Merge pull request #1416 from trapexit/docs

Add FAQ section on common perceived problems

mkdocs/docs/faq/configuration_and_policies.md

@@ -79,28 +79,3 @@ If you wish to move files around or balance the pool you can:
    [mergerfs.balance](https://github.com/trapexit/mergerfs-tools/blob/master/src/mergerfs.balance). Keep
    in mind that this tool is really just an example of how to
    accomplish such a task.
-
-
-
-## Why are all my files ending up on 1 filesystem?!
-
-Did you start with empty filesystems? Did you explicitly configure a
-`category.create` policy? Are you using an `existing path` / `path
-preserving` policy?
-
-The default create policy is `epmfs`. That is a path preserving
-algorithm. With such a policy for `mkdir` and `create` with a set of
-empty filesystems it will select only 1 filesystem when the first
-directory is created. Anything, files or directories, created in that
-directory will be placed on the same branch because it is preserving
-paths.
-
-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 `rand`, `pfrd`, `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
-writing said data.

mkdocs/docs/faq/technical_behavior_and_limitations.md

@@ -2,21 +2,26 @@
 
 ## Do hardlinks work?
 
-Yes. See also the option `inodecalc` for how inode values are
-calculated.
-
-What mergerfs does not do is fake hard links across branches. Read
-the section "rename & link" for how it works.
-
-Remember that hardlinks will NOT work across devices. That includes
-between the original filesystem and a mergerfs pool, between two
-separate pools of the same underlying filesystems, or bind mounts of
-paths within the mergerfs pool. The latter is common when using Docker
-or Podman. Multiple volumes (bind mounts) to the same underlying
-filesystem are considered different devices. There is no way to link
-between them. You should mount in the highest directory in the
-mergerfs pool that includes all the paths you need if you want links
-to work.
+Yes. There is no option to enable or disable links. They are
+fundimentally supported in compatible situations. That said the inode
+of a file is not necessarily indicitive of two file names linking to
+the same underlying data. See also the option `inodecalc` for how
+inode values are calculated.
+
+What mergerfs does not do is fake hard links across branches. Read the
+section [rename & link](../config/rename_and_link.md) for how it
+works.
+
+Remember that hardlinks will **NOT** work across devices. That
+includes between the original filesystem and a mergerfs pool, between
+two separate pools of the same underlying filesystems, or bind mounts
+of paths within the mergerfs pool. The latter is common when using
+Docker or Podman. Multiple volumes (bind mounts) to the same
+underlying filesystem are considered different devices. There is no
+way to link or rename between them. You should mount in the highest
+directory in the mergerfs pool that includes all the paths you need if
+you want links and rename to work.
+
 
 ## How does mergerfs handle moving and copying of files?
 
@@ -27,8 +32,8 @@ individual filesystem functions.
 
 A "move" can include a "copy" so lets describe copy first.
 
-When an application copies a file from source to destination it can do
-so in a number of ways but the basics are the following.
+When an application copies a file from "source" to "destination" it
+can do so in a number of ways but the basics are the following.
 
 1. `open` the source file.
 2. `create` the destination file.
@@ -58,13 +63,16 @@ some low level settings for the operating system.
 All of this means that mergerfs can not make decisions when a file is
 created based on file size or the source of the data. That information
 is simply not available. At best mergerfs could respond to files
-reaching a certain size when writing data or when a file is closed.
+reaching a certain size when writing data, a file is closed, or
+renamed.
 
 Related: if a user wished to have mergerfs perform certain activities
 based on the name of a file it is common and even best practice for a
 program to write to a temporary file first and then rename to its
 final destination. That temporary file name will typically be random
-and have no indication of the type of file being written.
+and have no indication of the type of file being written. At best
+something could be done on rename.
+
 
 ## Does FICLONE or FICLONERANGE work?
 
@@ -75,26 +83,27 @@ an error back to the application making the request.
 
 Should FUSE gain the ability mergerfs will be updated to support it.
 
+
 ## Why do I get an "out of space" / "no space left on device" / ENOSPC error even though there appears to be lots of space available?
 
 First make sure you've read the sections above about policies, path
-preservation, branch filtering, and the options **minfreespace**,
-**moveonenospc**, **statfs**, and **statfs_ignore**.
+preservation, branch filtering, and the options `minfreespace`,
+`moveonenospc`, `statfs`, and `statfs_ignore`.
 
 mergerfs is simply presenting a union of the content within multiple
 branches. The reported free space is an aggregate of space available
-within the pool (behavior modified by **statfs** and
-**statfs_ignore**). It does not represent a contiguous space. In the
+within the pool (behavior modified by `statfs` and
+`statfs_ignore`). It does not represent a contiguous space. In the
 same way that read-only filesystems, those with quotas, or reserved
 space report the full theoretical space available.
 
