/ copy-on-write (CoW) features. If you're just looking to create a
union of filesystems and want flexibility in file/directory placement
then mergerfs offers that whereas unionfs-fuse is more for overlaying
read/write filesystems over read-only ones. Largely unionfs-fuse has
been replaced by overlayfs.
## overlayfs
overlayfs is similar to aufs and unionfs-fuse in that it also is
primarily used to layer a read/write filesystem over one or more
read-only filesystems. It does not have the ability to spread
files/directories across numerous filesystems.
files/directories across numerous filesystems. It is the successor to
unionfs, unionfs-fuse, and aufs and widely used by container platforms
such as Docker.
If your usecase is layering a writable filesystem on top of readonly
filesystems then you should look first to overlayfs.
#### RAID0, JBOD, drive concatenation, striping
## RAID0, JBOD, drive concatenation, striping
With simple JBOD / drive concatenation / stripping / RAID0 a single
drive failure will result in full pool failure. mergerfs performs a
@ -48,28 +63,25 @@ actually have contiguous space as large as if you used those other
technologies. Meaning you can't create a 2TB file on a pool of 2 1TB
filesystems.
When combined with something like [SnapRaid](http://www.snapraid.it)
and/or an offsite backup solution you can have the flexibility of JBOD
without the single point of failure.
#### UnRAID
UnRAID is a full OS and its storage layer, as I understand, is
proprietary and closed source. Users who have experience with both
have often said they prefer the flexibility offered by mergerfs and
for some the fact it is open source is important.
## UnRAID
There are a number of UnRAID users who use mergerfs as well though I'm
not entirely familiar with the use case.
UnRAID is a full OS and offers a (FUSE based?) filesystem which
provides a union of filesystems like mergerfs but with the addition of
live parity calculation and storage. Outside parity calculations
mergerfs offers more features and due to the lack of realtime parity
calculation can have high peak performance. Some users also prefer an
open source solution.
For semi-static data mergerfs + [SnapRaid](http://www.snapraid.it)
provides a similar solution.
#### ZFS
## ZFS
mergerfs is very different from ZFS. mergerfs is intended to provide
flexible pooling of arbitrary filesystems (local or remote), of
arbitrary sizes, and arbitrary filesystems. For `write once, read
arbitrary sizes, and arbitrary filesystems. Primarily in `write once, read
many` usecases such as bulk media storage. Where data integrity and
backup is managed in other ways. In those usecases ZFS can introduce a
number of costs and limitations as described
@ -77,24 +89,34 @@ number of costs and limitations as described
[here](https://markmcb.com/2020/01/07/five-years-of-btrfs/), and
[here](https://utcc.utoronto.ca/~cks/space/blog/solaris/ZFSWhyNoRealReshaping).
#### StableBit's DrivePool
## StableBit's DrivePool
DrivePool works only on Windows so not as common an alternative as
other Linux solutions. If you want to use Windows then DrivePool is a
good option. Functionally the two projects work a bit
differently. DrivePool always writes to the filesystem with the most
free space and later rebalances. mergerfs does not offer rebalance but
chooses a branch at file/directory create time. DrivePool's
rebalancing can be done differently in any directory and has file
pattern matching to further customize the behavior. mergerfs, not
having rebalancing does not have these features, but similar features
are planned for mergerfs v3. DrivePool has builtin file duplication
which mergerfs does not natively support (but can be done via an
external script.)
free space and later rebalances. mergerfs does not currently offer
rebalance but chooses a branch at file/directory create
time. DrivePool's rebalancing can be done differently in any directory
and has file pattern matching to further customize the
behavior. mergerfs, not having rebalancing does not have these
features, but similar features are planned for mergerfs v3. DrivePool
has builtin file duplication which mergerfs does not natively support
(but can be done via an external script.)
There are a lot of misc differences between the two projects but most
features in DrivePool can be replicated with external tools in
combination with mergerfs.
Additionally, DrivePool is a closed source commercial product vs
mergerfs a ISC licensed OSS project.
mergerfs a ISC licensed open source project.
## Plan9 binds
Plan9 has the native ability to bind multiple paths/filesystems
together which can be compared to a simplified union filesystem. Such
bind mounts choose files in a "first found" in the order they are
listed similar to mergerfs' `ff` policy. File creation is limited
to... FILL ME IN. REFERENCE DOCS.

