mergerfs - a featureful union filesystem


% mergerfs(1) mergerfs user manual % Antonio SJ Musumeci [email protected] % 2021-10-25


mergerfs - a featureful union filesystem


mergerfs -o


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.


  • 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


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 drives when creating new files so you can mix read-write and read-only drives. It also does NOT split data across drives. It is not RAID0 / striping. It is simply a union of other filesystems.


  • 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.


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)


You don't need mmap


See the mergerfs wiki for real world deployments for comparisons / ideas.


These options are the same regardless you use them with the mergerfs commandline program, used in fstab, or in a config file.

mount options

  • config: Path to a config file. Same arguments as below in key=val / ini style format.
  • branches: Colon delimited list of branches.
  • allow_other: A libfuse option which allows users besides the one which ran mergerfs to see the filesystem. This is required for most use-cases.
  • minfreespace=SIZE: The minimum space value used for creation policies. Can be overridden by branch specific option. Understands 'K', 'M', and 'G' to represent kilobyte, megabyte, and gigabyte respectively. (default: 4G)
  • moveonenospc=BOOL|POLICY: When enabled if a write fails with ENOSPC (no space left on device) or EDQUOT (disk quota exceeded) the policy selected will run to find a new location for the file. An attempt to move the file to that branch will occur (keeping all metadata possible) and if successful the original is unlinked and the write retried. (default: false, true = mfs)
  • use_ino: Causes mergerfs to supply file/directory inodes rather than libfuse. While not a default it is recommended it be enabled so that linked files share the same inode value.
  • inodecalc=passthrough|path-hash|devino-hash|hybrid-hash: Selects the inode calculation algorithm. (default: hybrid-hash)
  • dropcacheonclose=BOOL: 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 cache.files=partial|full|auto-full to limit double caching. (default: false)
  • symlinkify=BOOL: When enabled and a file is not writable and its mtime or ctime is older than symlinkify_timeout files will be reported as symlinks to the original files. Please read more below before using. (default: false)
  • symlinkify_timeout=UINT: Time to wait, in seconds, to activate the symlinkify behavior. (default: 3600)
  • nullrw=BOOL: Turns reads and writes into no-ops. The request will succeed but do nothing. Useful for benchmarking mergerfs. (default: false)
  • ignorepponrename=BOOL: Ignore path preserving on rename. Typically rename and link act differently depending on the policy of create (read below). Enabling this will cause rename and link to always use the non-path preserving behavior. This means files, when renamed or linked, will stay on the same drive. (default: false)
  • security_capability=BOOL: If false return ENOATTR when xattr security.capability is queried. (default: true)
  • xattr=passthrough|noattr|nosys: Runtime control of xattrs. Default is to passthrough xattr requests. 'noattr' will short circuit as if nothing exists. 'nosys' will respond with ENOSYS as if xattrs are not supported or disabled. (default: passthrough)
  • link_cow=BOOL: When enabled if a regular file is opened which has a link count > 1 it will copy the file to a temporary file and rename over the original. Breaking the link and providing a basic copy-on-write function similar to cow-shell. (default: false)
  • statfs=base|full: Controls how statfs works. 'base' means it will always use all branches in statfs calculations. 'full' is in effect path preserving and only includes drives where the path exists. (default: base)
  • statfs_ignore=none|ro|nc: 'ro' will cause statfs calculations to ignore available space for branches mounted or tagged as 'read-only' or 'no create'. 'nc' will ignore available space for branches tagged as 'no create'. (default: none)
  • nfsopenhack=off|git|all: A workaround for exporting mergerfs over NFS where there are issues with creating files for write while setting the mode to read-only. (default: off)
  • follow-symlinks=never|directory|regular|all: Turns symlinks into what they point to. (default: never)
  • link-exdev=passthrough|rel-symlink|abs-base-symlink|abs-pool-symlink: When a link fails with EXDEV optionally create a symlink to the file instead.
  • rename-exdev=passthrough|rel-symlink|abs-symlink: When a rename fails with EXDEV optionally move the file to a special directory and symlink to it.
  • posix_acl=BOOL: Enable POSIX ACL support (if supported by kernel and underlying filesystem). (default: false)
  • async_read=BOOL: Perform reads asynchronously. If disabled or unavailable the kernel will ensure there is at most one pending read request per file handle and will attempt to order requests by offset. (default: true)
  • fuse_msg_size=UINT: Set the max number of pages per FUSE message. Only available on Linux >= 4.20 and ignored otherwise. (min: 1; max: 256; default: 256)
  • threads=INT: Number of threads to use in multithreaded mode. When set to zero it will attempt to discover and use the number of logical cores. If the lookup fails it will fall back to using 4. If the thread count is set negative it will look up the number of cores then divide by the absolute value. ie. threads=-2 on an 8 core machine will result in 8 / 2 = 4 threads. There will always be at least 1 thread. NOTE: higher number of threads increases parallelism but usually decreases throughput. (default: 0)
  • fsname=STR: 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.action=POLICY: Sets policy of all FUSE functions in the action category. (default: epall)
  • category.create=POLICY: Sets policy of all FUSE functions in the create category. (default: epmfs)
  • Sets policy of all FUSE functions in the search category. (default: ff)
  • '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)
  • cache.entry=UINT: File name lookup cache timeout in seconds. (default: 1)
  • cache.negative_entry=UINT: Negative file name lookup cache timeout in seconds. (default: 0)
  • cache.files=libfuse|off|partial|full|auto-full: File page caching mode (default: libfuse)
  • cache.writeback=BOOL: Enable kernel writeback caching (default: false)
  • cache.symlinks=BOOL: Cache symlinks (if supported by kernel) (default: false)
  • cache.readdir=BOOL: Cache readdir (if supported by kernel) (default: false)
  • 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.

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.

Value Types

  • BOOL = 'true' | 'false'
  • UINT = [0,MAX_INT]
  • SIZE = 'NNM'; NN = INT, M = 'K' | 'M' | 'G' | 'T'
  • STR = string
  • FUNC = filesystem function
  • CATEGORY = function category
  • POLICY = mergerfs function policy


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 drives nor does it matter the filesystem (within reason). 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.

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 type

  • 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.


Same purpose as the global option but specific to the branch. If not set the global value is used.


To make it easier to include multiple branches mergerfs supports globbing. The globbing tokens MUST be escaped when using via the shell else the shell itself will apply the glob itself.

# mergerfs -o allow_other,use_ino /mnt/disk\*:/mnt/cdrom /media/drives

The above line will use all mount points in /mnt prefixed with disk and the cdrom.

To have the pool mounted at boot or otherwise accessible from related tools use /etc/fstab.

          /mnt/disk*:/mnt/cdrom /mnt/pool fuse.mergerfs allow_other,use_ino 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.

NOTE: for mounting via fstab to work you must have mount.fuse installed. For Ubuntu/Debian it is included in the fuse package.


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 will 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 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 regarding 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").

In the future the use_ino option will probably be removed as this feature should replace the original libfuse inode calculation strategy. Currently you still need to use use_ino in order to enable inodecalc.


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 from 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 4.20 a new feature was added allowing the negotiation of the max message size. Since the size is in multiples of pages 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 its 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 / max_pages, outside a minor bump in RAM usage due to larger message buffers, mergerfs defaults the value to 256. On kernels before 4.20 the value has no effect. The reason the value is configurable is to enable experimentation and benchmarking. See the BENCHMARKING section for examples.


This feature, when enabled, will cause symlinks to be interpreted by mergerfs as their target (depending on the mode).

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.

  • never: Behave as normal. Symlinks are treated as such.
  • directory: Resolve symlinks only which point to directories.
  • regular: Resolve symlinks only which point to regular files.
  • all: Resolve all symlinks to that which they point to.

Symlinks which do not point to anything are left as is.

WARNING: This feature works but there might be edge cases yet found. If you find any odd behaviors please file a ticket on github.


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.

  • passthrough: Return EXDEV as normal.
  • rel-symlink: A relative path from the newpath.
  • abs-base-symlink: A absolute value using the underlying branch.
  • abs-pool-symlink: A absolute value using the mergerfs mount point.

NOTE: It is possible that some applications check the file they link. In those cases it is possible it will error or complain.


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.

  • passthrough: Return EXDEV as normal.
  • rel-symlink: A relative path from the newpath.
  • abs-symlink: A absolute value using the mergerfs mount point.

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.


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 drives 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.


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 section for suggestions on how to test.


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.


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 it it applying anytime a readonly file which is empty is opened for writing.


The POSIX filesystem API is made up of a number of functions. creat, stat, chown, etc. For ease of configuration in mergerfs most of the core functions are grouped into 3 categories: action, create, and search. These functions 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. readdir has no real need for a policy given the purpose is merely to return a list of entries in a directory. statfs's behavior can be modified via other options.

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 question. Meaning that mounts in the branch won't be considered in the space calculations. The reason is that it doesn't really work for non-path preserving policies and can lead to non-obvious behaviors.

NOTE: While any policy can be assigned to a function or category though some may not be very useful in practice. For instance: rand (random) may be useful for file creation (create) but could lead to very odd behavior if used for chmod if there were more than one copy of the file.

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
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.


A policy is the algorithm used to choose a branch or branches for a function to work on. Think of them as ways to filter and sort branches.

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.


Policies basically search branches and create a list of files / paths for functions to work on. The policy is responsible for filtering and sorting the branches. Filters include minfreespace, whether or not a branch is mounted read-only, and the branch tagging (RO,NC,RW). These filters are applied across all policies unless otherwise noted.

  • No search function policies filter.
  • All action function policies filter out branches which are mounted read-only or tagged as RO (read-only).
  • All create function policies filter out branches which are mounted read-only, tagged RO (read-only) or NC (no create), or has available space less than minfreespace.

Policies may have their own additional filtering such as those that require existing paths to be present.

If all branches are filtered an error will be returned. Typically EROFS (read-only filesystem) or ENOSPC (no space left on device) depending on the most recent reason for filtering a branch. ENOENT will be returned if no elegible branch is found.

Path Preservation

Policies, as described below, are of two basic types. path preserving and non-path preserving.

All policies which start with ep (epff, eplfs, eplus, epmfs, eprand) are path preserving. ep stands for existing path.

A path preserving policy will only consider drives where the relative path being accessed already exists.

When using non-path preserving policies paths will be cloned to target drives as necessary.

