% mergerfs(1) mergerfs user manual
% Antonio SJ Musumeci trapexit@spawn.link
% 2023-02-19

NAME

mergerfs - a featureful union filesystem

SYNOPSIS

mergerfs -o<options> <branches> <mountpoint>

DESCRIPTION

mergerfs is a union filesystem geared towards simplifying storage
and management of files across numerous commodity storage devices. It
is similar to mhddfs, unionfs, and aufs.

FEATURES

• 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

HOW IT WORKS

mergerfs logically merges multiple paths together. Think a union of
sets. The file/s or directory/s acted on or presented through mergerfs
are based on the policy chosen for that particular action. Read more
about policies below.

A         +      B        =       C
/disk1           /disk2           /merged
|                |                |
+-- /dir1        +-- /dir1        +-- /dir1
|   |            |   |            |   |
|   +-- file1    |   +-- file2    |   +-- file1
|                |   +-- file3    |   +-- file2
+-- /dir2        |                |   +-- file3
|   |            +-- /dir3        |
|   +-- file4        |            +-- /dir2
|                     +-- file5   |   |
+-- file6                         |   +-- file4
                                  |
                                  +-- /dir3
                                  |   |
                                  |   +-- file5
                                  |
                                  +-- file6

mergerfs does NOT support the copy-on-write (CoW) or whiteout
behaviors found in aufs and overlayfs. You can not mount a
read-only filesystem and write to it. However, mergerfs will ignore
read-only 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.

TERMINOLOGY

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

BASIC SETUP

If you don't already know that you have a special use case then just
start with one of the following option sets.

You need mmap (used by rtorrent and many sqlite3 base software)

cache.files=partial,dropcacheonclose=true,category.create=mfs

You don't need mmap

cache.files=off,dropcacheonclose=true,category.create=mfs

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

OPTIONS

These options are the same regardless of whether you use them with the mergerfs commandline program, 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.
• 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)
• 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|per-process 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)
• branches-mount-timeout=UINT: Number of seconds to wait at
  startup for branches to be a mount other than the mountpoint's
  filesystem. (default: 0)
• 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. When used alone
  (process-thread-count=-1) it sets the number of threads reading
  and processing FUSE messages. When used together it sets the number
  of threads reading from FUSE. When set to zero it will attempt to
  discover and use the number of logical cores. 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. If set to
  -1 in combination with process-thread-count then it will try to
  pick reasonable values based on CPU thread count. NOTE: higher
  number of threads increases parallelism but usually decreases
  throughput. (default: 0)
• read-thread-count=INT: Alias for threads.
• process-thread-count=INT: Enables separate thread pool to
  asynchronously process FUSE requests. In this mode
  read-thread-count refers to the number of threads reading FUSE
  messages which are dispatched to process threads. -1 means disabled
  otherwise acts like read-thread-count. (default: -1)
• pin-threads=STR: Selects a strategy to pin threads to CPUs
  (default: unset)
• scheduling-priority=INT: Set mergerfs' scheduling
  priority. Valid values range from -20 to 19. See setpriority man
  page for more details. (default: -10)
• 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)
• category.search=POLICY: Sets policy of all FUSE functions in the
  search category. (default: ff)
• cache.open=UINT: 'open' policy cache timeout in
  seconds. (default: 0)
• cache.statfs=UINT: 'statfs' cache timeout in seconds. (default:
  0)
• cache.attr=UINT: File attribute cache timeout in
  seconds. (default: 1)
• 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|per-process: File
  page caching mode (default: libfuse)
• cache.files.process-names=LIST: A pipe | delimited list of
  process comm
  names to enable page caching for when
  cache.files=per-process. (default: "rtorrent|qbittorrent-nox")
• 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'
• INT = [MIN_INT,MAX_INT]
• UINT = [0,MAX_INT]
• SIZE = 'NNM'; NN = INT, M = 'K' | 'M' | 'G' | 'T'
• STR = string (may refer to an enumerated value, see details of
  argument)
• FUNC = filesystem function
• CATEGORY = function category
• POLICY = mergerfs function policy

branches

The 'branches' argument is a colon (':') delimited list of paths to be pooled together. It does not matter if the paths are on the same or different 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.

minfreespace

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

globbing

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

# mergerfs /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.

# <file system>        <mount point>  <type>         <options>             <dump>  <pass>
/mnt/disk*:/mnt/cdrom  /mnt/pool      fuse.mergerfs  minfreespace=16G      0       0

NOTE: the globbing is done at mount or when updated using the runtime API. If a new directory is added matching the glob after the fact it will not be automatically included.