 Due to path preservation, branch tagging, read-only status, and
-**minfreespace** settings it is perfectly valid that `ENOSPC` / "out
-of space" / "no space left on device" be returned. It is doing what
-was asked of it: filtering possible branches due to those
-settings. Only one error can be returned and if one of the reasons for
-filtering a branch was **minfreespace** then it will be returned as
-such. **moveonenospc** is only relevant to writing a file which is too
+`minfreespace` settings it is perfectly valid that `ENOSPC` / "out of
+space" / "no space left on device" be returned. It is doing what was
+asked of it: filtering possible branches due to those settings. Only
+one error can be returned and if one of the reasons for filtering a
+branch was `minfreespace` then it will be returned as
+such. `moveonenospc` is only relevant to writing a file which is too
 large for the filesystem it's currently on.
 
 It is also possible that the filesystem selected has run out of
@@ -107,6 +116,7 @@ looking for. The reason it's not default is because it was originally
 set to `epmfs` and changing it now would change people's setup. Such a
 setting change will likely occur in mergerfs 3.
 
+
 ## Why does the total available space in mergerfs not equal outside?
 
 Are you using ext2/3/4? With reserve for root? mergerfs uses available
@@ -115,6 +125,7 @@ it won't show up.
 
 You can remove the reserve by running: `tune2fs -m 0 <device>`
 
+
 ## I notice massive slowdowns of writes when enabling cache.files.
 
 When file caching is enabled in any form (`cache.files!=off`) it will
@@ -141,32 +152,6 @@ To work around this situation mergerfs offers a few solutions.
    `mmap` it's probably simpler to just disable it altogether. The
    kernel won't send the requests when caching is disabled.
 
-## Why can't I see my files / directories?
-
-It's almost always a permissions issue. Unlike mhddfs and
-unionfs-fuse, which runs as root and attempts to access content as
-such, mergerfs always changes its credentials to that of the
-caller. This means that if the user does not have access to a file or
-directory than neither will mergerfs. However, because mergerfs is
-creating a union of paths it may be able to read some files and
-directories on one filesystem but not another resulting in an
-incomplete set.
-
-Whenever you run into a split permission issue (seeing some but not
-all files) try using
-[mergerfs.fsck](https://github.com/trapexit/mergerfs-tools) tool to
-check for and fix the mismatch. If you aren't seeing anything at all
-be sure that the basic permissions are correct. The user and group
-values are correct and that directories have their executable bit
-set. A common mistake by users new to Linux is to `chmod -R 644` when
-they should have `chmod -R u=rwX,go=rX`.
-
-If using a network filesystem such as NFS or SMB (Samba) be sure to
-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?
 

