mergerfs is similar to mhddfs, unionfs, and aufs. Like mhddfs in that it too uses [FUSE](http://en.wikipedia.org/wiki/Filesystem_in_Userspace). Like aufs in that it provides multiple policies for how to handle behavior.
**mergerfs** is similar to **mhddfs**, **unionfs**, and **aufs**. Like **mhddfs** in that it too uses **FUSE**. Like **aufs** in that it provides multiple policies for how to handle behavior.
Why create mergerfs when those exist? mhddfs isn't really maintained or flexible. There are also security issues when with running as root. aufs is more flexible than mhddfs but contains some hard to debug inconsistencies in behavior on account of it being a kernel driver. Neither support file attributes ([chattr](http://linux.die.net/man/1/chattr)).
Why create **mergerfs** when those exist? **mhddfs** has not been updated in some time nor very flexible. There are also security issues when with running as root. **aufs** is more flexible than **mhddfs** but contains some hard to debug inconsistencies in behavior on account of it being a kernel driver. Neither support file attributes ([chattr](http://linux.die.net/man/1/chattr)).
# OPTIONS
###options###
* `defaults` is a shortcut for `auto_cache`. `big_writes`, `atomic_o_trunc`, `splice_read`, `splice_write`, and `splice_move` are in effect also enabled (by asking FUSE internally for such features) but if unavailable will be ignored. These options seem to provide the best performance.
* `minfreespace` (defaults to `4G`) is the minimum space value used for the `lfs` and `fwfs` policies. Understands 'K', 'M', and 'G' to represent kilobyte, megabyte, and gigabyte respectively.
* All [FUSE](http://fuse.sourceforge.net) functions which have a category (see below) are option keys. The syntax being `func.<func>=<policy>`.
* To set all function policies in a category use `category.<category>=<policy>` such as `category.create=mfs`.
* They 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.
* **defaults** is a shortcut for **auto_cache**. **big_writes**, **atomic_o_trunc**, **splice_read**, **splice_write**, and **splice_move** are in effect also enabled (by asking **FUSE** internally for such features) but if unavailable will be ignored. These options seem to provide the best performance.
* **minfreespace** (defaults to **4G**) is the minimum space value used for the **lfs** and **fwfs** policies. Understands 'K', 'M', and 'G' to represent kilobyte, megabyte, and gigabyte respectively.
* All FUSE functions which have a category (see below) are option keys. The syntax being **func.<func>=<policy>**. Example: **func.getattr=newest**.
* To set all function policies in a category use **categor.<category>=<policy>**. Example: **category.create=mfs**.
* 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.
###srcpoints###
The source points argument is a colon (':') delimited list of paths. To make it simplier to include multiple source points without having to modify your [fstab](http://linux.die.net/man/5/fstab) we also support [globbing](http://linux.die.net/man/7/glob).
The source points argument is a colon (':') delimited list of paths. To make it simpler to include multiple source points without having to modify your [fstab](http://linux.die.net/man/5/fstab) we also support [globbing](http://linux.die.net/man/7/glob).
```
$ mergerfs /mnt/disk*:/mnt/cdrom /media/drives
@ -47,7 +47,7 @@ In /etc/fstab it'd look like the following:
# POLICIES
Filesystem calls are broken up into 3 categories: action, create, search. There are also some calls which have no policy attached due to state being kept between calls. These categories can be assigned a policy which dictates how [mergerfs](http://github.com/trapexit/mergerfs) behaves. Any policy can be assigned to a category though some aren't terribly practical. For instance: rand (Random) may be useful for **create** but could lead to very odd behavior if used for **search**.
Filesystem calls are broken up into 3 categories: **action**, **create**, **search**. There are also some calls which have no policy attached due to state being kept between calls. These categories can be assigned a policy which dictates how **mergerfs** behaves. Any policy can be assigned to a category though some aren't terribly practical. For instance: **rand** (Random) may be useful for **create** but could lead to very odd behavior if used for **search**.
#### Functional classifications ####
@ -55,10 +55,10 @@ Filesystem calls are broken up into 3 categories: action, create, search. There
`ioctl` behaves differently if its acting on a directory. It'll use the `getattr` policy to find and open the directory before issuing the `ioctl`. In other cases where something may be searched (to confirm a directory exists across all source mounts) then `getattr` will be used.