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

inodecalc

Inodes (st_ino) are unique identifiers within a filesystem. Each mounted filesystem has device ID (st_dev) as well and together they can uniquely identify a file on the whole of the system. Entries on the same device with the same inode are in fact references to the same underlying file. It is a many to one relationship between names and an inode. Directories, however, do not have multiple links on most systems due to the complexity they add.

FUSE allows the server (mergerfs) to set inode values but not device IDs. Creating an inode value is somewhat complex in mergerfs' case as files aren't really in its control. If a policy changes what directory or file is to be selected or something changes out of band it becomes unclear what value should be used. Most software does not to care what the values are but those that do often break if a value changes unexpectedly. The tool find will abort a directory walk if it sees a directory inode change. NFS 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").

As of version 2.35.0 the use_ino option has been removed. mergerfs should always be managing inode values.

pin-threads

Simple strategies for pinning read and/or process threads. If process
threads are not enabled than the strategy simply works on the read
threads. Invalid values are ignored.

• R1L: All read threads pinned to a single logical CPU.
• R1P: All read threads pinned to a single physical CPU.
• RP1L: All read and process threads pinned to a single logical CPU.
• RP1P: All read and process threads pinned to a single physical CPU.
• R1LP1L: All read threads pinned to a single logical CPU, all process
  threads pinned to a (if possible) different logical CPU.
• R1PP1P: All read threads pinned to a single physical CPU, all
  process threads pinned to a (if possible) different logical CPU.
• RPSL: All read and process threads are spread across all logical CPUs.
• RPSP: All read and process threads are spread across all physical CPUs.
• R1PPSP: All read threads are pinned to a single physical CPU while
  process threads are spread across all other phsycial CPUs.

fuse_msg_size

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.

follow-symlinks

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.

link-exdev

If using path preservation and a link fails with EXDEV make a call to symlink where the target is the oldlink and the linkpath is the newpath. The target value is determined by the value of link-exdev.

• 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.

rename-exdev

If using path preservation and a rename fails with EXDEV:

1. Move file from /branch/a/b/c to /branch/.mergerfs_rename_exdev/a/b/c.
2. symlink the rename's newpath to the moved file.

The target value is determined by the value of rename-exdev.

• 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.

symlinkify

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.

nullrw

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.

xattr

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.

nfsopenhack

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

FUNCTIONS, CATEGORIES and POLICIES

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.

Policies

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.

Filtering

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 eligible 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: For mkdir, mknod, and symlink it will apply to all branches. create works like ff.
epall (existing path, all)	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.
ff (first found)	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)	Pick the drive with the least available free space.
lus (least used space)	Pick the drive with the least used space.
mfs (most free space)	Pick the drive with the most available free space.
msplfs (most shared path, least free space)	Like eplfs but if it fails to find a branch it will try again with the parent directory. Continues this pattern till finding one.
msplus (most shared path, least used space)	Like eplus but if it fails to find a branch it will try again with the parent directory. Continues this pattern till finding one.
mspmfs (most shared path, most free space)	Like epmfs but if it fails to find a branch it will try again with the parent directory. Continues this pattern till finding one.
msppfrd (most shared path, percentage free random distribution)	Like eppfrd but if it fails to find a branch it will try again with the parent directory. Continues this pattern till finding one.
newest	Pick the file / directory with the largest mtime.
pfrd (percentage free random distribution)	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 branch.

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.

Defaults

Category	Policy
action	epall
create	epmfs
search	ff

ioctl

When ioctl is used with an open file then it will use the file handle which was created at the original open call. However, when using ioctl with a directory mergerfs will use the open policy to find the directory to act on.

rename & 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

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.