With the msp or most shared path policies they are defined as path preserving for the purpose of controlling link and rename's behaviors since ignorepponrename is available to disable that behavior. In mergerfs v3.0 the path preserving behavior of rename and link will likely be separated from the policy all together.

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 offer certain policies because they are literally the same as others but it makes things a bit more uniform. In mergerfs 3.0 this might change.

Policy Description
all Search: Same as epall. Action: Same as epall. Create: for mkdir, mknod, and symlink it will apply to all branches. create works like ff.
epall (existing path, all) Search: Same as epff (but more expensive because it doesn't stop after finding a valid branch). Action: apply to all found. Create: for mkdir, mknod, and symlink it will apply to all found. create works like epff (but more expensive because it doesn't stop after finding a valid branch).
epff (existing path, first found) Given the order of the branches, as defined at mount time or configured at runtime, act on the first one found where the relative path exists.
eplfs (existing path, least free space) Of all the branches on which the relative path exists choose the drive with the least free space.
eplus (existing path, least used space) Of all the branches on which the relative path exists choose the drive with the least used space.
epmfs (existing path, most free space) Of all the branches on which the relative path exists choose the drive with the most free space.
eppfrd (existing path, percentage free random distribution) Like pfrd but limited to existing paths.
eprand (existing path, random) Calls epall and then randomizes. Returns 1.
erofs Exclusively return -1 with errno set to EROFS (read-only filesystem).
ff (first found) Search: Same as epff. Action: Same as epff. Create: Given the order of the drives, as defined at mount time or configured at runtime, act on the first one found.
lfs (least free space) Search: Same as eplfs. Action: Same as eplfs. Create: Pick the drive with the least available free space.
lus (least used space) Search: Same as eplus. Action: Same as eplus. Create: Pick the drive with the least used space.
mfs (most free space) Search: Same as epmfs. Action: Same as epmfs. Create: Pick the drive with the most available free space.
msplfs (most shared path, least free space) Search: Same as eplfs. Action: Same as eplfs. Create: like eplfs but walk back the path if it fails to find a branch at that level.
msplus (most shared path, least used space) Search: Same as eplus. Action: Same as eplus. Create: like eplus but walk back the path if it fails to find a branch at that level.
mspmfs (most shared path, most free space) Search: Same as epmfs. Action: Same as epmfs. Create: like epmfs but walk back the path if it fails to find a branch at that level.
msppfrd (most shared path, percentage free random distribution) Search: Same as eppfrd. Action: Same as eppfrd. Create: Like eppfrd but will walk back the path if it fails to find a branch at that level.
newest Pick the file / directory with the largest mtime.
pfrd (percentage free random distribution) Search: Same as eppfrd. Action: Same as eppfrd. Create: Chooses a branch at random with the likelihood of selection based on a branch's available space relative to the total.
rand (random) Calls all and then randomizes. Returns 1.

NOTE: If you are using an underlying filesystem that reserves blocks such as ext2, ext3, or ext4 be aware that mergerfs respects the reservation by using f_bavail (number of free blocks for unprivileged users) rather than f_bfree (number of free blocks) in policy calculations. df does NOT use f_bavail, it uses f_bfree, so direct comparisons between df output and mergerfs' policies is not appropriate.


Category Policy
action epall
create epmfs
search ff


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 & 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 drives 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 drive 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 drives with no source file
      • Remove the source from all drives 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 drive != target drive:
        • Clone target path from target drive to source drive
      • 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 drives with no source file
      • Remove the source from all drives which failed to rename

The 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.


readdir is different from all other filesystem functions. While it could have its 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.

statfs / statvfs

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 its space. 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.


POSIX filesystem functions offer a single return code meaning that there is some complication regarding the handling of multiple branches as mergerfs does. It tries to handle errors in a way that would generally return meaningful values for that particular function.

chmod, chown, removexattr, setxattr, truncate, utimens

  1. if no error: return 0 (success)
  2. if no successes: return first error
  3. if one of the files acted on was the same as the related search function: return its value
  4. return 0 (success)

While doing this increases the complexity and cost of error handling, particularly step 3, this provides probably the most reasonable return value.

unlink, rmdir

  1. if no errors: return 0 (success)
  2. return first error

Older version of mergerfs would return success if any success occurred but for unlink and rmdir there are downstream assumptions that, while not impossible to occur, can confuse some software.


For search functions there is always a single thing acted on and as such whatever return value that comes from the single function call is returned.

For create functions mkdir, mknod, and symlink which don't return a file descriptor and therefore can have all or epall policies it will return success if any of the calls succeed and an error otherwise.


NOTE: Prebuilt packages can be found at and recommended for most users: NOTE: Only tagged releases are supported. master and other branches should be considered works in progress.

First get the code from github.

$ git clone
$ # or
$ wget


Debian / Ubuntu

$ cd mergerfs
$ sudo tools/install-build-pkgs
$ make deb
$ sudo dpkg -i ../mergerfs_


RHEL / CentOS /Fedora

$ su -
# cd mergerfs
# tools/install-build-pkgs
# make rpm
# rpm -i rpmbuild/RPMS/



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


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.

Add nonempty to your mergerfs option list and call 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.

$ sudo mount /mnt/mergerfs
$ mount | grep mergerfs
media on /mnt/mergerfs type fuse.mergerfs (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)
media on /mnt/mergerfs type fuse.mergerfs (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)

A problem with this approach is that the underlying instance will continue to run even if the software using it stop or are 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 pseudo file



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 calls.

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).


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.


Same as the command line.


Used to query or modify the list of branches. When modifying there are several shortcuts to easy manipulation of the list.

Value Description
[list] set
+<[list] prepend
+>[list] append
-[list] remove all values provided
-< remove first in list
-> remove last in list

xattr -w user.mergerfs.branches +

The =NC, =RO, =RW syntax works just as on the command line.

[trapexit:/mnt/mergerfs] $ getfattr -d .mergerfs

[trapexit:/mnt/mergerfs] $ getfattr -n .mergerfs"ff"

[trapexit:/mnt/mergerfs] $ setfattr -n -v newest .mergerfs
[trapexit:/mnt/mergerfs] $ getfattr -n .mergerfs"newest"

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 served. To access the values you will need to issue a getxattr for one of the following:

  • user.mergerfs.basepath: the base mount point for the file given the current getattr policy
  • user.mergerfs.relpath: the relative path of the file from the perspective of the mount point
  • 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


    • mergerfs.ctl: A tool to make it easier to query and configure mergerfs at runtime
    • mergerfs.fsck: Provides permissions and ownership auditing and the ability to fix them
    • mergerfs.dedup: Will help identify and optionally remove duplicate files
    • mergerfs.dup: Ensure there are at least N copies of a file across the pool
    • mergerfs.balance: Rebalance files across drives by moving them from the most filled to the least filled
    • mergerfs.consolidate: move files within a single mergerfs directory to the drive with most free space
    • scorch: A tool to help discover silent corruption of files and keep track of files
    • bbf (bad block finder): a tool to scan for and 'fix' hard drive bad blocks and find the files using those blocks


page caching


  • 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.

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 the what underlying filesystems would be supported and complicates the code.

kernel documentation:

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).

policy caching

Policies are run every time a function (with a policy as mentioned above) is called. These policies can be expensive depending on mergerfs' setup and client usage patterns. Generally we wouldn't want to cache policy results because it may result in stale responses if the underlying drives are used directly.

The open policy cache will cache the result of an open policy for a particular input for seconds or until the file is unlinked. Each file close (release) will randomly chose to clean up the cache of expired entries.

This cache is really only useful in cases where you have a large number of branches and open is called on the same files repeatedly (like Transmission which opens and closes a file on every read/write presumably to keep file handle usage low).

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 drive 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 drive 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 its 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 drive could help with a typical mergerfs setup.

  1. Fast network, slow drives, many readers: You've a 10+Gbps network with many readers and your regular drives can't keep up.
  2. Fast network, slow drives, small'ish bursty writes: You have a 10+Gbps network and wish to transfer amounts of data less than your cache drive but wish to do so quickly.

With #1 its 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 drives (see the mergerfs.dup tool) and setting, 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 drives and one which has both the fast drives (SSD,NVME,etc.) and slow drives.
  2. The 'cache' pool should have the cache drives 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 drive(s) are far smaller than the backing drives. 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 tag the slow drives as =NC though that'd mean if the cache drives 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 drive if not larger. This way in the worst case the whole of the cache drive(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 drive. Not the pool with the cache drive. You could have data loss if the source is the cache pool.

" exit 1 fi CACHE="${1}" BACKING="${2}" N=${3} find "${CACHE}" -type f -atime +${N} -printf '%P\n' | \ rsync --files-from=- -axqHAXWES --preallocate --remove-source-files "${CACHE}/" "${BACKING}/" ">

if [ $# != 3 ]; then
  echo "usage: $0 
  exit 1


find "${CACHE}" -type f -atime +${N} -printf '%P\n' | \
  rsync --files-from=- -axqHAXWES --preallocate --remove-source-files "${CACHE}/" "${BACKING}/"

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 drive. Not the pool with the cache drive. You could have data loss if the source is the cache pool.

" exit 1 fi CACHE="${1}" BACKING="${2}" PERCENTAGE=${3} set -o errexit while [ $(df --output=pcent "${CACHE}" | grep -v Use | cut -d'%' -f1) -gt ${PERCENTAGE} ] do FILE=$(find "${CACHE}" -type f -printf '%[email protected] %P\n' | \ sort | \ head -n 1 | \ cut -d' ' -f2-) test -n "${FILE}" rsync -axqHAXWESR --preallocate --remove-source-files "${CACHE}/./${FILE}" "${BACKING}/" done ">

if [ $# != 3 ]; then
  echo "usage: $0 
  exit 1


set -o errexit
while [ $(df --output=pcent "${CACHE}" | grep -v Use | cut -d'%' -f1) -gt ${PERCENTAGE} ]
    FILE=$(find "${CACHE}" -type f -printf '%[email protected] %P\n' | \
                  sort | \
                  head -n 1 | \
                  cut -d' ' -f2-)
    test -n "${FILE}"
    rsync -axqHAXWESR --preallocate --remove-source-files "${CACHE}/./${FILE}" "${BACKING}/"



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. Drive 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

  • enable (or disable) splice_move, splice_read, and splice_write
  • disable security_capability and/or xattr
  • increase cache timeouts cache.attr, cache.entry, cache.negative_entry
  • enable (or disable) page caching (cache.files)
  • enable cache.writeback
  • enable
  • 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 drives
  • 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 so he may investigate further.