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@ -0,0 +1,189 @@
# QuickStart
## Install
First ensure you have the [latest version installed](setup/installation.md).
## Configuration
mergerfs has many options and effectively all of them are functional
in nature. What that means is that there is no "best" or "fastest"
configuration. No "make faster" options. Everything changes
behavior. Sometimes those changes in behavior affect performance.
That said: If you don't already know that you have a special use case
then use one of the following option sets as it will cover most casual
usecases.
### You use Linux v6.6 or above
* cache.files=off
* category.create=mfs
* dropecacheonclose=false
In previous versions of Linux it was unable to support `mmap` if page
caching was disabled (ie: `cache.files=off`). However, it now will
enable page caching if needed for a particular file if mmap is
requested.
`mmap` is needed by certain software to read and write to a
file. However, many software could work without it and fail to have
proper error handling. Many programs that use sqlite3 will require
`mmap` despite sqlite3 working perfectly fine without it (and in some
cases can be more performant with regular file IO.)
### You use Linux v6.5 or below
#### You need `mmap` (used by rtorrent and many sqlite3 base software)
* cache.files=auto-full
* category.create=mfs
* dropcacheonclose=true
#### You don't need `mmap`
* cache.files=off
* category.create=mfs
* dropcacheonclose=false
## Usage
### Command Line
```
mergerfs -o cache.files=off,dropcacheonclose=false,category.create=mfs /mnt/hdd0:/mnt/hdd1 /media
```
### /etc/fstab
```
/mnt/hdd0:/mnt/hdd1 /media mergerfs cache.files=off,dropcacheonclose=false,category.create=mfs 0 0
```
### /etc/fstab w/ config file
For more complex setups it can be useful to separate out the config.
#### /etc/fstab
```
/etc/mergerfs/branches/media/* /media mergerfs config=/etc/mergerfs/config/media.ini
```
#### /etc/mergerfs/config/media.ini
```ini title="media.ini" linenums="1"
cache.files=off
category.create=mfs
dropcacheonclose=false
```
#### /etc/mergerfs/branches/media/
Create a bunch of symlinks to point to the branch. mergerfs will
resolve the symlinks and use the real path.
`ls -lh /etc/mergerfs/branches/media/*`
```text
lrwxrwxrwx 1 root root 21 Aug 4 2023 hdd00 -> /mnt/hdd/hdd00
lrwxrwxrwx 1 root root 21 Aug 4 2023 hdd01 -> /mnt/hdd/hdd01
lrwxrwxrwx 1 root root 21 Aug 4 2023 hdd02 -> /mnt/hdd/hdd02
lrwxrwxrwx 1 root root 21 Aug 4 2023 hdd03 -> /mnt/hdd/hdd03
```
### systemd (simple)
`/etc/systemd/system/mergerfs-media.service`
```systemd title="mergerfs-media.service" linenums="1"
[Unit]
Description=mergerfs /media service
After=local-fs.target network.target
[Service]
Type=simple
KillMode=none
ExecStart=/usr/bin/mergerfs \
-f \
-o cache.files=off \
-o category.create=mfs \
-o dropcacheonclose=false \
/mnt/hdd0:/mnt/hdd1 \
/media
ExecStop=/bin/fusermount -uz /media
Restart=on-failure
[Install]
WantedBy=default.target
```
### systemd (w/ setup script)
Since it isn't well documented otherwise: if you wish to do some setup before
you mount mergerfs follow this example.
#### setup-for-mergerfs
`/usr/local/bin/setup-for-mergerfs`
```shell title="setup-for-mergerfs" linenums="1"
#!/usr/bin/env sh
# Perform setup
/bin/sleep 10
# Report back to systemd that things are ready
/bin/systemd-notify --ready
```
#### setup-for-mergerfs.service
`/etc/systemd/system/setup-for-mergerfs.service`
```systemd title="setup-for-mergerfs.service" linenums="1"
[Unit]
Description=mergerfs setup service
[Service]
Type=notify
RemainAfterExit=yes
ExecStart=/usr/local/bin/setup-for-mergerfs
[Install]
WantedBy=default.target
```
#### mergerfs-media.service
`/etc/systemd/system/mergerfs-media.service`
```systemd title="mergerfs-media.service" linenums="1"
[Unit]
Description=mergerfs /media service
Requires=setup-for-mergerfs.service
After=local-fs.target network.target prepare-for-mergerfs.service
[Service]
Type=simple
KillMode=none
ExecStart=/usr/bin/mergerfs \
-f \
-o cache.files=off \
-o category.create=mfs \
-o dropcacheonclose=false \
/mnt/hdd0:/mnt/hdd1 \
/media
ExecStop=/bin/fusermount -uz /media
Restart=on-failure
[Install]
WantedBy=default.target
```

56
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@ -0,0 +1,56 @@
# Related Projects
## Projects using mergerfs
* [Lakka.tv](https://lakka.tv/): A turnkey software emulation Linux
distribution. Used to pool user and local storage. Also includes my
other project [Opera](https://retroarch.com/). A 3DO emulator.