**ioctl** behaves differently if its acting on a directory. It'll use the **getattr** policy to find and open the directory before issuing the **ioctl**. In other cases where something may be searched (to confirm a directory exists across all source mounts) then **getattr** will be used.
#### Policy descriptions ####
@ -68,11 +68,11 @@ Filesystem calls are broken up into 3 categories: action, create, search. There
| ffwp (first found w/ permissions) | Given the order of the paths act on the first one found which you have access (stat does not error with EACCES). |
| newest (newest file) | If multiple files exist return the one with the most recent mtime. |
| mfs (most free space) | Use the drive with the most free space available. |
| epmfs (existing path, most free space) | If the path exists in multiple locations use the one with the most free space. Otherwise fall back to mfs. |
| fwfs (first with free space) | Pick the first path which has at least `minfreespace`. |
| lfs (least free space) | Pick the path with least available space but more than `minfreespace`. |
| epmfs (existing path, most free space) | If the path exists in multiple locations use the one with the most free space. Otherwise fall back to **mfs**. |
| fwfs (first with free space) | Pick the first path which has at least **minfreespace**. |
| lfs (least free space) | Pick the path with least available space but more than **minfreespace**. |
| rand (random) | Pick an existing destination at random. |
| all | Applies action to all found. For searches it will behave like first found `ff`. |
| all | Applies action to all found. For searches it will behave like first found **ff**. |
#### Defaults ####
@ -84,66 +84,59 @@ Filesystem calls are broken up into 3 categories: action, create, search. There
#### readdir ####
[readdir](http://linux.die.net/man/3/readdir) is very different from most functions in this realm. It certainly could have it's own set of policies to tweak its behavior. At this time it provides a simple `first found` merging of directories and file found. That is: only the first file or directory found for a directory is returned. Given how FUSE works though the data representing the returned entry comes from `getattr`.
[readdir](http://linux.die.net/man/3/readdir) is very different from most functions in this realm. It certainly could have it's own set of policies to tweak its behavior. At this time it provides a simple **first found** merging of directories and file found. That is: only the first file or directory found for a directory is returned. Given how FUSE works though the data representing the returned entry comes from **getattr**.
It could be extended to offer the ability to see all files found. Perhaps concatinating `#` and a number to the name. But to really be useful you'd need to be able to access them which would complicate file lookup.
It could be extended to offer the ability to see all files found. Perhaps concatenating **#** and a number to the name. But to really be useful you'd need to be able to access them which would complicate file lookup.
#### statvfs ####
[statvfs](http://linux.die.net/man/2/statvfs) normalizes the source drives 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 drive so multiple points on the same drive will not result in double counting it's space.
**NOTE:** Since we can not (easily) replicate the atomicity of an `mkdir` or `mknod` without side effects those calls will first do a scan to see if the file exists and then attempts a create. This means there is a slight race condition. Worse case you'd end up with the directory or file on more than one mount.
**NOTE:** Since we can not (easily) replicate the atomicity of an **mkdir** or **mknod** without side effects those calls will first do a scan to see if the file exists and then attempts a create. This means there is a slight race condition. Worse case you'd end up with the directory or file on more than one mount.
# BUILDING
* Need to install FUSE development libraries (libfuse-dev).
* Optionally need libattr1 (libattr1-dev).
First get the code from [github](http://github.com/trapexit/mergerfs).
Have pkg-config, pandoc, libfuse, libattr1 installed.
```
[trapexit:~/dev/mergerfs] $ make help
usage: make
make XATTR_AVAILABLE=0 - to build program without xattrs functionality (auto discovered otherwise)
$ cd mergerfs
$ make
$ make man
$ sudo make install
```
# Runtime Settings
# RUNTIME
#### /.mergerfs pseudo file ####
#### .mergerfs pseudo file ####
```
<mountpoint>/.mergerfs
```
There is a pseudo file available at the mountpoint which allows for the runtime modification of certain mergerfs options. The file will not show up in readdirs but can be stat'ed and manipulated via [{list,get,set}xattrs](http://linux.die.net/man/2/listxattr) calls.
There is a pseudo file available at the mountpoint which allows for the runtime modification of certain **mergerfs** options. The file will not show up in **readdir** but can be **stat**'ed and manipulated via [{list,get,set}xattrs](http://linux.die.net/man/2/listxattr) calls.
Even if xattrs are disabled the [{list,get,set}xattrs](http://linux.die.net/man/2/listxattr) calls will still work.