Filesystems are complicated. They do many things and many of those are interconnected. Additionally, the OS, drivers, hardware, etc. all can impact performance. Therefore, when benchmarking, it is necessary that the test focus as narrowly as possible.

For most throughput is the key benchmark. To test throughput dd is useful but must be used with the correct settings in order to ensure the filesystem or device is actually being tested. The OS can and will cache data. Without forcing synchronous reads and writes and/or disabling caching the values returned will not be representative of the device's true performance.

When benchmarking through mergerfs ensure you only use 1 branch to remove any possibility of the policies complicating the situation. Benchmark the underlying filesystem first and then mount mergerfs over it and test again. If you're experience speeds below your expectation you will need to narrow down precisely which component is leading to the slowdown. Preferably test the following in the order listed (but not combined).

  1. Enable nullrw mode with nullrw=true. This will effectively make reads and writes no-ops. Removing the underlying device / filesystem from the equation. This will give us the top theoretical speeds.
  2. Mount mergerfs over tmpfs. tmpfs is a RAM disk. Extremely high speed and very low latency. This is a more realistic best case scenario. Example: mount -t tmpfs -o size=2G tmpfs /tmp/tmpfs
  3. Mount mergerfs over a local drive. NVMe, SSD, HDD, etc. If you have more than one I'd suggest testing each of them as drives and/or controllers (their drivers) could impact performance.
  4. Finally, if you intend to use mergerfs with a network filesystem, either as the source of data or to combine with another through mergerfs, test each of those alone as above.

Once you find the component which has the performance issue you can do further testing with different options to see if they impact performance. For reads and writes the most relevant would be: cache.files, async_read, splice_move, splice_read, splice_write. Less likely but relevant when using NFS or with certain filesystems would be security_capability, xattr, and posix_acl. If you find a specific system, drive, filesystem, controller, etc. that performs poorly contact trapexit so he may investigate further.

Sometimes the problem is really the application accessing or writing data through mergerfs. Some software use small buffer sizes which can lead to more requests and therefore greater overhead. You can test this out yourself by replace bs=1M in the examples below with ibs or obs and using a size of 512 instead of 1M. In one example test using nullrw the write speed dropped from 4.9GB/s to 69.7MB/s when moving from 1M to 512. Similar results were had when testing reads. Small writes overhead may be improved by leveraging a write cache but in casual tests little gain was found. More tests will need to be done before this feature would become available. If you have an app that appears slow with mergerfs it could be due to this. Contact trapexit so he may investigate further.

write benchmark

$ dd if=/dev/zero of=/mnt/mergerfs/1GB.file bs=1M count=1024 oflag=nocache conv=fdatasync status=progress

read benchmark

$ dd if=/mnt/mergerfs/1GB.file of=/dev/null bs=1M count=1024 iflag=nocache conv=fdatasync status=progress

other benchmarks

If you are attempting to benchmark other behaviors you must ensure you clear kernel caches before runs. In fact it would be a good deal to run before the read and write benchmarks as well just in case.

echo 3 | sudo tee /proc/sys/vm/drop_caches


  • This document is very literal and thorough. Unless there is a bug things work as described. 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. Few distros have the latest version.
  • use_ino will only work when used with mergerfs 2.18.0 and above.
  • Run mergerfs as root (with allow_other) unless you're merging paths which are owned exclusively and fully by the same user otherwise strange permission issues may arise. mergerfs is designed and intended to be run as root.
  • 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 drive 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 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, Plex, Subsonic, etc. can use directory mtime 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 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 the 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. However, the flexibility is there if needed.


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 drives.

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 drive 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 does not like out of band changes. That is especially true of inode values.

Be sure to use the following options:

  • noforget
  • use_ino
  • inodecalc=path-hash

rtorrent fails with ENODEV (No such device)

Be sure to set cache.files=partial|full|auto-full or turn off direct_io. rtorrent and some other applications use 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.

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 & 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 drive 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 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.

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 tool mergerfs.mktrash.

tar: Directory renamed before its status could be extracted

Make sure to use the use_ino option.

Supplemental user groups

Due to the overhead of getgroups/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 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.

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.

mergerfs or libfuse crashing

First... always upgrade to the latest version unless told otherwise.

If using mergerfs below 2.22.0:

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 and fuse from the repo of a newer release.

If using mergerfs at or above 2.22.0:

First upgrade if possible, check the known bugs section, and contact trapexit.

mergerfs appears to be crashing or exiting

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 it's not a proper crash. Debugging of the issue is still ongoing and can be followed via the fuse-devel thread.

rm: fts_read failed: No such file or directory

Please update. This is only happened to mergerfs versions at or below v2.25.x and will not occur in more recent versions.


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 includes mergerfs as its sole solution for pooling drives. The author of mergerfs had it running for over 300 days managing 16+ drives 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. All of which are fixed in stable releases.

Can mergerfs be used with drives which already have data / are in use?

Yes. MergerFS is a proxy and does NOT interfere with the normal form or function of the drives / mounts / paths it manages.

MergerFS is not a traditional filesystem. MergerFS is not RAID. It does not manipulate the data that passes through it. It does not shard data across drives. It merely shards some behavior and aggregates others.

Can mergerfs be removed without affecting the data?

See the previous question's answer.

What policies should I use?

Unless you're doing something more niche the average user is probably best off using mfs for category.create. It will spread files out across your branches based on available space. Use mspmfs if you want to try to colocate the data a bit more. You may want to use lus if you prefer a slightly different distribution of data if you have a mix of smaller and larger drives. Generally though mfs, lus, or even rand are good for the general use case. If you are starting with an imbalanced pool you can use the tool mergerfs.balance to redistribute files across the pool.

If you really wish to try to colocate files based on directory you can set func.create to epmfs or similar and func.mkdir to rand or eprand depending on if you just want to colocate generally or on specific branches. Either way the need to colocate is rare. For instance: if you wish to remove the drive regularly and want the data to predictably be on that drive or if you don't use backup at all and don't wish to replace that data piecemeal. In which case using path preservation can help but will require some manual attention. Colocating after the fact can be accomplished using the mergerfs.consolidate tool. If you don't need strict colocation which the ep policies provide then you can use the msp based policies which will walk back the path till finding a branch that works.

Ultimately there is no correct answer. It is a preference or based on some particular need. mergerfs is very easy to test and experiment with. I suggest creating a test setup and experimenting to get a sense of what you want.

The reason mfs is not the default category.create policy is historical. When/if a 3.X gets released it will be changed to minimize confusion people often have with path preserving policies.

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 reach out and the documentation will be improved.

That said, for the average person, the following should be fine:

-o use_ino,cache.files=off,dropcacheonclose=true,allow_other,category.create=mfs

Why are all my files ending up on 1 drive?!

Did you start with empty drives? 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 drives it will select only 1 drive when the first 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. If you do not care about path preservation and wish your files to be spread across all your drives 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 drives 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 together directory content by drive.

Do hardlinks work?

Yes. You need to use use_ino to support proper reporting of inodes but they work regardless. See also the option inodecalc.

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.

Can I use mergerfs without SnapRAID? SnapRAID without mergerfs?

Yes. They are completely unreleated pieces of software.

Can mergerfs run via Docker, Podman, Kubernetes, etc.

Yes. With Docker you'll need to include --cap-add=SYS_ADMIN --device=/dev/fuse --security-opt=apparmor:unconfined or similar with other container runtimes. You should also be running it as root or given sufficient caps to change user and group identity as well as have root like filesystem permissions.

Keep in mind that you MUST consider identity when using containers. For example: supplemental groups will be picked up from the container unless you properly manage users and groups by sharing relevant /etc files or by using some other means to share identity across containers. Similarly if you use "rootless" containers and user namespaces to do uid/gid translations you MUST consider that while managing shared files.

Also, as mentioned by hotio, 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 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.

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 drive 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 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, SMB, CIFS (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.

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 libfuse embedded into mergerfs?

  1. A significant number of users use mergerfs on distros with old versions of libfuse which have serious bugs. Requiring updated versions of libfuse on those distros isn't practical (no package offered, user inexperience, etc.). The only practical way to provide a stable runtime on those systems was to "vendor" / embed the library into the project.
  2. mergerfs was written to use the high level API. There are a number of limitations in the HLAPI that make certain features difficult or impossible to implement. While some of these features could be patched into newer versions of libfuse without breaking the public API some of them would require hacky code to provide backwards compatibility. While it may still be worth working with upstream to address these issues in future versions, since the library needs to be vendored for stability and compatibility reasons it is preferable / easier to modify the API. Longer term the plan is to rewrite mergerfs to use the low level API.

Why did support for system libfuse get removed?

See above first.

If/when mergerfs is rewritten to use the low-level API then it'll be plausible to support system libfuse but till then it's simply too much work to manage the differences across the versions.

Why use mergerfs over mhddfs?

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.

Below is an example of mhddfs and mergerfs setup to work similarly.

mhddfs -o mlimit=4G,allow_other /mnt/drive1,/mnt/drive2 /mnt/pool

mergerfs -o minfreespace=4G,allow_other,category.create=ff /mnt/drive1:/mnt/drive2 /mnt/pool

Why use mergerfs over aufs?

aufs is mostly abandoned and no longer available in many 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 and overlayfs have.

Why use mergerfs over unionfs?

UnionFS is more like aufs than mergerfs in that it offers overlay / CoW features. If you're just looking to create a union of drives and want flexibility in file/directory placement then mergerfs offers that whereas unionfs is more for overlaying RW filesystems over RO ones.

Why use mergerfs over overlayfs?

Same reasons as with unionfs.

Why use mergerfs over LVM/ZFS/BTRFS/RAID0 drive concatenation / striping?

With simple JBOD / drive concatenation / stripping / RAID0 a single drive failure will result in full pool failure. mergerfs performs a similar function without the possibility of catastrophic failure and the difficulties in recovery. Drives may fail, however, all other data will continue to be accessible.

When combined with something like SnapRaid and/or an offsite backup solution you can have the flexibility of JBOD without the single point of failure.

Why use mergerfs over ZFS?

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 bulk media storage. Where data integrity and backup is managed in other ways. In that situation ZFS can introduce a number of costs and limitations as described here, here, and here.

Why use mergerfs over 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 said they prefer the flexibility offered by mergerfs and for some the fact it is free and open source is important.

