**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**.
* **defaults**: a shortcut for FUSE's **atomic_o_trunc**, **auto_cache**, **big_writes**, **default_permissions**, **splice_move**, **splice_read**, and **splice_write**. These options seem to provide the best performance.
* **direct_io**: causes FUSE to bypass caching which can increase write speeds at the detriment of reads. Note that not enabling `direct_io` will cause double caching of files and therefore less memory for caching generally. However, `mmap` does not work when `direct_io` is enabled.
* **minfreespace**: the minimum space value used for creation policies. Understands 'K', 'M', and 'G' to represent kilobyte, megabyte, and gigabyte respectively. (default: 4G)
* **moveonenospc**: when enabled (set to **true**) if a **write** fails with **ENOSPC** or **EDQUOT** a scan of all drives will be done looking for the drive with most free space which is at least the size of the file plus the amount which failed to write. An attempt to move the file to that drive will occur (keeping all metadata possible) and if successful the original is unlinked and the write retried. (default: false)
* **use_ino**: causes mergerfs to supply file/directory inodes rather than libfuse. While not a default it is generally recommended it be enabled so that hard linked files share the same inode value.
* **dropcacheonclose**: when a file is requested to be closed call `posix_fadvise` on it first to instruct the kernel that we no longer need the data and it can drop its cache. Recommended when **direct_io** is not enabled to limit double caching. (default: false)
* **fsname**: sets the name of the filesystem as seen in **mount**, **df**, etc. Defaults to a list of the source paths concatenated together with the longest common prefix removed.
* **func.<func>=<policy>**: sets the specific FUSE function's policy. See below for the list of value types. Example: **func.getattr=newest**
* **category.<category>=<policy>**: Sets policy of all FUSE functions in the provided category. Example: **category.create=mfs**
**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.
The srcmounts (source mounts) argument is a colon (':') delimited list of paths to be included in the pool. It does not matter if the paths are on the same or different drives nor does it matter the filesystem. Used and available space will not be duplicated for paths on the same device and any features which aren't supported by the underlying filesystem (such as file attributes or extended attributes) will return the appropriate errors.
To make it easier to include multiple source mounts 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 expand it.**
**NOTE:** the globbing is done at mount or xattr update time (see below). If a new directory is added matching the glob after the fact it will not be automatically included.
The POSIX filesystem API has a number of functions. **creat**, **stat**, **chown**, etc. In mergerfs these functions are grouped into 3 categories: **action**, **create**, and **search**. Functions and categories can be assigned a policy which dictates how **mergerfs** behaves. Any policy can be assigned to a function or category though some are not very practical. For instance: **rand** (random) may be useful for file creation (create) but could lead to very odd behavior if used for `chmod` (though only if there were more than one copy of the file).
Policies, when called to create, will ignore drives which are readonly or have less than **minfreespace**. This allows for read/write and readonly drives to be mixed together and keep drives which may remount as readonly on error from further affecting the pool.
Due to FUSE limitations **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) **getattr** will also be used.
| all | Search category: acts like **ff**. Action category: apply to all found. Create category: for **mkdir**, **mknod**, and **symlink** it will apply to all found. **create** works like **ff**. It will exclude readonly drives and those with free space less than **minfreespace**. |
| epall (existing path, all) | Search category: acts like **epff**. Action category: apply to all found. Create category: for **mkdir**, **mknod**, and **symlink** it will apply to all existing paths found. **create** works like **epff**. It will exclude readonly drives and those with free space less than **minfreespace**. |
| epff | Given the order of the drives, as defined at mount time or when configured via the xattr interface, act on the first one found where the path already exists. For **create** category it will exclude readonly drives and those with free space less than **minfreespace** (unless there is no other option). Falls back to **ff**. |
| eplfs (existing path, least free space) | If the path exists on multiple drives use the one with the least free space. For **create** category it will exclude readonly drives and those with free space less than **minfreespace**. Falls back to **lfs**. |
| eplus (existing path, least used space) | If the path exists on multiple drives use the one with the least used space. For **create** category it will exclude readonly drives and those with free space less than **minfreespace**. Falls back to **lus**. |
| epmfs (existing path, most free space) | If the path exists on multiple drives use the one with the most free space. For **create** category it will exclude readonly drives and those with free space less than **minfreespace**. Falls back to **mfs**. |
| erofs | Exclusively return **-1** with **errno** set to **EROFS**. By setting **create** functions to this you can in effect turn the filesystem readonly. |
| ff (first found) | Given the order of the drives, as defined at mount time or when configured via xattr interface, act on the first one found. For **create** category it will exclude readonly drives and those with free space less than **minfreespace** (unless there is no other option). |
| lfs (least free space) | Pick the drive with the least available free space. For **create** category it will exclude readonly drives and those with free space less than **minfreespace**. Falls back to **mfs**. |
| lus (least used space) | Pick the drive with the least used space. For **create** category it will exclude readonly drives and those with free space less than **minfreespace**. Falls back to **mfs**. |
| mfs (most free space) | Pick the drive with the most available free space. For **create** category it will exclude readonly drives. Falls back to **ff**. |
| newest (newest file) | Pick the file / directory with the largest mtime. For **create** category it will exclude readonly drives and those with free space less than **minfreespace** (unless there is no other option). |
**epff**, **eplfs**, **eplus**, and **epmf** are path preserving policies. As the descriptions above explain they will only consider drives where the path being accessed exists. Non-path preserving policies will clone paths as necessary.