##### Keys #####
* user.mergerfs.srcmounts
* user.mergerfs.minfreespace
* user.mergerfs.category.action
* user.mergerfs.category.create
* user.mergerfs.category.search
* user.mergerfs.func.access
* user.mergerfs.func.chmod
* user.mergerfs.func.chown
* user.mergerfs.func.create
* user.mergerfs.func.getattr
* user.mergerfs.func.getxattr
* user.mergerfs.func.link
* user.mergerfs.func.listxattr
* user.mergerfs.func.mkdir
* user.mergerfs.func.mknod
* user.mergerfs.func.open
* user.mergerfs.func.readlink
* user.mergerfs.func.removexattr
* user.mergerfs.func.rename
* user.mergerfs.func.rmdir
* user.mergerfs.func.setxattr
* user.mergerfs.func.symlink
* user.mergerfs.func.truncate
* user.mergerfs.func.unlink
* user.mergerfs.func.utimens
Use `xattr -l /mount/point/.mergerfs` to see all supported keys.
##### Example #####
@ -201,22 +194,21 @@ For **user.mergerfs.srcmounts** there are several instructions available for man
| Instruction | Description |
|--------------|-------------|
| +[list] | append |
| +<[list] | prepend |
| +>[list] | append |
| -[list] | remove all values provided |
| -< | remove first in list |
| -> | remove last in list |
| =[list] | set |
| [list] | set |
| [list] | set |
| +<[list] | prepend |
| +>[list] | append |
| -[list] | remove all values provided |
| -< | remove first in list |
| -> | remove last in list |
##### misc #####
Categories and funcs take a policy as described in the previous section. When reading funcs you'll get the policy string. However, with categories you'll get a coma separated list of policies for each type found. For example: if all search functions are `ff` except for `access` which is `ffwp` the value for `user.mergerfs.category.search` will be `ff,ffwp`.
Categories and funcs take a policy as described in the previous section. When reading funcs you'll get the policy string. However, with categories you'll get a comma separated list of policies for each type found. For example: if all search functions are **ff** except for **access** which is **ffwp** the value for **user.mergerfs.category.search** will be **ff,ffwp**.
#### mergerfs file xattrs ####
While they won't show up when using [listxattr](http://linux.die.net/man/2/listxattr) mergerfs offers a number of special xattrs to query information about the files served. To access the values you will need to issue a [getxattr](http://linux.die.net/man/2/getxattr) for one of the following:
While they won't show up when using [listxattr](http://linux.die.net/man/2/listxattr) **mergerfs** offers a number of special xattrs to query information about the files served. To access the values you will need to issue a [getxattr](http://linux.die.net/man/2/getxattr) for one of the following:
* **user.mergerfs.basepath:** the base mount point for the file given the current search policy
* **user.mergerfs.relpath:** the relative path of the file from the perspective of the mount point
@ -237,27 +229,25 @@ A B C
/mnt/b/full/path/to/A
```
# Tips / Notes
# TIPS / NOTES
* The recommended options are `defaults,allow_other`. The `allow_other` is to allow users who are not the one which executed mergerfs access to the mountpoint. `defaults` is described above and should offer the best performance. It's possible that if you're running on an older platform the `splice` features aren't available and could error. In that case simply use the other options manually.
* Remember that 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](http://github.com/trapexit/mergerfs) but that they are far more likely to occur on account of attempt to merge together multiple sources of data which could be out of sync due to the different policies.
* An example: [Kodi](http://kodi.tv) can apparently use directory [mtime](http://linux.die.net/man/2/stat) to more efficiently determine whether or not to scan for new content rather than simply performing a full scan. If using the current default `getattr` policy of `ff` it's possible Kodi will miss an update on account of it returning the first directory found's `stat` info and its 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.
* Due to previously mentioned issues its 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).
* The recommended options are **defaults,allow_other**. The **allow_other** is to allow users who are not the one which executed mergerfs access to the mountpoint. **defaults** is described above and should offer the best performance. It's possible that if you're running on an older platform the **splice** features aren't available and could error. In that case simply use the other options manually.
* Remember that 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 attempt to merge together multiple sources of data which could be out of sync due to the different policies.
* An example: [Kodi](http://kodi.tv) can apparently use directory [mtime](http://linux.die.net/man/2/stat) to more efficiently determine whether or not to scan for new content rather than simply performing a full scan. If using the current default **getattr** policy of **ff** its possible **Kodi** will miss an update on account of it returning the first directory found's **stat** info and its 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.