There are a number of UnRAID users who use mergerfs as well though I'm not entirely familiar with the use case.

What should mergerfs NOT be used for?

  • 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.

  • 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).

  • 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.

Can drives be written to directly? Outside of mergerfs while pooled?

Yes, however it's not recommended to use the same file from within the pool and from without at the same time (particularly writing). Especially if using caching of any kind (cache.files, cache.entry, cache.attr, cache.negative_entry, cache.symlinks, cache.readdir, etc.) as there could be a conflict between cached data and not.

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.

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 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 large for the drive its currently on.

It is also possible that the filesystem selected has run out of inodes. Use df -i to list the total and available inodes per filesystem.

If you don't care about path preservation then simply change the create policy to one which isn't. mfs is probably what most are 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 space for statfs calculations. If you've reserved space for root then it won't show up.

You can remove the reserve by running: tune2fs -m 0

Can mergerfs mounts be exported over NFS?

Yes, however if you do anything which may changes files out of band (including for example using the newest policy) it will result in "stale file handle" errors unless properly setup.

Be sure to use the following options:

  • noforget
  • use_ino
  • inodecalc=path-hash

Can mergerfs mounts be exported over Samba / SMB?

Yes. While some users have reported problems it appears to always be related to how Samba is setup in relation to permissions.

Can mergerfs mounts be used over SSHFS?


I notice massive slowdowns of writes when enabling cache.files.

When file caching is enabled in any form (cache.files!=off or direct_io=false) it will issue getxattr requests for security.capability prior to every single write. This will usually result in a performance degradation, especially when using a network filesystem (such as NFS or CIFS/SMB/Samba.) Unfortunately at this moment the kernel is not caching the response.

To work around this situation mergerfs offers a few solutions.

  1. Set security_capability=false. It will short circuit any call and return ENOATTR. This still means though that mergerfs will receive the request before every write but at least it doesn't get passed through to the underlying filesystem.
  2. Set xattr=noattr. Same as above but applies to all calls to getxattr. Not just security.capability. This will not be cached by the kernel either but mergerfs' runtime config system will still function.
  3. Set xattr=nosys. Results in mergerfs returning ENOSYS which will be cached by the kernel. No future xattr calls will be forwarded to mergerfs. The downside is that also means the xattr based config and query functionality won't work either.
  4. Disable file caching. If you aren't using applications which use mmap it's probably simpler to just disable it all together. The kernel won't send the requests when caching is disabled.

What are these .fuse_hidden files?

Please upgrade. mergerfs >= 2.26.0 will not have these temporary files. See the notes on unlink.

It's mentioned that there are some security issues with mhddfs. What are they? How does mergerfs address them?

mhddfs manages running as root by calling getuid() and if it returns 0 then it will 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 this is to use seteuid and setegid, in effect becoming the 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 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.


Filesystems are complex and difficult to debug. mergerfs, while being just a proxy of sorts, is also very difficult to debug given the large number of possible settings it can have itself and the massive number of environments it can run in. When reporting on a suspected issue please, please include as much of the below information as possible otherwise it will be difficult or impossible to diagnose. Also please make sure to read all of the above documentation as it includes nearly every known system or user issue previously encountered.

Please make sure you are using the latest release 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 your bug may have been addressed already.

Information to include in bug reports

  • Version of mergerfs: mergerfs -V
  • mergerfs settings / arguments: from fstab, systemd unit, command line, etc.
  • Version of the OS: uname -a
  • List of branches, their filesystem types, sizes (before and after issue): df -h
  • All information about the relevant branches and paths: permissions, ownership, etc.
  • All information about the client app making the requests: version, uid/gid
  • Runtime environment: mostly are things running inside containers or not
  • A strace of the app having problems:
    • strace -fvTtt -s 256 -o /tmp/app.strace.txt
  • A strace of mergerfs while the program is trying to do whatever it's failing to do:
    • strace -fvTtt -s 256 -p -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

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  • Problems using Git repos hosted on pool which is NFS-exported

    Problems using Git repos hosted on pool which is NFS-exported

    General description

    I am running the latest stable version of Openmediavault with 2 drives pooled with mergerfs.

    I have a single shared folder on the pool which is exported over NFS to clients.

    My NFS export looks like: /export/media *(fsid=1,rw,subtree_check,insecure,no_root_squash,anonuid=1000)

    To rule out permission issues I have made everything very permissive: my shared folder has "Everyone read/write" in OMV's settings so real-world perms end up as:

    [email protected]:/export# ls -la
    total 12
    drwxr-xr-x  3 root root  4096 May 18 23:05 .
    drwxr-xr-x 24 root root  4096 May 11 14:34 ..
    drwxrwsrwx 22 root users 4096 May 19 14:54 media
    [email protected]:/srv# ls -la ecaf2ef9-aa68-47d9-99ad-ac21d64d8764/
    total 28
    drwxr-xr-x  4 root root   4096 May 15 14:18 .
    drwxr-xr-x  7 root root   4096 May 18 23:04 ..
    drwx------  2 root root  16384 May 15 14:15 lost+found
    drwxrwsrwx 22 root users  4096 May 19 14:54 media

    Permissions inherit down throughout the tree - 2777 all the way (including the setgid that OMV seems to put in too).

    My client in question mounts the NFS export from fstab with: x.x.x.x:/export/media /omv nfs rw,soft,intr,noauto,x-systemd.automount 0 0

    From an NFS client (Ubuntu 19.04), I am trying to run some git commands to create/fetch a repo I wish to store and run the resulting software from the NFS mount-point, and I am hitting "permission denied" errors (specifically with a "git fetch --all" command)

    The error is:

    fatal: write error: Permission denied
    fatal: index-pack failed
    error: Could not fetch from origin

    If I bypass NFS and run the "git fetch" directly from the pool/shared folder on the OMV server's console or into a local non-NFS folder on the client, it succeeds without error. Only over NFS to this mergerfs pool do I see these errors.

    I am doing all sorts of other things from this (and other) NFS clients to the pool manipulating data, creating/deleting files and it all works perfectly, just not git for some reason.

    Expected behavior

    The git fetch command should work as expected without permission errors.

    Actual behavior

    "Permission denied" messages in relation to "git fetch" command

    Precise steps to reproduce the behavior

    The NFS exported pool is at "/omv" in this example:

    On the Ubuntu 19.04 NFS client:

    cd /omv
    mkdir Medusa
    cd Medusa
    git init
    git remote add origin
    git fetch --all

    System information

    Please provide as much of the following information as possible:

    • [ ] mergerfs version: mergerfs -V 2.26.2

    • [ ] mergerfs settings: cat /etc/fstab or the command line arguments /srv/dev-disk-by-label-WD6TBBAY1:/srv/dev-disk-by-id-usb-WD_Elements_25A1_575833314434383746543045-0-0-part1 /srv/ecaf2ef9-aa68-47d9-99ad-ac21d64d8764 fuse.mergerfs defaults,allow_other,direct_io,use_ino,noforget,category.create=eplfs,minfreespace=10M 0 0

    • [ ] Linux version: uname -a Linux omv 4.19.0-0.bpo.4-amd64 #1 SMP Debian 4.19.28-2~bpo9+1 (2019-03-27) x86_64 GNU/Linux

    • [ ] Versions of any additional software being used On the OMV server:

    • FUSE library version: 2.9.7-mergerfs_2.26.0
    • fusermount version: 2.9.7
    • using FUSE kernel interface version 7.27
    • git version 2.11.0

    On the NFS client:

    • git version 2.20.1
    • libnfs12:amd64 version 3.0.0-2
    • libnfsidmap2:amd64 0.25-5.1ubuntu1
    • nfs-common 1:1.3.4-2.3ubuntu3
    • [ ] List of drives, filesystems, & sizes: df -h
    label-WD6TBBAY1:id-usb-WD_Elements_25A1_575833314434383746543045-0-0-part1  9.0T  5.6T  3.3T  64% /srv/ecaf2ef9-aa68-47d9-99ad-ac21d64d8764
    /dev/sda1                                                                   5.5T  2.5T  3.0T  45% /srv/dev-disk-by-label-WD6TBBAY1
    /dev/sdc1                                                                   3.6T  3.2T  225G  94% /srv/dev-disk-by-id-usb-WD_Elements_25A1_575833314434383746543045-0-0-part1
    • [ ] strace of application having problem: strace -f -o /tmp/app.strace.txt <cmd> or strace -f -p <appPID> -o /tmp/app.strace.txt git.strace.txt

    • [ ] strace of mergerfs while app tried to do it's thing: strace -f -p <mergerfsPID> -o /tmp/mergerfs.strace.txt mergerfs.strace.txt

    opened by rodhull 92
  • Permission denied when creating files w/ O_EXCL over NFS

    Permission denied when creating files w/ O_EXCL over NFS

    I'm getting a permission error on creating files with the O_EXCL flag on NFS mounts.

    It works fine if I...

    • Create the file directly on the file server in the mergerfs pool
    • Use a NFS mount that is not sourced by mergerfs (to disprove some google suggestions that NFS itself may not work with O_EXCL. It had worked with flexraid storage pool fine so fairly sure it's not NFS)
    • Squash all the nfs access to be under root

    I believe the above suggest that there is something in mergerfs that is causing this issue.

    A simple example is when using vim as it tries to create the temporary swap files with the O_EXCL flag, and vim will complain that the swap file cannot be opened.