ERROR HANDLING

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.

others

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.

BUILD / UPDATE

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

First get the code from github.

$ git clone https://github.com/trapexit/mergerfs.git
$ # or
$ wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-<ver>.tar.gz

Debian / Ubuntu

$ cd mergerfs
$ sudo tools/install-build-pkgs
$ make deb
$ sudo dpkg -i ../mergerfs_<version>_<arch>.deb

RHEL / CentOS /Fedora

$ su -
# cd mergerfs
# tools/install-build-pkgs
# make rpm
# rpm -i rpmbuild/RPMS/<arch>/mergerfs-<version>.<arch>.rpm

Generically

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

UPGRADE

mergerfs can be upgraded live by mounting on top of the previous instance. Simply install the new version of mergerfs and follow the instructions below.

Run mergerfs again or if using /etc/fstab call for it to mount again. Existing open files and such will continue to work fine though they won't see runtime changes since any such change would be the new mount. If you plan on changing settings with the new mount you should / could apply those before mounting the new version.

$ 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_mountpoint>.

RUNTIME CONFIG

.mergerfs pseudo file

<mountpoint>/.mergerfs

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).

Keys

Use getfattr -d /mountpoint/.mergerfs or xattr -l /mountpoint/.mergerfs to see all supported keys. Some are informational and therefore read-only. setxattr will return EINVAL (invalid argument) on read-only keys.

Values

Same as the command line.

user.mergerfs.branches

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 +</mnt/drive3 /mnt/pool/.mergerfs

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

Example

[trapexit:/mnt/mergerfs] $ getfattr -d .mergerfs
user.mergerfs.branches="/mnt/a=RW:/mnt/b=RW"
user.mergerfs.minfreespace="4294967295"
user.mergerfs.moveonenospc="false"
...

[trapexit:/mnt/mergerfs] $ getfattr -n user.mergerfs.category.search .mergerfs
user.mergerfs.category.search="ff"

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

TOOLING

• https://github.com/trapexit/mergerfs-tools
  • 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
• https://github.com/trapexit/scorch
  • scorch: A tool to help discover silent corruption of files and keep track of files
• https://github.com/trapexit/bbf
  • bbf (bad block finder): a tool to scan for and 'fix' hard drive bad blocks and find the files using those blocks

CACHING

page caching

https://en.wikipedia.org/wiki/Page_cache

• cache.files=off: Disables page caching. Underlying files cached,
  mergerfs files are not.
• cache.files=partial: Enables page caching. Underlying files cached,
  mergerfs files cached while open.
• cache.files=full: Enables page caching. Underlying files cached,
  mergerfs files cached across opens.
• cache.files=auto-full: Enables page caching. Underlying files
  cached, mergerfs files cached across opens if mtime and size are
  unchanged since previous open.
• cache.files=libfuse: follow traditional libfuse direct_io,
  kernel_cache, and auto_cache arguments.
• cache.files=per-process: Enable page caching only for processes
  which 'comm' name matches one of the values defined in
  cache.files.process-names.

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: https://www.kernel.org/doc/Documentation/filesystems/fuse-io.txt

entry & attribute caching

Given the relatively high cost of FUSE due to the kernel <-> userspace round trips there are kernel side caches for file entries and attributes. The entry cache limits the lookup calls to mergerfs which ask if a file exists. The attribute cache limits the need to make getattr calls to mergerfs which provide file attributes (mode, size, type, etc.). As with the page cache these should not be used if the underlying filesystems are being manipulated at the same time as it could lead to odd behavior or data corruption. The options for setting these are cache.entry and cache.negative_entry for the entry cache and cache.attr for the attributes cache. cache.negative_entry refers to the timeout for negative responses to lookups (non-existent files).

writeback caching

When cache.files is enabled the default is for it to perform writethrough caching. This behavior won't help improve performance as each write still goes one for one through the filesystem. By enabling the FUSE writeback cache small writes may be aggregated by the kernel and then sent to mergerfs as one larger request. This can greatly improve the throughput for apps which write to files inefficiently. The amount the kernel can aggregate is limited by the size of a FUSE message. Read the fuse_msg_size section for more details.