[rename](http://man7.org/linux/man-pages/man2/rename.2.html) is a tricky function in a merged system. Normally 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**. The atomic rename is most critical for replacing files in place atomically (such as securing writing to a temp file and then replacing a target). The problem is that by merging multiple paths you can have N instances of the source and destinations on different drives. This can lead to several undesirable situtations with or without errors and it's not entirely obvious what to do when an error occurs.
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 complient with POSIX requirements. However, many applications fail to handle EXDEV at all and treat it as a normal error or they only partially support EXDEV (don't respond the same as `mv` would). 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.
The above behavior will help minimize the likelihood of EXDEV being returned but it will still be possible. To remove the possibility all together mergerfs would need to perform the as **mv** does when it receives EXDEV normally.
[readdir](http://linux.die.net/man/3/readdir) is different from all other filesystem functions. While it could have it's own set of policies to tweak its behavior at this time it provides a simple union of files and directories found. Remember that any action or information queried about these files and directories come from the respective function. For instance: an **ls** is a **readdir** and for each file/directory returned **getattr** is called. Meaning the policy of **getattr** is responsible for choosing the file/directory which is the source of the metadata you see in an **ls**.
[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 sources on the same drive will not result in double counting it's space.
There is a pseudo file available at the mount point 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 for mergerfs the [{list,get,set}xattrs](http://linux.die.net/man/2/listxattr) calls against this pseudo file will still work.
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 (fstab).
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:
* https://github.com/trapexit/backup-and-recovery-howtos : A set of guides / howtos on creating a data storage system, backing it up, maintaining it, and recovering from failure.
* If you don't see some directories / files you expect in a merged point be sure the user has permission to all the underlying directories. Use `mergerfs.fsck` to audit the drive for out of sync permissions.
* Do *not* use `direct_io` if you expect applications (such as rtorrent) to [mmap](http://linux.die.net/man/2/mmap) files. It is not currently supported in FUSE w/ `direct_io` enabled.
* Since POSIX gives you only error or success on calls its difficult to determine the proper behavior when applying the behavior to multiple targets. **mergerfs** will return an error only if all attempts of an action fail. Any success will lead to a success returned. This means however that some odd situations may arise.
* [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** 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.
* 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.
* For consistency 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). However, the flexibility is there if needed.
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 far 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 drives. That alone is far more expensive than `ff` but would also possibly spin up sleeping drives.
If you always want the directory information from the one with the most recent mtime then use the `newest` policy for `getattr`.
Use the `direct_io` option as described above. Due to what mergerfs is doing there ends up being two caches of a file under normal usage. One from the underlying filesystem and one from mergerfs. Enabling `direct_io` removes the mergerfs cache. This saves on memory but means the kernel needs to communicate with mergerfs more often and can therefore result in slower speeds.
Since enabling `direct_io` disables `mmap` this is not an ideal situation however write speeds should be increased.
Be sure to turn off `direct_io`. rtorrent and some other applications use [mmap](http://linux.die.net/man/2/mmap) to read and write to files and offer no failback to traditional methods. FUSE does not currently support mmap while using `direct_io`. There will 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.