    Ubuntu 16.04 NFSv4 mergerfs 2.17

    NFS Server

    Hostname: fileserver


    /mnt/hd0:/mnt/hd1 /mnt/dump  fuse.mergerfs  defaults,allow_other,minfreespace=10G  0       0


    # Broken
    /mnt/dump     -rw,sync,no_subtree_check,fsid=1 10.X.X.X/24
    # Working - but all files created are under root
    /mnt/dump      -rw,sync,no_subtree_check,anonuid=0,anongid=0,all_squash,fsid=1 10.X.X.X/24

    ps aux - Shows mergerfs is running under root

    root       573  0.6  0.5 138432 21188 ?        S<sl 08:27   5:36 mergerfs /mnt/hd0:/mnt/hd1 /mnt/dump -o rw,allow_other,minfreespace=10G,dev,suid

    dpkg -s mergerfs

    Package: mergerfs
    Status: install ok installed
    Priority: optional
    Section: utils
    Installed-Size: 193
    Maintainer: Antonio SJ Musumeci <[email protected]>
    Architecture: i386
    Version: 2.17.0~ubuntu-xenial

    NFS Client


    fileserver:/mnt/dump     /mnt/dump     nfs     defaults,_netdev 0 0

    Output of mount

    fileserver:/mnt/dump on /mnt/dump type nfs4 (rw,relatime,vers=4.0,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=10.X.X.2,local_lock=none,addr=10.X.X.1,_netdev)

    Snippet of strace

    open("", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE|O_NOFOLLOW, 0600) = -1 EACCES (Permission denied)

    Steps to repro

    Assuming that the NFS mounts is setup correctly with the /etc/exports labeled as broken

    1. cd /mnt/dump
    2. vim myfile Note error message that pops up concerning error opening the swap file

    Alternative example

    1. cd /mnt/dump Create a python file named with the following contents:
    import os
    filename = 'mydummyfile'
    flags = os.O_RDWR | os.O_CREAT | os.O_EXCL
    #flags = os.O_RDWR | os.O_CREAT
    fd =, flags, 0600)

    Execute: python Note the error:

    Traceback (most recent call last):
      File "", line 6, in <module>
        fd =, flags, 0600)
    OSError: [Errno 13] Permission denied: 'myfile'

    Note the file mode on myfile is 000 Before running again, delete mydummyfile Note that removing the os.O_EXCL flag works fine

    opened by dreamnid 54
  • Not enough space when using path  preserving policy

    Not enough space when using path preserving policy

    General description

    I have total of 10 disks with most being at or close to minfreespace limit of 50gb. I am using path preserving policy as outlined below because I want to minimize disk access which causes wake up and delay in data retrieval due to spin up.

    Expected behavior

    I was under the impression that when the drive with existing path is full even with path preserving policy new path would be created on a free disk. Is this not how it should behave?

    Actual behavior

    If I copy a file to directory x "disk2" is where it is copied despite it being at or below minfreespace limit of 50gb and disk0, disk1, disk3 and others having directory x as well as disk3 having 300gb+ free space. I am able to copy directly to disk3 so it should not be a permission issue. I also ran mergerfs.dedup and mergerfs.fsck scripts with no change.

    Precise steps to reproduce the behavior

    Copy file to an existing directory x with config below. Note I also tried epff and epall policies and some files will still get copied to disks that are over 50gb limit until they are completely full. What am I doing wrong?

    System information

    Please provide as much of the following information as possible:

    • [ ] mergerfs version: mergerfs -V mergerfs version: 2.29.0 FUSE library version: 2.9.7-mergerfs_2.29.0 fusermount version: 2.9.7-mergerfs_2.29.0 using FUSE kernel interface version 7.31
    • [ ] mergerfs settings: cat /etc/fstab or the command line arguments

    LABEL=disk0 /mnt/disk0 ext4 defaults,nofail 0 2 LABEL=disk1 /mnt/disk1 ext4 defaults,nofail 0 2 LABEL=disk2 /mnt/disk2 ext4 defaults,nofail 0 2 LABEL=disk3 /mnt/disk3 ext4 defaults,nofail 0 2 LABEL=disk4 /mnt/disk4 ext4 defaults,nofail 0 2 LABEL=disk5 /mnt/disk5 ext4 defaults,nofail 0 2 LABEL=disk6 /mnt/disk6 ext4 defaults,nofail 0 2 LABEL=disk7 /mnt/disk7 ext4 defaults,nofail 0 2 LABEL=disk8 /mnt/disk8 ext4 defaults,nofail 0 2 LABEL=disk9 /mnt/disk9 ext4 defaults,nofail 0 2

    /mnt/disk* /mnt/pool fuse.mergerfs defaults,allow_other,direct_io,use_ino,fsname=mergerfs,minfreespace=50G,category.create=eplfs,moveonenospc=true 0 0

    /mnt/disk* /mnt/pool fuse.mergerfs defaults,allow_other,direct_io,use_ino,fsname=mergerfs,minfreespace=50G,category.create=eplfs,moveonenospc=true 0 0
    * [  ] Linux version: `uname -a`
    Linux Server 5.6.13-200.fc31.x86_64 #1 SMP Thu May 14 23:26:14 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
    * [ ] Versions of any additional software being used
    * [ ] List of drives, filesystems, & sizes:
    [[email protected] ~]$ df -h

    Filesystem Size Used Avail Use% Mounted on devtmpfs 3.4G 0 3.4G 0% /dev tmpfs 3.4G 20K 3.4G 1% /dev/shm tmpfs 3.4G 1.5M 3.4G 1% /run /dev/nvme0n1p3 901G 84G 771G 10% / tmpfs 3.4G 44K 3.4G 1% /tmp /dev/nvme0n1p2 488M 220M 233M 49% /boot mergerfs 105T 97T 8.1T 93% /mnt/pool backing 105T 97T 8.1T 93% /mnt/backing /dev/nvme0n1p1 256M 8.4M 248M 4% /boot/efi /dev/sda1 962G 702G 261G 73% /mnt/downloads /dev/sdi1 7.3T 7.2T 61G 100% /mnt/disk3 /dev/sdh1 11T 11T 23G 100% /mnt/disk1 /dev/sdf1 7.3T 7.2T 54G 100% /mnt/disk9 /dev/sdk1 11T 11T 51G 100% /mnt/disk2 /dev/sdc1 15T 14T 639G 96% /mnt/disk4 /dev/sdj1 13T 13T 277G 98% /mnt/disk0 /dev/sdd1 15T 7.7T 6.8T 54% /mnt/disk6 tmpfs 694M 0 694M 0% /run/user/1000 tmpfs 694M 0 694M 0% /run/user/420 /dev/sdg1 7.3T 7.2T 48G 100% /mnt/disk8 /dev/sdb1 13T 13T 108G 100% /mnt/disk5 /dev/sde1 7.3T 7.2T 49G 100% /mnt/disk7

    opened by DominicMe 53
  • Mergerfs Segfault

    Mergerfs Segfault

    Describe the bug

    • MergerFS segfaults with error 4.
    • Jan 07 08:36:38 beast kernel: mergerfs[345]: segfault at 10 ip 000055dfac03ccb2 sp 00007fa1f77c53e0 error 4 in mergerfs[55dfabff3000+58000]
    • beast kernel: [149142.660381] mergerfs[345]: segfault at 10 ip 000055dfac03ccb2 sp 00007fa1f77c53e0 error 4 in mergerfs[55dfabff3000+58000] Jan 7 08:36:38 beast kernel: [149142.660396] Code: 80 7b 02 00 0f 85 70 fe ff ff 49 8d 6f 60 31 db 48 89 ef e8 20 7d fb ff 4c 89 ee 4c 89 ff e8 f5 d6 ff ff 48 89 ef 48 8b 40 20 <4c> 8b 68 10 e8 d5 77 fb ff e9 42 fe ff ff 48 89 df e8 c8 77 fb ff
    • umount -f /mnt/storage (mergerfs mountpoint) prompts a message that the device is busy.
    • mount -o remount /mnt/storage prompts the error
    • fuse: bad mount point /mnt/storage "Input/output error"

    The master branch is not to be considered production ready. Feel free to file bug reports but do so indicating clearly that you are testing unreleased code.

    To Reproduce

    Im not exactly sure how to reproduce the problem. It seems fairly random. I cant force it to happen. Ill log into a client system and notice the SMB mount isn't working for example. Ill log onto the server and ls -l /mnt/storage (mergerfs volume) and the permissions and other information for the mergerfs mount are peppered with ? characters.

    Expected behavior

    Mount point operates normally. The mount point is becoming unavailable.

    System information:

    • OS, kernel version: uname -a
    • Linux beast 5.11.0-44-generic #48~20.04.2-Ubuntu SMP Tue Dec 14 15:36:44 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
    • mergerfs version: mergerfs -V
    • mergerfs version: 2.33.3
    • mergerfs settings
    • /mnt/disk* /mnt/storage fuse.mergerfs defaults,nonempty,allow_other,noforget,use_ino,inodecalc=path-hash,cache.files=off,moveonenospc=true,dropcacheonclose=true,minfreespace=200G,fsname=mergerfs 0 0
    • List of drives, filesystems, & sizes:
      • df -h Filesystem Size Used Avail Use% Mounted on udev 7.8G 0 7.8G 0% /dev tmpfs 1.6G 4.7M 1.6G 1% /run /dev/sda2 49G 24G 23G 52% / tmpfs 7.9G 0 7.9G 0% /dev/shm tmpfs 5.0M 4.0K 5.0M 1% /run/lock tmpfs 7.9G 0 7.9G 0% /sys/fs/cgroup mergerfs 29T 3.3T 25T 13% /mnt/storage /dev/loop0 66M 66M 0 100% /snap/gtk-common-themes/1515 /dev/sda1 511M 5.3M 506M 2% /boot/efi /dev/sdi1 2.7T 168G 2.4T 7% /mnt/disk4 /dev/sdb1 2.7T 174G 2.4T 7% /mnt/disk3 /dev/sdd1 7.3T 1.5T 5.5T 22% /mnt/disk1 /dev/sdc1 7.3T 1.5T 5.4T 22% /mnt/disk2 /dev/sdh1 9.1T 21G 8.6T 1% /mnt/disk5 /dev/sde1 13T 1.5T 11T 13% /mnt/parity1 /dev/loop2 66M 66M 0 100% /snap/gtk-common-themes/1519 /dev/loop3 9.2M 9.2M 0 100% /snap/canonical-livepatch/119 /dev/loop1 128K 128K 0 100% /snap/bare/5 /dev/loop5 9.2M 9.2M 0 100% /snap/canonical-livepatch/126 /dev/loop4 56M 56M 0 100% /snap/core18/2253 /dev/loop6 100M 100M 0 100% /snap/core/11798 /dev/loop7 56M 56M 0 100% /snap/core18/2246 /dev/loop8 219M 219M 0 100% /snap/gnome-3-34-1804/72 /dev/loop9 248M 248M 0 100% /snap/gnome-3-38-2004/87 /dev/loop10 43M 43M 0 100% /snap/snapd/14066 /dev/loop11 44M 44M 0 100% /snap/snapd/14295 /dev/loop12 55M 55M 0 100% /snap/snap-store/558 /dev/loop13 62M 62M 0 100% /snap/core20/1270 /dev/loop14 100M 100M 0 100% /snap/core/11993 /dev/loop15 51M 51M 0 100% /snap/snap-store/547 /dev/loop16 219M 219M 0 100% /snap/gnome-3-34-1804/77 /dev/loop17 62M 62M 0 100% /snap/core20/1242
      • lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT loop0 7:0 0 65.1M 1 loop /snap/gtk-common-themes/1515 loop1 7:1 0 4K 1 loop /snap/bare/5 loop2 7:2 0 65.2M 1 loop /snap/gtk-common-themes/1519 loop3 7:3 0 9M 1 loop /snap/canonical-livepatch/119 loop4 7:4 0 55.5M 1 loop /snap/core18/2253 loop5 7:5 0 9M 1 loop /snap/canonical-livepatch/126 loop6 7:6 0 99.5M 1 loop /snap/core/11798 loop7 7:7 0 55.5M 1 loop /snap/core18/2246 loop8 7:8 0 219M 1 loop /snap/gnome-3-34-1804/72 loop9 7:9 0 247.9M 1 loop /snap/gnome-3-38-2004/87 loop10 7:10 0 42.2M 1 loop /snap/snapd/14066 loop11 7:11 0 43.3M 1 loop /snap/snapd/14295 loop12 7:12 0 54.2M 1 loop /snap/snap-store/558 loop13 7:13 0 61.9M 1 loop /snap/core20/1270 loop14 7:14 0 99.4M 1 loop /snap/core/11993 loop15 7:15 0 51M 1 loop /snap/snap-store/547 loop16 7:16 0 219M 1 loop /snap/gnome-3-34-1804/77 loop17 7:17 0 61.9M 1 loop /snap/core20/1242 sda 8:0 0 50G 0 disk ├─sda1 8:1 0 512M 0 part /boot/efi └─sda2 8:2 0 49.5G 0 part / sdb 8:16 0 2.7T 0 disk └─sdb1 8:17 0 2.7T 0 part /mnt/disk3 sdc 8:32 0 7.3T 0 disk └─sdc1 8:33 0 7.3T 0 part /mnt/disk2 sdd 8:48 0 7.3T 0 disk └─sdd1 8:49 0 7.3T 0 part /mnt/disk1 sde 8:64 0 12.8T 0 disk └─sde1 8:65 0 12.8T 0 part /mnt/parity1 sdf 8:80 0 7.3T 0 disk ├─sdf1 8:81 0 7.3T 0 part └─sdf9 8:89 0 8M 0 part sdg 8:96 0 7.3T 0 disk ├─sdg1 8:97 0 7.3T 0 part └─sdg9 8:105 0 8M 0 part sdh 8:112 0 9.1T 0 disk └─sdh1 8:113 0 9.1T 0 part /mnt/disk5 sdi 8:128 0 2.7T 0 disk └─sdi1 8:129 0 2.7T 0 part /mnt/disk4 sr0 11:0 1 1024M 0 rom
    • A strace of the application having a problem:
      • strace -fvTtt -s 256 -o /tmp/app.strace.txt <cmd>
      • strace -fvTtt -s 256 -o /tmp/app.strace.txt -p <appPID>
    • strace of mergerfs while app tried to do it's thing:
      • strace -fvTtt -s 256 -p <mergerfsPID> -o /tmp/mergerfs.strace.txt

    Additional context

    Add any other context about the problem here. I rebooted the machine after the issue arose a second time in ~2 weeks this morning. If it happens again, I will work to get the strace data for you. Apologies for not being able to file all of the requested data.

    bug investigating 
    opened by Go0oSer 47
  • nfs mount point mtime changed nfs client got nfs file stale error

    nfs mount point mtime changed nfs client got nfs file stale error

    General description

    mergerfs /test1/assets:/test2/assets /test/assets -o rw,umask=0000,allow_other,use_ino,noforget,category.action=eplus,category.create=eplus,,func.utimens=ff,dev,suid

    when mergerfs start,path /test1/assets mtime is newer than /test2/assets if i touch a file through /test/assets,then /test2/assets mtime is newer than /test1/assets

    nfs client will got nfs file stale error.

    System information

    Please provide as much of the following information as possible:

    • [x] mergerfs version: mergerfs -V
    mergerfs -V
    mergerfs version: 2.28.1-6-g52e4d60
    FUSE library version: 2.9.7-mergerfs_2.28.0
    fusermount version: 2.9.2
    using FUSE kernel interface version 7.29
    • [x] Linux version: uname -a
    uname -a
    Linux store001 5.4.3-1.el7.elrepo.x86_64 #1 SMP Fri Dec 13 07:27:20 EST 2019 x86_64 x86_64 x86_64 GNU/Linux
    opened by swimfish09 47
  • Slow Speeds since last update 2.26+ with rclone mount

    Slow Speeds since last update 2.26+ with rclone mount

    General description

    I've been using rclone along with mergerfs for some time now. With the last update, my performance seems to have tanked.

    Expected behavior

    I'd expect the mergerfs mount to perform the same as the rclone mount:


    Actual behavior

    It's dropping to 1-2MB a second.

    Precise steps to reproduce the behavior

    I am testing via just using rsync progress to copy a file over.

    System information

    Please provide as much of the following information as possible: ArchLinux - This package:

    [[email protected] etc]# mergerfs -V mergerfs version: 4147b9e FUSE library version: 2.9.7-mergerfs_2.27.0 fusermount version: 2.9.9 using FUSE kernel interface version 7.27

    mount command:

    root       977     1  0 May23 ?        00:39:56 mergerfs /data/local:/GD /gmedia -o rw,sync_read,auto_cache,use_ino,allow_other,func.getattr=newest,category.action=all,category.create=ff,dev,suid
    opened by animosity22 47
  • Slow speed on write/read to/from MergerFS

    Slow speed on write/read to/from MergerFS

    General description

    Mounted MergerFS but speed is slow (Compared with other server).

    Expected behavior

    I was expecting some better speeds on this type of server.

    Actual behavior

    The speed is like strange for me.

    Precise steps to reproduce the behavior

    So here is some information. I'm owning 2 servers.

    1. Ryzen 5 3600 / 64GB RAM / 4x1 TB NVMe SSD.
    2. E-2288G / 32GB RAM / 2x960 GB NVMe SSD.

    When I'm launching the tests I'm getting a big difference.

    Let's take a look at the first server.

    (1:169)# dd if=/dev/zero of=/home/user/rclone/gmedia/benchmark ibs=1M obs=1M count=1024 iflag=dsync,nocache oflag=dsync,nocache conv=fdatasync status=progress
    1042284544 bytes (1.0 GB, 994 MiB) copied, 17 s, 61.3 MB/s
    1024+0 records in
    1024+0 records out
    1073741824 bytes (1.1 GB, 1.0 GiB) copied, 17.5234 s, 61.3 MB/s
    dd if=/home/user/rclone/gmedia/Media/benchmark of=/dev/null
    917504+0 records in
    917504+0 records out
    469762048 bytes (470 MB, 448 MiB) copied, 4.42408 s, 106 MB/s

    And now the second one

    (1:504)# dd if=/dev/zero of=/home/user/rclone/gmedia/benchmark ibs=1M obs=1M count=1024 iflag=dsync,nocache oflag=dsync,nocache conv=fdatasync status=progress
    1038090240 bytes (1.0 GB, 990 MiB) copied, 2 s, 519 MB/s
    1024+0 records in
    1024+0 records out
    1073741824 bytes (1.1 GB, 1.0 GiB) copied, 2.08218 s, 516 MB/s
    dd if=/home/user/rclone/gmedia/Media/benchmark of=/dev/null
    917504+0 records in
    917504+0 records out
    469762048 bytes (470 MB, 448 MiB) copied, 2.28686 s, 205 MB/s

    So as you can see there is a big writing speed difference. I'm not telling that they should be the same, but why this might happen? Is there any way to improve the speed?

    System information

    Please provide as much of the following information as possible: For both servers i use same configurations.

    mergerfs version: 2.28.3
    FUSE library version: 2.9.7-mergerfs_2.29.0
    fusermount version: 2.9.9
    using FUSE kernel interface version 7.29

    I'm running it through services as mount. (I'm using MergerFS for Plex and gdrive) So options witch I'm using is this:


    Or you need the full command?

    Linux amd 4.19.96-xanmod46 #1.200114 SMP PREEMPT Tue Jan 14 21:54:52 -03 2020 x86_64 GNU/Linux

    Raid 0. 70GB on "/" all ther in "/home" location.

    opened by Prosecutor 45
  • mergerfs 2.30 on stretch armhf - Unhandled fault: alignment fault (0x92000021) at 0x00000000f4801d3b

    mergerfs 2.30 on stretch armhf - Unhandled fault: alignment fault (0x92000021) at 0x00000000f4801d3b

    Describe the bug

    I try to install the latest mergerfs on a vero4k. For now the only available version in the repos is the 2.18.0. The vero4k is on stretch on an armhf architecture.

    When I install And try to mount my media like I do on the 2.18.0, a simple "ls" break.

    To Reproduce Steps to reproduce the behavior. List all steps to reproduce. All settings.

    • install mergerfs (2.18) from debian/stretch on armhf
    • install the package from
    • mount my media:
    sudo mergerfs -o defaults,allow_other,category.create=mfs,use_ino -d /media/\*/media /mnt/media
    • ls /mnt/media

    Expected behavior The mergerfs should not crash at all. The package is for jessie armhf, so I except this distribution is supported.