* Due to previously mentioned issues its 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).
# FAQ
#### 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) trys to handle being run as `root` by calling [getuid()](https://github.com/trapexit/mhddfs/blob/cae96e6251dd91e2bdc24800b4a18a74044f6672/src/main.c#L319) 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 many other situtations. Rather than attempting to simulate POSIX ACL behaviors the proper behavior is to use [seteuid](http://linux.die.net/man/2/seteuid) and [setegid](http://linux.die.net/man/2/setegid), become the user making the original call and perform the action as them. This is how [mergerfs](https://github.com/trapexit/mergerfs) handles things.
If you are familiar with POSIX standards you'll know that this behavior poses a problem. `seteuid` and `setegid` affect the whole process and `libfuse` is multithreaded by default. We'd need to lock access to `seteuid` and `setegid` with a mutex so that the several threads aren't stepping on one another and files end up with weird permissions and ownership. This however wouldn't scale well. With lots of calls the contention on that mutex would be extremely high. Thankfully on Linux and OSX we have a better solution.
*It's mentioned that there are some security issues with mhddfs. What are they? How does mergerfs address them?*
OSX has a [non-portable pthread extension](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/pthread_setugid_np.2.html) for per-thread user and group impersonation. When building on OSX mergerfs will use this without any mutexes.
[mhddfs](https://github.com/trapexit/mhddfs) tries to handle being run as **root** by calling [getuid()](https://github.com/trapexit/mhddfs/blob/cae96e6251dd91e2bdc24800b4a18a74044f6672/src/main.c#L319) 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 many other situations. Rather than attempting to simulate POSIX ACL behaviors the proper behavior is to use [seteuid](http://linux.die.net/man/2/seteuid) and [setegid](http://linux.die.net/man/2/setegid), become the user making the original call and perform the action as them. This is how [mergerfs](https://github.com/trapexit/mergerfs) handles things.
Linux does not support [pthread_setugid_np](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/pthread_setugid_np.2.html) but user and group IDs are a per-thread attribute though documention on that fact or how to manipulate them is not well distributed. From the `4.00` release of the Linux man-pages project for [setuid](http://man7.org/linux/man-pages/man2/setuid.2.html)
If you are familiar with POSIX standards you'll know that this behavior poses a problem. **seteuid** and **setegid** affect the whole process and **libfuse** is multithreaded by default. We'd need to lock access to **seteuid** and **setegid** with a mutex so that the several threads aren't stepping on one another and files end up with weird permissions and ownership. This however wouldn't scale well. With lots of calls the contention on that mutex would be extremely high. Thankfully on Linux and OSX we have a better solution.
> At the kernel level, user IDs and group IDs are a per-thread attribute. However, POSIX requires that all threads in a process share the same credentials. The NPTL threading implementation handles the POSIX requirements by providing wrapper functions for the various system calls that change process UIDs and GIDs. These wrapper functions (including the one for setuid()) employ a signal-based technique to ensure that when one thread changes credentials, all of the other threads in the process also change their credentials. For details, see nptl(7).
OSX has a [non-portable pthread extension](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/pthread_setugid_np.2.html) for per-thread user and group impersonation. When building on OSX mergerfs will use this without any mutexes.
Turns out the setreuid syscalls apply only to the thread. GLIBC hides this away using RT signals and other tricks. 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 privilages be needed (for instance: to clone paths).
Linux does not support [pthread_setugid_np](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/pthread_setugid_np.2.html) but user and group IDs are a per-thread attribute though documentation on that fact or how to manipulate them is not well distributed. From the **4.00** release of the Linux man-pages project for [setuid](http://man7.org/linux/man-pages/man2/setuid.2.html)
> At the kernel level, user IDs and group IDs are a per-thread attribute. However, POSIX requires that all threads in a process share the same credentials. The NPTL threading implementation handles the POSIX requirements by providing wrapper functions for the various system calls that change process UIDs and GIDs. These wrapper functions (including the one for setuid()) employ a signal-based technique to ensure that when one thread changes credentials, all of the other threads in the process also change their credentials. For details, see nptl(7).
Turns out the setreuid syscalls apply only to the thread. GLIBC hides this away using RT signals and other tricks. 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).
Antonio SJ Musumeci
10 years ago