* [OpenMediaVault](https://www.openmediavault.org): A network attached
storage (NAS) solution based on Debian Linux. They provide plugins
to manage mergerfs.
* [CasaOS](https://casaos.io): "A simple, easy to use, elegant open
source home cloud system." Has added initial integration with
mergerfs to create pools from existing filesystems.
* [ZimaOS](https://github.com/IceWhaleTech/zimaos-rauc): A more
commercially focused NAS OS by the authors of CasaOS at [Ice
Whale](https://www.zimaboard.com/).
* [Cosmos Cloud](https://cosmos-cloud.io/): Cosmos "take the chore out
of selfhosting, with automated maintenance and fully secured setup
out of the box. It even integrates to your existing setup."
## Software and services commonly used with mergerfs
* [snapraid](https://www.snapraid.it/)
* [rclone](https://rclone.org/)
* rclone's [union](https://rclone.org/union/) feature is based on
mergerfs policies
* [ZFS](https://openzfs.org/): Common to use ZFS w/ mergerfs
* [UnRAID](https://unraid.net): While UnRAID has its own union
filesystem it isn't uncommon to see UnRAID users leverage mergerfs
given the differences in the technologies.
* For a time there were a number of Chia miners recommending mergerfs
* [cloudboxes.io](https://cloudboxes.io/wiki/how-to/apps/set-up-mergerfs-using-ssh)
## Distributions including mergerfs
mergerfs can be found in the
[repositories](https://pkgs.org/download/mergerfs) of [many
Linux](https://repology.org/project/mergerfs/versions) (and maybe
FreeBSD) distributions.
Note: Any non-rolling release based distro is likely to have
out-of-date versions.
* [Debian](https://packages.debian.org/bullseye/mergerfs)
* [Ubuntu](https://launchpad.net/ubuntu/+source/mergerfs)
* [Fedora](https://rpmsphere.github.io/)
* [T2](https://t2sde.org/packages/mergerfs)
* [Alpine](https://pkgs.alpinelinux.org/packages?name=mergerfs&branch=edge&repo=&arch=&maintainer=)
* [Gentoo](https://packages.gentoo.org/packages/sys-fs/mergerfs)
* [Arch (AUR)](https://aur.archlinux.org/packages/mergerfs)
* [Void](https://voidlinux.org/packages/?arch=x86_64&q=mergerfs)
* [NixOS](https://search.nixos.org/packages?channel=22.11&show=mergerfs&from=0&size=50&sort=relevance&type=packages&query=mergerfs)
* [Guix]()
* [Slackware](https://slackbuilds.org/repository/15.0/system/mergerfs/?search=mergerfs)

39
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@ -4,17 +4,16 @@ Many users ask about compatibility with remote filesystems. This
section is to describe any known issues or quirks when using mergerfs
with common remote filesystems.
Keep in mind that, like with caching, it is not a good idea to change
the contents of the remote filesystem
Keep in mind that, like with caching, it is **NOT** a good idea to
change the contents of the remote filesystem
[out-of-band](https://en.wikipedia.org/wiki/Out-of-band). Meaning that
you really shouldn't change the contents of the underlying
filesystems or mergerfs on the server hosting the remote
filesystem. Doing so can lead to weird behavior, inconsistency,
errors, and even data corruption should multiple programs try to write
or read the same data at the same time. This isn't to say you can't do
it or that data corruption is likely but it _could_ happen. It is
better to always use the remote filesystem. Even on the machine
serving it.
you should not change the contents of the underlying filesystems or
mergerfs on the server hosting the remote filesystem. Doing so can
lead to weird behavior, inconsistency, errors, and even data
corruption should multiple programs try to write or read the same data
at the same time. This isn't to say you can't do it or that data
corruption is likely but it _could_ happen. It is better to always use
the remote filesystem. Even on the machine serving it.