There is a small side effect as a result of enabling writeback caching. Underlying files won't ever be opened with O_APPEND or O_WRONLY. The former because the kernel then manages append mode and the latter because the kernel may request file data from mergerfs to populate the write cache. The O_APPEND change means that if a file is changed outside of mergerfs it could lead to corruption as the kernel won't know the end of the file has changed. That said any time you use caching you should keep from using the same file outside of mergerfs at the same time.

Note that if an application is properly sizing writes then writeback caching will have little or no effect. It will only help with writes of sizes below the FUSE message size (128K on older kernels, 1M on newer).

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 cache.open 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 it's arguable if you should be using mergerfs at all. RAID would probably be the better solution. If you're going to use mergerfs there are other tactics that may help: spreading the data across drives (see the mergerfs.dup tool) and setting func.open=rand, using symlinkify, or using dm-cache or a similar technology to add tiered cache to the underlying device.

With #2 one could use dm-cache as well but there is another solution which requires only mergerfs and a cronjob.

1. Create 2 mergerfs pools. One which includes just the slow 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.

#!/bin/bash

if [ $# != 3 ]; then
  echo "usage: $0 <cache-drive> <backing-pool> <days-old>"
  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}/"

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.

#!/bin/bash

if [ $# != 3 ]; then
  echo "usage: $0 <cache-drive> <backing-pool> <percentage>"
  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 "${CACHE}" -type f -printf '%A@ %P\n' | \
                  sort | \
                  head -n 1 | \
                  cut -d' ' -f2-)
    test -n "${FILE}"
    rsync -axqHAXWESR --preallocate --remove-source-files "${CACHE}/./${FILE}" "${BACKING}/"
done

PERFORMANCE

mergerfs is at its core just a proxy and therefore its theoretical max performance is that of the underlying devices. However, given it is a FUSE filesystem working from userspace there is an increase in overhead relative to kernel based solutions. That said the performance can match the theoretical max but it depends greatly on the system's configuration. Especially when adding network filesystems into the mix there are many variables which can impact performance. 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 cache.open
• 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 cache.files is enabled increase readahead: echo "1024" > /sys/class/bdi/0:$(stat -c%d /MOUNT)/read_ahead_kb
• increase readahead on all devices: ls -1 /sys/class/bdi/*/read_ahead_kb | xargs -n1 -I{} sh -c "echo 1024 > {}"

If you come across a setting that significantly impacts performance please contact trapexit so he may investigate further.

BENCHMARKING

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.

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

TIPS / NOTES

• 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.
• Run mergerfs as root 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 it's a later directory on another mount which had the mtime recently updated. To fix this you will want to set func.getattr=newest. Remember though that this is just stat. If the file is later open'ed or unlink'ed and the policy is different for those then a completely different file or directory could be acted on.
• Some policies mixed with some functions may result in strange behaviors. Not that some of these behaviors and race conditions couldn't happen outside mergerfs but that they are far more likely to occur on account of the attempt to merge 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.

KNOWN ISSUES / BUGS

kernel issues & bugs

https://github.com/trapexit/mergerfs/wiki/Kernel-Issues-&-Bugs

directory mtime is not being updated

Remember that the default policy for getattr is ff. The information for the first directory found will be returned. If it wasn't the directory which had been updated then it will appear outdated.

The reason this is the default is because any other policy would be more expensive and for many applications it is unnecessary. To always return the directory with the most recent mtime or a faked value based on all found would require a scan of all 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
• 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.

This applies to other software: Radarr, Sonarr, Lidarr, Jellyfin, etc.

I would recommend reaching out to the developers of the software
you're having troubles with and asking them to add a fallback to
regular file IO when mmap is unavailable.

If the issue is that scanning doesn't seem to pick up media then be
sure to set func.getattr=newest though generally a full scan will
pick up all media anyway.

When a program tries to move or rename a file it fails

Please read the section above regarding rename & 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.

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.

FAQ

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 cache.files=off,dropcacheonclose=true,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. See also the option inodecalc for how inode values are calculated.