    System information:

    • OS, kernel version: uname -a Linux vero4kp 3.14.29-158-osmc #1 SMP Thu Apr 2 18:40:24 UTC 2020 aarch64 GNU/Linux

    • mergerfs version: mergerfs -V

    normal version: mergerfs version: 2.18.0 FUSE library version: 2.9.7 fusermount version: 2.9.7 using FUSE kernel interface version 7.19

    updated version: mergerfs version: 2.30.0 FUSE library version: 2.9.7-mergerfs_2.30.0 fusermount version: 2.9.7-mergerfs_2.30.0 using FUSE kernel interface version 7.31

    • mergerfs settings defaults,allow_other,category.create=mfs,use_ino

    • List of drives, filesystems, & sizes: df -h

    Filesystem           Size  Used Avail Use% Mounted on
    devtmpfs             757M     0  757M   0% /dev
    tmpfs                879M   28M  851M   4% /run
    /dev/vero-nand/root   14G  5.1G  8.1G  39% /
    tmpfs                879M  4.0K  879M   1% /dev/shm
    tmpfs                5.0M     0  5.0M   0% /run/lock
    tmpfs                879M     0  879M   0% /sys/fs/cgroup
    tmpfs                176M     0  176M   0% /run/user/1000
    /dev/sdb1            916G  133G  738G  16% /media/ext2
    /dev/sdc1            932G  801G  131G  86% /media/WD1042-1TB-a
    /dev/sdd1            932G  828G  104G  89% /media/TB2017-2TB-a
    /dev/sdd2            932G  692G  240G  75% /media/TB2017-2TB-b
    /dev/sda1            932G  5.0G  927G   1% /media/WD10JM-1TB-a
    backup               3.7T  2.3T  1.4T  63% /mnt/backup
    media                4.6T  2.5T  2.1T  54% /mnt/media
    • A strace of the application having a problem: strace -f -o /tmp/app.strace.txt <cmd> or strace -f -p <appPID> -o /tmp/app.strace.txt none. I simplify do a "ls"

    • strace of mergerfs while app tried to do it's thing: strace -f -p <mergerfsPID> -o /tmp/mergerfs.strace.txt strace.txt

    Additional context

    bug investigating 
    opened by vincent-ogury 44
  • Move Queue

    Move Queue

    Hi, Im currently trying to write a script which moves files to the correct path policy folder over the night. I have ignorepponrename set to true, so that i can move files over samba.

    But i would still like to move every file to the correct harddrive via a cronjob.

    Is it somehow possible to create a simple log entry for every file that was not moved to the correct path because of the ignorepponrename opition? I thought about layering another fuse fs on top of mergerfs. To just log this one operation, but this seems kinda overkill for this task. Anyother options that you could think of?

    opened by dgalli1 40
  • Mergerfs and Sonarr and Hardlink

    Mergerfs and Sonarr and Hardlink

    General description

    Hi, I have 2 hdds


    rtorrent download folder for movies is: I already created this folder myself so it should downkload here /mnt/storage/downloads/incomplete/ located in /mnt/disk1/downloads/incomplete/ /mnt/storage/downloads/completed/ located in /mnt/disk1/downloads/completed/
    Using Sonarr docker to import or even add new movie to download I have tried many different combination and policies, I have been using EPFF for majority of times, and tried FF last night; same result.

    Expected behavior

    Hardlink movie after download is finished

    /mnt/storage/downloads/completed => /mnt/storage/tv located in /mnt/disk1/tv

    Actual behavior

    It actually copies, so I have 2 x of the same file, verified it with using : ln -i 2 Different inode

    Precise steps to reproduce the behavior

    Explicitly list all steps to reproduce. Preferably create a minimal example of the problem using standard command-line tools. The more variables (apps, settings, etc.) that are involved the more difficult it is to debug. Also, please be sure to have read all of the README. It contains a lot of information regarding known system and user issues.

    System information

    Please provide as much of the following information as possible:

    • [ ] mergerfs version: 2.29.0
    • [ ] mergerfs settings: /mnt/disk1:/mnt/disk2 /mnt/storage fuse.mergerfs use_ino,cache.files=partial,dropcacheonclose=true,allow_other,category.create=epff 0 0
    • [ ] Linux version: Linux len 4.19.0-9-amd64 #1 SMP Debian 4.19.118-2+deb10u1 (2020-06-07) x86_64 GNU/Linux
    • [ ] Versions of any additional software being used
    • [ ] List of drives, filesystems, & sizes:
    Filesystem      Size  Used Avail Use% Mounted on
    udev            2.8G     0  2.8G   0% /dev
    tmpfs           563M  9.0M  554M   2% /run
    /dev/sda1        68G   11G   54G  17% /
    tmpfs           2.8G     0  2.8G   0% /dev/shm
    tmpfs           5.0M  4.0K  5.0M   1% /run/lock
    tmpfs           2.8G     0  2.8G   0% /sys/fs/cgroup
    1:2             9.1T  6.4T  2.8T  70% /mnt/storage
    /dev/sdc1       7.3T  6.1T  1.2T  84% /mnt/disk1
    /dev/sdb1       1.8T  288G  1.6T  16% /mnt/disk2
    tmpfs           563M  4.0K  563M   1% /run/user/115
    tmpfs           563M     0  563M   0% /run/user/1000
    • [ ] strace of application having problem: strace -f -o /tmp/app.strace.txt <cmd> or strace -f -p <appPID> -o /tmp/app.strace.txt
    • [ ] strace of mergerfs while app tried to do it's thing: strace -f -p <mergerfsPID> -o /tmp/mergerfs.strace.txt

    EDITS= Formatting

    opened by ghost 40
  • No space left on device

    No space left on device

    Having some trouble with mergerfs. A bit of context:

    Linux, Fedora 26 I have 14 8TB data drives, all mounted at /export/Archive_01 through /export/Archive_14

    2 mergerfs mounts: /storage Mounted with /export/Archive_* to aggregate all of the drives. create policy mfs.

    /fill Mounted with all the drives minus Archive_14, which is my currently filling drive where all new files are downloaded to. (I only have 01-13 as snapraid data drives to keep the parity from changing all the time. Files only ever get removed from 01-13 with this setup, so if I lose one of these drives, I won't lose anything, only restore something that was deleted since the last sync).

    Attached is the current df --o output and tune2fs outputs for all the drives. I have plenty of inodes, and zero reserved space. ~250GB free per drive, shouldn't be an issue. Additionally, I can create directories and create files on all the drives individually, so this is an issue with mergerfs.

    I use an rsync command to move files from /export/Archive_14 to /fill: sudo ionice -c3 rsync -Ahal --remove-source-files --progress --log-file=/home/server/rsync_media_fill_date +%Y%m%d%H%M%S.txt --preallocate --inplace --exclude='**.part'"

    Attached is /etc/fstab, df output, and tune2fs outputs for all the drives.

    When running the rsync command, I get the following output: rsync: open "/fill/TV/file.mp4" failed: No space left on device (28)




    opened by qweasdzxc787 40
  • Broken symbolic links to small files on XFS with symlinkify=true

    Broken symbolic links to small files on XFS with symlinkify=true

    Got broken symbolic links to files smaller than 4096b. Like "test2 -> /HDD2/TEST/a" instead of "test2 -> /HDD2/TEST/abcde/qwerty/zxcvb/test2".

    To Reproduce

    On mounted xfs file systems (/HDD1, /HDD2) create folder TEST:

    mkdir /HDD1/TEST
    mkdir /HDD2/TEST

    merge them in folder /TEST:

    mergerfs -o allow_other,use_ino,cache.files=partial,symlinkify=true,cache.symlinks=true,cache.statfs=true,cache.readdir=true,,dropcacheonclose=true,category.create=mfs,minfreespace=50G /HDD1/TEST:/HDD2/TEST /TEST

    Create test directory and files:

    mkdir -p /TEST/abcde/qwerty/zxcvb
    echo "sdadfadf" > /TEST/abcde/qwerty/zxcvb/test1
    echo "sdfgsdfgsdf" > /TEST/abcde/qwerty/zxcvb/test2
    echo "yeasdf" > /TEST/abcde/qwerty/zxcvb/test3
    echo "zbxzcsd" > /TEST/abcde/qwerty/zxcvb/test4
    chmod a-w /TEST/abcde/qwerty/zxcvb/*

    Results after symlinkify:

    ls -lt /TEST/abcde/qwerty/zxcvb/    
    total 16
    lr--r--r-- 1 root root  8 окт 26 09:26 test4 -> /HDD2/TE
    lr--r--r-- 1 root root  7 окт 26 09:26 test3 -> /HDD2/T
    lr--r--r-- 1 root root 12 окт 26 09:26 test2 -> /HDD2/TEST/a
    lr--r--r-- 1 root root  9 окт 26 09:25 test1 -> /HDD2/TES

    System information:

    • OS, kernel version: 5.19.13-gentoo-dist #1 SMP PREEMPT_DYNAMIC Wed Oct 5 16:16:56 +07 2022 x86_64 Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz GenuineIntel GNU/Linux
    • mergerfs version: mergerfs version: 2.33.4
    • mergerfs settings: allow_other,use_ino,cache.files=partial,symlinkify=true,cache.symlinks=true,cache.statfs=true,cache.readdir=true,,dropcacheonclose=true,category.create=mfs,minfreespace=50G
    • List of drives, filesystems, & sizes: /dev/sdc1 xfs 7,3T(2,4T used) /HDD2 (defaults,noatime) /dev/sdg1 xfs 7,3T(2,4T used) /HDD1 (defaults,noatime)
    bug investigating 
    opened by altwazar 4
  • Prevent no branches

    Prevent no branches

    as discussed in #1073, this seems to fix it.

    returns "not supported" ENOTSUP when a change would result in no branches. incidentally, the code for branches from_string was already raising/erroring out in case of setting completely empty branches, that's fixed. I've added unit test cases to test all that.

    On my system, the -fsanitize=undefined option in the compiler added some problems that seem to be fixed by also adding the -fsanitize=undefined flag on linking, not sure why, that's why the change is in there,

    opened by rienafairefr 9
  • feature request: create policy msp smallest total space

    feature request: create policy msp smallest total space

    Is your feature request related to a problem? Please describe. Using SSD as tiered storage for a bunch of drives. ff or epff will not switch to next branch if first found is full. so first tier fills up and errors out

    Describe the solution you'd like msp with smallest total space would allow for SSD (most often the smallest drive) to fill with some sort of path preservation and move to next branch if first branch is filled up to the limit set. or is there a way to use mspff with moving down the list if the first branch is filled to the limit.

    maybe i am not understanding the limits of mergerfs completely - so i would appreciate being educated :)

    feature investigating 
    opened by sluggathor 22
  • Crash when using compsize on btrfs drives

    Crash when using compsize on btrfs drives

    Describe the bug

    MergerFS crashes when calling compsize on any directory in the pool (compsize /mnt/raid/). It does not crash when called on a file (compsize /mnt/raid/file), but compsize still fails and prints All empty or still-delalloced files.

    Interestingly official btrfs-progs simply say ERROR: not a btrfs filesystem: /mnt/raid and refuse to do anything with the MergerFS pool... but it would be fantastic if compsize could actually work (i.e. MergerFS would forward the appropriate calls or something?)