## NFS
@ -26,22 +25,21 @@ with it.
It should be noted that NFS and FUSE (the technology mergerfs uses) do
not work perfectly with one another due to certain design choices in
FUSE (and mergerfs.) Due to these issues, it is generally recommended
to use SMB when possible till situations change. That said mergerfs
should generally work as an export of NFS and issues discovered should
still be reported.
to use SMB when possible. That said mergerfs should generally work as
an export of NFS and issues discovered should still be reported.
To ensure compatibility between mergerfs and NFS use the following
settings.
mergerfs settings:
- noforget
- inodecalc=path-hash
* `noforget`
* `inodecalc=path-hash`
NFS export settings:
- fsid=UUID
- no_root_squash
* `fsid=UUID`
* `no_root_squash`
`noforget` is needed because NFS uses the `name_to_handle_at` and
`open_by_handle_at` functions which allow a program to keep a
@ -70,7 +68,10 @@ line tool `uuid` or `uuidgen` or through a website such as
[uuidgenerator.net](https://www.uuidgenerator.net/).
`no_root_squash` is not strictly necessary but can lead to confusing
permission and ownership issues if root squashing is enabled.
permission and ownership issues if root squashing is enabled. mergerfs
needs to run certain commands as `root` and if root squash is enabled
it NFS will tell mergerfs a non-root user is making certain calls.
## SMB / CIFS
@ -90,6 +91,7 @@ not been extensively tested. If you use mergerfs with CIFSD or other
SMB servers please submit your experiences so these docs can be
updated.
## SSHFS
[SSHFS](https://en.wikipedia.org/wiki/SSHFS) is a FUSE filesystem
@ -107,6 +109,7 @@ may vary.
More info can be found
[here](https://ideatrash.net/2016/08/odds-and-ends-optimizing-sshfs-moving.html).
## Other
There are other remote filesystems but none popularly used to serve

23
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@ -1,8 +1,8 @@
# RUNTIME INTERFACES
# Runtime Interfaces
## RUNTIME CONFIG
## Runtime Config
#### .mergerfs pseudo file
### .mergerfs pseudo file
```
<mountpoint>/.mergerfs
@ -18,18 +18,21 @@ Any changes made at runtime are **not** persisted. If you wish for
values to persist they must be included as options wherever you
configure the mounting of mergerfs (/etc/fstab).
##### Keys
#### Keys
Use `getfattr -d /mountpoint/.mergerfs` or `xattr -l
/mountpoint/.mergerfs` to see all supported keys. Some are
informational and therefore read-only. `setxattr` will return EINVAL
(invalid argument) on read-only keys.
##### Values
#### Values
Same as the command line.
###### user.mergerfs.branches
##### user.mergerfs.branches
Used to query or modify the list of branches. When modifying there are
several shortcuts to easy manipulation of the list.
@ -64,7 +67,7 @@ user.mergerfs.category.search="ff"
user.mergerfs.category.search="newest"
```
#### file / directory xattrs
### file / directory xattrs
While they won't show up when using `getfattr` **mergerfs** offers a
number of special xattrs to query information about the files
@ -77,7 +80,7 @@ following:
- **user.mergerfs.fullpath**: the full path of the original file given the getattr policy
- **user.mergerfs.allpaths**: a NUL ('\0') separated list of full paths to all files found
## SIGNALS
## Signals
- USR1: This will cause mergerfs to send invalidation notifications to
the kernel for all files. This will cause all unused files to be
@ -85,13 +88,13 @@ following:
- USR2: Trigger a general cleanup of currently unused memory. A more
thorough version of what happens every ~15 minutes.