What mergerfs does not do is fake hard links across branches. Read the section "rename & link" for how it works.

Remember that hardlinks will NOT work across devices. That includes between the original filesystem and a mergerfs pool, between two separate pools of the same underlying filesystems, or bind mounts of paths within the mergerfs pool. The latter is common when using Docker or Podman. Multiple volumes (bind mounts) to the same underlying filesystem are considered different devices. There is no way to link between them. You should mount in the highest directory in the mergerfs pool that includes all the paths you need if you want links to work.

Can I use mergerfs without SnapRAID? SnapRAID without mergerfs?

Yes. They are completely unrelated 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.

Why use FUSE? Why not a kernel based solution?

As with any two solutions to a problem there are advantages and disadvantages to each one.

A FUSE based solution has all the downsides of FUSE:

• Higher IO latency due to the trips in and out of kernel space
• Higher general overhead due to trips in and out of kernel space
• Double caching when using page caching
• Misc limitations due to FUSE's design

But FUSE also has a lot of upsides:

• Easier to offer a cross platform solution
• Easier forward and backward compatibility
• Easier updates for users
• Easier and faster release cadence
• Allows more flexibility in design and features
• Overall easier to write, secure, and maintain
• Ability to run without root access or need to change the kernel
• Much lower barrier to entry (getting code into the kernel takes a lot of time and effort initially)

FUSE was chosen because of all the advantages listed above. The negatives of FUSE do not outweigh the positives.

Is my OS's libfuse needed for mergerfs to work?

No. Normally mount.fuse is needed to get mergerfs (or any FUSE filesystem to mount using the mount command but in vendoring the libfuse library the mount.fuse app has been renamed to mount.mergerfs meaning the filesystem type in fstab can simply be mergerfs. That said there should be no harm in having it installed and continuing to using fuse.mergerfs as the type in /etc/fstab.

If mergerfs doesn't work as a type it could be due to how the mount.mergerfs tool was installed. Must be in /sbin/ with proper permissions.

Why was 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,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 <device>

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
• 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?

Yes.

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.

SUPPORT

Filesystems are complex and difficult to debug. mergerfs, while being
just a proxy of sorts, can be difficult to debug given the large
number of possible settings it can have itself and the number of
environments it can run in. When reporting on a suspected issue
please include as much of the below information as possible
otherwise it will be difficult or impossible to diagnose. Also please
read the above documentation as it provides details on many previously
encountered questions/issues.

Please make sure you are using the latest
release 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.

For commercial support or feature requests please contact me directly.

Information to include in bug reports

• Information about the broader problem along with any attempted
  solutions.
• Solution already ruled out and why.
• Version of mergerfs: mergerfs -V
• mergerfs settings / arguments: from fstab, systemd unit, command
  line, OMV plugin, etc.
• Version of the OS: uname -a and lsb_release -a
• List of branches, their filesystem types, sizes (before and after issue): df -h
• All information about the relevant paths and files: permissions, ownership, etc.
• All information about the client app making the requests: version, uid/gid
• Runtime environment:
  • Is mergerfs running within a container?
  • Are the client apps using mergerfs running in a container?
• A strace of the app having problems:
  • strace -fvTtt -s 256 -o /tmp/app.strace.txt <cmd>
• A strace of mergerfs while the program is trying to do whatever it is failing to do:
  • strace -fvTtt -s 256 -p <mergerfsPID> -o /tmp/mergerfs.strace.txt
• Precise directions on replicating the issue. Do not leave anything out.
• Try to recreate the problem in the simplest way using standard programs: ln, mv, cp, ls, dd, etc.

Contact / Issue submission

• github.com: https://github.com/trapexit/mergerfs/issues
• email: trapexit@spawn.link
• discord: https://discord.gg/MpAr69V
• twitter: https://twitter.com/_trapexit
• reddit: https://www.reddit.com/user/trapexit

Support development

This software is released under the very liberal ISC license and is
therefore free to use for personal or commercial uses. That said if
you like this software and have the means please consider supporting
its development.

https://github.com/trapexit/support

LINKS

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