mkdocs/docs/faq/why_isnt_it_working.md

@@ -0,0 +1,112 @@
+# "Why isn't it working?"
+
+## I modified mergerfs' config but it still behaves the same.
+
+mergerfs, like most filesystems, are given their options/arguments
+at mount time. Unlike most filesystems, mergerfs also has the ability
+to modify certain options at [runtime](../runtime_interfaces.md).
+
+That said: mergerfs does not actively try to monitor typical methods
+of configuration (nor should it.) As such if changes are made to
+`/etc/fstab`, a systemd unit file, etc. it will have no knowledge of
+those changes. It is the user's responsibility to
+[restart](../setup/upgrade.md) mergerfs to pick up the changes or use
+the [runtime interface](../runtime_interfaces.md).
+
+
+## Why are all my files ending up on 1 filesystem?!
+
+Did you start with empty filesystems? Did you explicitly configure a
+`category.create` policy? Are you using an `existing path` / `path
+preserving` policy?
+
+The default create policy is `epmfs`. That is a path preserving
+algorithm. With such a policy for `mkdir` and `create` with a set of
+empty filesystems it will select only 1 filesystem when the first
+directory is created. Anything, files or directories, created in that
+directory will be placed on the same branch because it is preserving
+paths. That is the expected behavior.
+
+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 branches. If you do not care about path preservation ([most
+should
+not](configuration_and_policies.md#how-can-i-ensure-files-are-collocated-on-the-same-branch))
+and wish your files to be spread across all your filesystems change to
+`pfrd`, `rand`, `mfs` or similar
+[policy](../config/functions_categories_and_policies.md).
+
+
+## Why isn't the create policy working?
+
+It probably is. The policies rather straight forward and well tested.
+
+First, confirm the policy is configured as expected by using the
+[runtime interface](../runtime_interfaces.md).
+
+```shell
+$ getfattr -n user.mergerfs.category.create /mnt/mergerfs/.mergerfs
+# file: mnt/mergerfs/.mergerfs
+user.mergerfs.category.create="mfs"
+```
+
+Second, as discussed in the [support](../support.md) section, test the
+behavior using simple command line tools such as `touch` and then see
+where it was created.
+
+
+```shell
+$ touch /mnt/mergerfs/new-file
+$ getfattr -n user.mergerfs.allpaths /mnt/mergerfs/new-file
+# file: mnt/mergerfs/new-file
+user.mergerfs.allpaths="/mnt/hdd/drive1/new-file"
+```
+
+If the location of the file is where it should be according to the
+state of the system at the time and the policy selected then the
+"problem" lies elsewhere.
+
+[Keep in
+mind](technical_behavior_and_limitations.md/#how-does-mergerfs-handle-moving-and-copying-of-files)
+that files, when created, have no size. If a number of files are
+created at the same time, for example by a program downloading
+numerous files like a BitTorrent client, then depending on the policy
+the files could be created on the same branch. As the files are
+written to, or resized immediately afterwards to the total size of the
+file being downloaded, the files will take up more space but there is
+no mechanism to move them as they grow. Nor would it be a good idea to
+do so as it would be expensive to continuously calculate their size and
+perform the move while the file is still being written to. As such an
+imbalance will occur that wouldn't if the files had been downloaded
+one at a time.
+
+If you wish to reduce the likelihood of this happening a policy that
+does not make decisions on available space alone such as `pfrd` or
+`rand` should be used.
+
+
+## Why can't I see my files / directories?
+
+It's almost always a permissions issue. Unlike mhddfs and
+unionfs-fuse, which accesses content as root, mergerfs always changes
+its credentials to that of the caller. This is done as it is the only
+properly secure way to manage permissions. This means that if the user
+does not have access to a file or directory than neither will
+mergerfs. However, because mergerfs is creating a union of paths it
+may be able to read some files and directories on one filesystem but
+not another resulting in an incomplete set. And if one of the branches
+it can access is empty then it will return an empty list.
+
+Try using [mergerfs.fsck](https://github.com/trapexit/mergerfs-tools)
+tool to check for and fix inconsistencies in permissions. If you
+aren't seeing anything at all be sure that the basic permissions are
+correct. The user and group values are correct and that directories
+have their executable bit set. A common mistake by users new to Linux
+is to `chmod -R 644` when they should have `chmod -R u=rwX,go=rX`.
+
+If using a [network filesystem](../remote_filesystems.md) such as NFS
+or SMB (Samba) be sure to 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.](compatibility_and_integration.md)

mkdocs/docs/setup/upgrade.md

@@ -4,11 +4,12 @@ mergerfs can be upgraded live by mounting on top of the previous
 instance. Simply install the new version of mergerfs and follow the
 instructions below.
 
-Run mergerfs again or if using `/etc/fstab` call for it to mount
-again. Existing open files and such will continue to work fine though
-they won't see runtime changes since any such change would be the new
-mount. If you plan on changing settings with the new mount you should
-/ could apply those before mounting the new version.
+Run mergerfs again or if using `/etc/fstab` (or systemd mount file)
+call for it to mount again. Existing open files and directories will
+continue to work fine though they won't see any differences that the
+new version would provide since it is still using the previous
+instance. If you plan on changing settings with the new mount you
+should / could apply those before mounting the new version.
 
 ```
 $ sudo mount /mnt/mergerfs
@@ -23,3 +24,6 @@ restarted. To work around this you can use a "lazy umount". Before
 mounting over top the mount point with the new instance of mergerfs
 issue: `umount -l <mergerfs_mountpoint>`. Or you can let mergerfs do
 it by setting the option `lazy-umount-mountpoint=true`.
+
+If the intent is to change settings at runtime then the [runtime
+interface](../runtime_interfaces.md) should be used.

mkdocs/mkdocs.yml

@@ -97,6 +97,7 @@ nav:
   - faq/compatibility_and_integration.md
   - faq/recommendations_and_warnings.md
   - faq/technical_behavior_and_limitations.md
+  - faq/why_isnt_it_working.md
   - faq/limit_drive_spinup.md
 - related_projects.md
 - media_and_publicity.md