## IOCTLS
## ioctl
Found in `fuse_ioctl.cpp`:
```C++
typedef char IOCTL_BUF[4096];
#define IOCTL_APP_TYPE 0xDF
#define IOCTL_APP_TYPE 0xDF
#define IOCTL_FILE_INFO _IOWR(IOCTL_APP_TYPE,0,IOCTL_BUF)
#define IOCTL_GC _IO(IOCTL_APP_TYPE,1)
#define IOCTL_GC1 _IO(IOCTL_APP_TYPE,2)

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@ -1,4 +1,4 @@
# BUILD
# Build
**NOTE:** Prebuilt packages can be found at and recommended for most
users: https://github.com/trapexit/mergerfs/releases
@ -14,7 +14,7 @@ $ # or
$ wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-<ver>.tar.gz
```
#### Debian / Ubuntu
## Debian / Ubuntu
```
$ cd mergerfs
@ -23,8 +23,34 @@ $ make deb
$ sudo dpkg -i ../mergerfs_<version>_<arch>.deb
```
#### RHEL / CentOS / Rocky / Fedora
## RHEL / CentOS / Rocky / Fedora
```
$ su -
# cd mergerfs
# tools/install-build-pkgs
# make rpm
# rpm -i rpmbuild/RPMS/<arch>/mergerfs-<version>.<arch>.rpm
```
## Generic
Have git, g++, make, python installed.
```
$ cd mergerfs
$ make
$ sudo make install
```
## Build options
```
$ make help
usage: make
make USE_XATTR=0 - build program without xattrs functionality
make STATIC=1 - build static binary
make LTO=1 - build with link time optimization
```

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@ -1,80 +1,100 @@
# INSTALL
# Installation
https://github.com/trapexit/mergerfs/releases
If you are using a non-rolling release Linux distro such as Debian or
Ubuntu then you are almost certainly going to have an old version of
mergerfs installed if you use the "official" package. For that reason
we provide packages for major stable released distros.
If your distribution's package manager includes mergerfs check if the
version is up to date. If out of date it is recommended to use
the latest release found on the release page. Details for common
distros are below.
Before reporting issues or bugs please be sure to upgrade to the
latest release to confirm they still exist.
#### Debian
All provided packages can be found at [https://github.com/trapexit/mergerfs/releases](https://github.com/trapexit/mergerfs/releases)
## Debian
Most Debian installs are of a stable branch and therefore do not have
the most up to date software. While mergerfs is available via `apt` it
is suggested that users install the most recent version available from
the [releases page](https://github.com/trapexit/mergerfs/releases).
#### prebuilt deb
### prebuilt deb
```
wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs_<ver>.debian-<rel>_<arch>.deb
dpkg -i mergerfs_<ver>.debian-<rel>_<arch>.deb
sudo dpkg -i mergerfs_<ver>.debian-<rel>_<arch>.deb
```
#### apt
### apt
```
sudo apt install -y mergerfs
```
#### Ubuntu
## Ubuntu
Most Ubuntu installs are of a stable branch and therefore do not have
the most up to date software. While mergerfs is available via `apt` it
is suggested that users install the most recent version available from
the [releases page](https://github.com/trapexit/mergerfs/releases).
#### prebuilt deb
### prebuilt deb
```
wget https://github.com/trapexit/mergerfs/releases/download/<version>/mergerfs_<ver>.ubuntu-<rel>_<arch>.deb
dpkg -i mergerfs_<ver>.ubuntu-<rel>_<arch>.deb
sudo dpkg -i mergerfs_<ver>.ubuntu-<rel>_<arch>.deb
```
#### apt
### apt
```
sudo apt install -y mergerfs
```
#### Raspberry Pi OS
Effectively the same as Debian or Ubuntu.
## Raspberry Pi OS
The same as Debian or Ubuntu.
#### Fedora
## Fedora
Get the RPM from the [releases page](https://github.com/trapexit/mergerfs/releases).
```
wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-<ver>.fc<rel>.<arch>.rpm
sudo rpm -i mergerfs-<ver>.fc<rel>.<arch>.rpm
```
#### CentOS / Rocky
## CentOS / Rocky
Get the RPM from the [releases page](https://github.com/trapexit/mergerfs/releases).
```
wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-<ver>.el<rel>.<arch>.rpm
sudo rpm -i mergerfs-<ver>.el<rel>.<arch>.rpm
```
#### ArchLinux
## ArchLinux
1. Setup AUR
2. Install `mergerfs`
2. `pacman -S mergerfs`
#### Other
## Other
Static binaries are provided for situations where native packages are
unavailable.
Get the tarball from the [releases page](https://github.com/trapexit/mergerfs/releases).