There [is a bug](https://lkml.org/lkml/2016/3/16/260) 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/2016/5/11/59).
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 situtation 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`.
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 it's 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 similar to older release of gvfs on Linux.
#### 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 upto 32 supplemental groups. Linux >= 2.6.3 allows upto 65535 groups per user but most other *nixs allow far less. NFS allowing 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.
If suddenly the mergerfs mount point disappears and `Transport endpoint is not connected` is returned when attempting to perform actions within the mount directory **and** the version of libfuse (use `mergerfs -v` to find the version) is older than `2.9.4` its likely due to a bug in libfuse. Affected versions of libfuse can be found in Debian Wheezy, Ubuntu Precise and others.
In order to fix this please install newer versions of libfuse. If using a Debian based distro (Debian,Ubuntu,Mint) you can likely just install newer versions of [libfuse](https://packages.debian.org/unstable/libfuse2) and [fuse](https://packages.debian.org/unstable/fuse) from the repo of a newer release.
There seems to be an issue with Linux version `4.9.0` and above in which an invalid message appears to be transmitted to libfuse (used by mergerfs) causing it to exit. No messages will be printed in any logs as its not a proper crash. Debugging of the issue is still ongoing and can be followed via the [fuse-devel thread](https://sourceforge.net/p/fuse/mailman/message/35662577).
There is a bug in the kernel. A work around appears to be turning off `splice`. Add `no_splice_write,no_splice_move,no_splice_read` to mergerfs' options. Should be placed after `defaults` if it is used since it will turn them on. This however is not guaranteed to work.
mhddfs is no longer maintained and has some known stability and security issues (see below). MergerFS provides a superset of mhddfs' features and should offer the same or maybe better performance.
While aufs can offer better peak performance mergerfs offers more configurability and is generally easier to use. mergerfs however does not offer the overlay / copy-on-write (COW) features which aufs and overlayfs have.
With simple JBOD / drive concatenation / stripping / RAID0 a single drive failure will lead to full pool failure. mergerfs performs a similar behavior without the catastrophic failure and general lack of recovery. Drives can fail and all other data will continue to be accessable.
When combined with something like [SnapRaid](http://www.snapraid.it) and/or an offsite full backup solution you can have the flexibilty of JBOD without the single point of failure.
MergerFS is not intended to be a replacement for ZFS. MergerFS is intended to provide flexible pooling of arbitrary drives (local or remote), of arbitrary sizes, and arbitrary filesystems. For `write once, read many` usecases such as media storage. And where data integrity and backup can be managed in other ways. In that situation ZFS can introduce major maintance and cost burdens as described [here](http://louwrentius.com/the-hidden-cost-of-using-zfs-for-your-home-nas.html).
#### Why do I get an "out of space" error even though the system says there's lots of space left?
Please reread the sections above about policies, path preserving, and the **moveonenospc** option. If the policy is path preserving and a drive is almost full and the drive the policy would pick then the writing of the file may fill the drive and receive ENOSPC errors. That is expected with those settings. If you don't want that: enable **moveonenospc** and don't use a path preserving policy.
While `ino_t` is 64 bits only a few filesystems use more than 32. Similarly, while `dev_t` is also 64 bits it was traditionally 16 bits. Bitwise or'ing them together should work most of the time. While totally unique inodes are preferred the overhead which would be needed does not seem to outweighted by the benefits.
[mhddfs](https://github.com/trapexit/mhddfs) manages running 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 behavior the proper way to manage this is to use [seteuid](http://linux.die.net/man/2/seteuid) and [setegid](http://linux.die.net/man/2/setegid), in effect becoming the user making the original call, and perform the action as them. This is what mergerfs does.
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 privileges be needed (for instance: to clone paths between drives).
For non-Linux systems mergerfs uses a read-write lock and changes credentials only when necessary. If multiple threads are to be user X then only the first one will need to change the processes credentials. So long as the other threads need to be user X they will take a readlock allowing multiple threads to share the credentials. Once a request comes in to run as user Y that thread will attempt a write lock and change to Y's credentials when it can. If the ability to give writers priority is supported then that flag will be used so threads trying to change credentials don't starve. This isn't the best solution but should work reasonably well assuming there are few users.