    To Reproduce

    The issue is present even with just a single drive in the "pool". The drive has a btrfs FS, made with mkfs.btrfs default values and is mounted with compress-force=zstd (see fstab entries below).

    1. Install btrfs-compsize
    2. Mount MergerFS pool with a btrfs drive
    3. Run compsize on any directory in the pool

    Expected behavior

    compsize should output the disk usage and compression ratio for the directory.

    System information:

    • OS, kernel version: Linux - 5.15.0-46-generic #49-Ubuntu SMP Thu Aug 4 18:03:25 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
    • mergerfs version: 2.33.5
    • List of drives, filesystems, & sizes:
    sdd      2.7T btrfs    2.7T    32% 2022-08#2 D4 /mnt/.data/4
    • mergerfs settings (fstab entries)
    /mnt/.data/4 /mnt/raid fuse.mergerfs allow_other,use_ino,fsname=SnapRAID-MergerFS,moveonenospc=mspmfs,category.create=mspmfs,minfreespace=20G 0 0
    /dev/disk/by-uuid/<uuid> /mnt/.data/4 auto nosuid,nodev,nofail,compress-force=zstd 0 0

    Attached files: Run on file (no crash, but doesn't work) compsize /mnt/raid/backup/2022-08-29_root_ssd file-mergerfs.strace.txt file-compsize.strace.txt

    Run on directory (crash) compsize /mnt/raid/backup/ dir-compsize.strace.txt dir-mergerfs.strace.txt

    • A strace of the application having a problem:
      • strace -fvTtt -s 256 -o /tmp/app.strace.txt <cmd>
      • strace -fvTtt -s 256 -o /tmp/app.strace.txt -p <appPID>
    • strace of mergerfs while app tried to do it's thing:
      • strace -fvTtt -s 256 -p <mergerfsPID> -o /tmp/mergerfs.strace.txt
    bug investigating 
    opened by Henri-J-Norden 3
  • Bind mounts from mergerfs filesystems require

    Bind mounts from mergerfs filesystems require

    Describe the bug

    When a bind mount is made from a directory, access control to the mount location is controlled by the permissions/ACLs set on the source directory/inode, and does not depend on access to parents of the source directory. However, mergerfs seems to require access to ancestors for unprivileged users to see/access the bind mount.

    To Reproduce

    Starting using root user, on Arch Linux system x86_64 with current upgrades.


    $ (dd if=/dev/zero of=testbed.img bs=16M count=1; mkfs.ext4 testbed.img) >/dev/null 2>&1
    $ mkdir testbed && mount testbed.img testbed
    $ cd testbed
    $ (dd if=/dev/zero of=disk.ext4 bs=4M count=1; mkfs.ext4 disk.ext4) >/dev/null 2>&1
    $ mkdir -p mnt/{ext4,mergerfs,target} && mount disk.ext4 mnt/ext4
    $ mkdir -p mnt/ext4/root-only/src
    $ chmod 700 mnt/ext4/root-only
    $ chmod 777 mnt/ext4/root-only/src
    $ mount -t mergerfs -o allow_other,use_ino,cache.files=partial,dropcacheonclose=true,category.create=mfs mnt/ext4 mnt/mergerfs
    State check with root user after prep

    $ ls -liR mnt
    total 3
                      2 drwxr-xr-x 4 root root 1024 Jun 18 17:07 ext4
    5424983562661234939 drwxr-xr-x 4 root root 1024 Jun 18 17:07 mergerfs
                   2052 drwxr-xr-x 2 root root 1024 Jun 18 17:07 target
    total 13
    11 drwx------ 2 root root 12288 Jun 18 17:07 lost+found
    12 drwx------ 3 root root  1024 Jun 18 17:07 root-only
    total 0
    total 1
    13 drwxrwxrwx 2 root root 1024 Jun 18 17:07 src
    total 0
    total 13
    10308613573065218086 drwx------ 2 root root 12288 Jun 18 17:07 lost+found
     9502623902895214744 drwx------ 3 root root  1024 Jun 18 17:07 root-only
    total 0
    total 1
    9471441698076499429 drwxrwxrwx 2 root root 1024 Jun 18 17:07 src
    total 0
    total 0

    State check with unprivileged user after prep

    $ sudo -u nobody ls -liR mnt
    total 3
                      2 drwxr-xr-x 4 root root 1024 Jun 18 17:07 ext4
    5424983562661234939 drwxr-xr-x 4 root root 1024 Jun 18 17:07 mergerfs
                   2052 drwxr-xr-x 2 root root 1024 Jun 18 17:07 target
    total 13
    11 drwx------ 2 root root 12288 Jun 18 17:07 lost+found
    12 drwx------ 3 root root  1024 Jun 18 17:07 root-only
    ls: cannot open directory 'mnt/ext4/lost+found': Permission denied
    ls: cannot open directory 'mnt/ext4/root-only': Permission denied
    total 13
    10308613573065218086 drwx------ 2 root root 12288 Jun 18 17:07 lost+found
     9502623902895214744 drwx------ 3 root root  1024 Jun 18 17:07 root-only
    ls: cannot open directory 'mnt/mergerfs/lost+found': Permission denied
    ls: cannot open directory 'mnt/mergerfs/root-only': Permission denied
    total 0

    Bind mount from ext4

    $ mount --bind mnt/ext4/root-only/src mnt/target
    State check with root user

    $ ls -li mnt mnt/target
    total 3
                      2 drwxr-xr-x 4 root root 1024 Jun 18 17:07 ext4
    5424983562661234939 drwxr-xr-x 4 root root 1024 Jun 18 17:07 mergerfs
                     13 drwxrwxrwx 2 root root 1024 Jun 18 17:07 target
    total 0

    State check with unprivileged user

    $ sudo -u nobody ls -li mnt mnt/target
    total 3
                      2 drwxr-xr-x 4 root root 1024 Jun 18 17:07 ext4
    5424983562661234939 drwxr-xr-x 4 root root 1024 Jun 18 17:07 mergerfs
                     13 drwxrwxrwx 2 root root 1024 Jun 18 17:07 target
    total 0

    Bind mount from mergerfs

    $ umount mnt/target
    $ mount --bind mnt/mergerfs/root-only/src mnt/target
    State check with root user

    $ ls -li mnt mnt/target
    total 3
                      2 drwxr-xr-x 4 root root 1024 Jun 18 17:07 ext4
    5424983562661234939 drwxr-xr-x 4 root root 1024 Jun 18 17:07 mergerfs
    9471441698076499429 drwxrwxrwx 2 root root 1024 Jun 18 17:07 target
    total 0

    State check with unprivileged user

    $ sudo -u nobody ls -li mnt mnt/target
    ls: mnt/target: No such file or directory
    ls: mnt/target: No such file or directory
    total 3
                      2 drwxr-xr-x 4 root root 1024 Jun 18 17:07 ext4
    5424983562661234939 drwxr-xr-x 4 root root 1024 Jun 18 17:07 mergerfs
    9471441698076499429 drwxrwxrwx 2 root root 1024 Jun 18 17:07 target
    ls: cannot open directory 'mnt/target': No such file or directory

    Granting access to unprivileged user

    $ chown nobody:nobody mnt/ext4/root-only
    State check with root user

    $ ls -li mnt mnt/target
    total 3
                      2 drwxr-xr-x 4 root root 1024 Jun 18 17:07 ext4
    5424983562661234939 drwxr-xr-x 4 root root 1024 Jun 18 17:07 mergerfs
    9471441698076499429 drwxrwxrwx 2 root root 1024 Jun 18 17:07 target
    total 0

    State check with unprivileged user

    $ sudo -u nobody ls -li mnt mnt/target
    total 3
                      2 drwxr-xr-x 4 root root 1024 Jun 18 17:07 ext4
    5424983562661234939 drwxr-xr-x 4 root root 1024 Jun 18 17:07 mergerfs
    9471441698076499429 drwxrwxrwx 2 root root 1024 Jun 18 17:07 target
    total 0

    Expected behavior

    Unprivileged users should have access to the bind mount, just like they do when the bind mount is done from a non-mergerfs filesystem, regardless of the ACLs on ancestors of the source directory.

    System information:

    $ uname -a
    Linux HPT-Server 5.18.3-arch1-1 #1 SMP PREEMPT_DYNAMIC Thu, 09 Jun 2022 16:14:10 +0000 x86_64 GNU/Linux
    $ mergerfs --version
    mergerfs version: 2.33.5

    See above for mergerfs settings. Test was done entirely using disk images; no physical disks.

    ls.strace.txt - run using strace -u nobody ... ls -li mnt mnt/target.


    Note that the straces are from a different run, paused at the Bind mount from mergerfs. Unclear why the directory shows as ????????? this time, but that tends to happen as well.

    bug investigating 
    opened by henryptung 1
  • Branch mounted in specific pool sub directory, with priority

    Branch mounted in specific pool sub directory, with priority

    Describe the solution you'd like A clear and concise description of what you want to happen. I have a huge data mergerfs file system, which is built upon four raids as branches. They are all rotating discs. I would like to use SSD storage for incoming data, in my case for a specific directory, but it could also be for specific directories (plural).

    Imagine raids being mounted in /mnt as /mnt/md[0-3], and then the pool mounted in /data. I would then like to have /mnt/ssd0 used for /data/incoming.

    I imagine this could be implemented by adding a new branch type, though I also assume that it might not be super easy, and might even be out of scope for the project. I am thinking a new branch type with syntax like this could be added: /mnt/ssd0=PO,/data/incoming,/data/incoming2 PO = Policy override, but there could probably be a much better name for it. I was first thinking PRIO (priority), which I find worse.

    Then every time mergerfs wants to create new data in those directories it should use ssd0 instead of following the configured policy.

    The next issue is that if/when the data is moved from the SSD, it shouldn't keep the data on the SSD, regardless of the policy set.

    Describe alternatives you've considered The best alternative I have found is to use mount --bind /mnt/ssd0 /data/incoming, though that has other downsides. Like being able to run out of space while the pool still has plenty of space. Or "hiding" underlying data in case the mount was missing for a while.

    I have also considered bcache, but that adds a whole lot of complexity that I don't want. Also I can't keep the SSD "cache" to just a single directory.

    Additional context If you feel that this is way too cumbersome or too much out of scope. Just close the request with a deny without further explanation. I am fine with that since I understand that this probably isn't a hack done in a day. I am also afraid it could have consequences I haven't been able to figure out myself.

    feature investigating 
    opened by magma1447 5
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