```
wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-static-linux_<arch>.tar.gz
sudo tar xvf mergerfs-static-linux_<arch>.tar.gz -C /

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@ -1,4 +1,4 @@
# UPGRADE
# Upgrade
mergerfs can be upgraded live by mounting on top of the previous
instance. Simply install the new version of mergerfs and follow the

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@ -0,0 +1,17 @@
# Sponsorship and Donations
[https://github.com/trapexit/support](https://github.com/trapexit/support)
Development and support of a project like mergerfs requires a
significant amount of time and effort. The software is released under
the very liberal [ISC](https://opensource.org/license/isc-license-txt)
license and is therefore free to use for personal or commercial uses.
If you are a non-commercial user and find mergerfs and its support valuable
and would like to support the project financially it would be very
much appreciated.
If you are using mergerfs commercially please consider sponsoring the
project to ensure it continues to be maintained and receive
updates. If custom features are needed feel free to [contact me
directly](mailto:support@spawn.link).

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@ -0,0 +1,45 @@
# Support
Filesystems are complex, as are the interactions software have with
them, and therefore difficult to debug. When reporting on a suspected
issue **please** include as much of the below information as possible
otherwise it will be difficult or impossible to diagnose. Also please
read the documentation as it provides details on many previously
encountered questions/issues.
**Please make sure you are using the [latest
release](https://github.com/trapexit/mergerfs/releases) or have tried
it in comparison. Old versions, which are often included in distros
like Debian and Ubuntu, are not ever going to be updated and the issue
you are encountering may have been addressed already.**
**For commercial support or feature requests please [contact me
directly.](mailto:support@spawn.link)**
### Information to include in bug reports
* [Information about the broader problem along with any attempted
solutions.](https://xyproblem.info)
* Solution already ruled out and why.
* Version of mergerfs: `mergerfs --version`
* mergerfs settings / arguments: from fstab, systemd unit, command
line, OMV plugin, etc.
* Version of the OS: `uname -a` and `lsb_release -a`
* List of branches, their filesystem types, sizes (before and after issue): `df -h`
* **All** information about the relevant paths and files: permissions, ownership, etc.
* **All** information about the client app making the requests: version, uid/gid
* Runtime environment:
* Is mergerfs running within a container?
* Are the client apps using mergerfs running in a container?
* A `strace` of the app having problems:
* `strace -fvTtt -s 256 -o /tmp/app.strace.txt <cmd>`
* A `strace` of mergerfs while the program is trying to do whatever it is failing to do:
* `strace -fvTtt -s 256 -p <mergerfsPID> -o /tmp/mergerfs.strace.txt`
* **Precise** directions on replicating the issue. Do not leave **anything** out.
* Try to recreate the problem in the simplest way using standard programs: `ln`, `mv`, `cp`, `ls`, `dd`, etc.
### Contact / Issue submission
* github.com: [https://github.com/trapexit/mergerfs/issues](https://github.com/trapexit/mergerfs/issues)
* discord: [https://discord.gg/MpAr69V](https://discord.gg/MpAr69V)
* reddit: [https://www.reddit.com/r/mergerfs](https://www.reddit.com/r/mergerfs)

43
mkdocs/docs/tips_notes.md
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@ -0,0 +1,43 @@
# Tips and Notes
* This document is literal and reasonably thorough. If a suspected
feature isn't mentioned it doesn't exist. If certain `libfuse`
arguments aren't listed they probably shouldn't be used.
* Ensure you're using the latest version. Especially before submitting
bug reports.
* Run mergerfs as `root`. mergerfs is designed and intended to be run
as `root` and may exhibit incorrect behavior if run otherwise.
* If you do not see some directories and files you expect, policies
seem to skip branches, you get strange permission errors, etc. be
sure the underlying filesystems' permissions are all the same. Use
`mergerfs.fsck` to audit the filesystem for out of sync permissions.
* If you still have permission issues be sure you are using POSIX ACL
compliant filesystems. mergerfs doesn't generally make exceptions
for FAT, NTFS, or other non-POSIX filesystem.
* Unless using Linux v6.6 or above do **not** use `cache.files=off` if
you expect applications (such as rtorrent) to use
[mmap](http://linux.die.net/man/2/mmap). Enabling `dropcacheonclose`
is recommended when `cache.files=auto-full`.
* [Kodi](http://kodi.tv), [Plex](http://plex.tv),
[Subsonic](http://subsonic.org), etc. can use directory
[mtime](http://linux.die.net/man/2/stat) to more efficiently
determine whether to scan for new content rather than simply
performing a full scan. If using the default `getattr` policy of
`ff` it's possible those programs will miss an update on account of
it returning the first directory found's `stat` info and it is a
later directory on another mount which had the `mtime` recently
updated. To fix this you will want to set
`func.getattr=newest`. Remember though that this is just `stat`. If
the file is later `open`'ed or `unlink`'ed and the policy is
different for those then a completely different file or directory
could be acted on.
* Some policies mixed with some functions may result in strange
behaviors. Not that some of these behaviors and race conditions
couldn't happen outside mergerfs but that they are far more
likely to occur on account of the attempt to merge multiple sources
of data which could be out of sync due to the different policies.
* For consistency it's generally best to set `category` wide policies
rather than individual `func`'s. This will help limit the
confusion of tools such as
[rsync](http://linux.die.net/man/1/rsync). However, the flexibility
is there if needed.

42
mkdocs/docs/pages/documentation/tooling.md → mkdocs/docs/tooling.md
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@ -1,4 +1,4 @@
# TOOLING
# Tooling
## preload.so
@ -7,14 +7,11 @@ EXPERIMENTAL
For some time there has been work to enable passthrough IO in
FUSE. Passthrough IO would allow for near native performance with
regards to reads and writes (at the expense of certain mergerfs
features.) However, there have been several complications which have
kept the feature from making it into the mainline Linux kernel. Until
that feature is available there are two methods to provide similar
functionality. One method is using the LD_PRELOAD feature of the
dynamic linker. The other leveraging ptrace to intercept
syscalls. Each has their disadvantages. At the moment only a preload
based tool is available. A ptrace based tool may be developed later if
there is a need.
features.) In Linux v6.9 that feature made its way into the kernel
however in a somewhat limited form which is incompatible with aspects
of how mergerfs currently functions. While work will continue to
support passthrough IO in mergerfs this library was created to offer
similar functionality in a more limited way.
`/usr/lib/mergerfs/preload.so`
@ -44,24 +41,36 @@ Thank you to
[nohajc](https://github.com/nohajc/mergerfs-io-passthrough) for
prototyping the idea.
### general usage
### casual usage
```sh
LD_PRELOAD=/usr/lib/mergerfs/preload.so touch /mnt/mergerfs/filename
```
### Docker usage
Or run `export LD_PRELOAD=/usr/lib/mergerfs/preload.so` in your shell
or place it in your shell config file to have it be picked up by all
software ran from your shell.
### Docker and Podman usage
Assume `/mnt/fs0` and `/mnt/fs1` are pooled with mergerfs at `/media`.
All mergerfs branch paths _must_ be bind mounted into the container at
the same path as found on the host so the preload library can see them.
the same path as found on the host so the preload library can see
them.
**NOTE:** Since a container can have its own OS setup there is no
guarentee that `preload.so` from the host install will be compatible
with the loader found in the container. If that is true it simply
won't work and shouldn't cause any issues.
```sh
docker run \
-e LD_PRELOAD=/usr/lib/mergerfs/preload.so \
-v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \
-v /media:/data \
-v /media:/media \
-v /mnt:/mnt \
ubuntu:latest \
bash
@ -73,19 +82,20 @@ or more explicitly
docker run \
-e LD_PRELOAD=/usr/lib/mergerfs/preload.so \
-v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \
-v /media:/data \
-v /media:/media \
-v /mnt/fs0:/mnt/fs0 \
-v /mnt/fs1:/mnt/fs1 \
ubuntu:latest \
bash
```
### systemd unit
Use the `Environment` option to set the LD_PRELOAD variable.
- https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Command%20lines
- https://serverfault.com/questions/413397/how-to-set-environment-variable-in-systemd-service
* [https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Command%20lines](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Command%20lines)
* [https://serverfault.com/questions/413397/how-to-set-environment-variable-in-systemd-service](https://serverfault.com/questions/413397/how-to-set-environment-variable-in-systemd-service)
```
[Service]

129
mkdocs/mkdocs.yml
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@ -1,6 +1,6 @@
site_name: mergerfs
site_description: mergerfs - a featureful union filesystem
site_url: https://trapexit.github.io/mergerfs
site_url: https://trapexit.github.io/mergerfs/
repo_name: mergerfs
repo_url: https://github.com/trapexit/mergerfs
edit_uri: tree/master/mkdocs/docs/
@ -10,43 +10,94 @@ theme:
logo: logo.jpeg
features:
- content.action.edit
- content.action.view
- content.code.copy
- content.code.select
- navigation.footer
- navigation.instant
- navigation.instant.prefetch
- navigation.path
- navigation.top
- navigation.tracking
- optimize
- search.share
- toc.follow
palette:
- media: "(prefers-color-scheme)"
toggle:
icon: material/brightness-auto
name: Switch to light mode
- media: "(prefers-color-scheme: light)"
scheme: default
toggle:
icon: material/brightness-7
name: Switch to dark mode
- media: "(prefers-color-scheme: dark)"
scheme: slate
toggle:
icon: material/brightness-4
name: Switch to system preference
markdown_extensions:
- footnotes
- pymdownx.highlight:
auto_title: false
linenums_style: pymdownx-inline
anchor_linenums: true
line_spans: __span
pygments_lang_class: true
- pymdownx.inlinehilite
- pymdownx.snippets
- pymdownx.superfences
nav:
- About: index.md
- Documentation:
- pages/documentation/how_it_works.md
- pages/documentation/terminology.md
- pages/documentation/basic_setup.md
- pages/documentation/options.md
- pages/documentation/functions_categories_and_policies.md
- pages/documentation/error_handling.md
- pages/documentation/install.md
- pages/documentation/build.md
- pages/documentation/upgrade.md
- pages/documentation/runtime_interfaces.md
- pages/documentation/tooling.md
- pages/documentation/caching.md
- pages/documentation/performance.md
- pages/documentation/benchmarking.md
- pages/documentation/tips_notes.md
- pages/documentation/known_issues_bugs.md
- pages/documentation/remote_filesystems.md
- pages/documentation/mergerfs_versus_x.md
- pages/documentation/support.md
- pages/documentation/links.md
- Home: index.md
- QuickStart: quickstart.md
- Installation:
- setup/installation.md
- setup/upgrade.md
- setup/build.md
- Config:
- config/terminology.md
- config/options.md
- config/deprecated_options.md
- config/branches.md
- config/functions_categories_and_policies.md
- config/func_readdir.md
- config/rename_and_link.md
- config/cache.md
- config/readahead.md
- config/inodecalc.md
- config/threads.md
- config/pin-threads.md
- config/link_cow.md
- config/fuse_msg_size.md
- config/follow-symlinks.md
- config/link-exdev.md
- config/rename-exdev.md
- config/symlinkify.md
- config/nullrw.md
- config/xattr.md
- config/nfsopenhack.md
- config/statfs.md
- config/flush-on-close.md
- config/export-support.md
- error_handling.md
- runtime_interfaces.md
- remote_filesystems.md
- tips_notes.md
- known_issues_bugs.md
- project_comparisons.md
- performance.md
- benchmarking.md
- tooling.md
- FAQ:
- pages/faq/compatibility_and_integration.md
- pages/faq/configuration_and_policies.md
- pages/faq/general_information_and_overview.md
- pages/faq/recommendations_and_warnings.md
- pages/faq/technical_behavior_and_limitations.md
- pages/faq/usage_and_functionality.md
- Wiki:
- pages/wiki/projects_using_mergerfs.md
- pages/wiki/installing_mergerfs_on_a_synology_nas.md
- pages/wiki/testimonials.md
- pages/wiki/real_world_deployments.md
- pages/wiki/systemd.md
- pages/wiki/kernel_issues_and_bugs.md
- pages/wiki/featured_media_and_publicity.md
- pages/wiki/limit_drive_spinup.md
- pages/wiki/links.md
- faq/reliability_and_scalability.md
- faq/usage_and_functionality.md
- faq/configuration_and_policies.md
- faq/compatibility_and_integration.md
- faq/recommendations_and_warnings.md
- faq/technical_behavior_and_limitations.md
- faq/limit_drive_spinup.md
- related_projects.md
- media_and_publicity.md
- support.md
- sponsorship_and_donations.md

2
mkdocs/requirements.txt
View File

@ -1,2 +0,0 @@
mkdocs==1.6.1
mkdocs-material==9.5.49
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