% mergerfs(1) mergerfs user manual

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 filesystems when creating new files so you can mix
read-write and read-only filesystems. It also does not split data
across filesystems. 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

Command Line

mergerfs -o cache.files=partial,dropcacheonclose=true,category.create=mfs /mnt/hdd0:/mnt/hdd1 /media

/etc/fstab

/mnt/hdd0:/mnt/hdd1 /media fuse.mergerfs cache.files=partial,dropcacheonclose=true,category.create=mfs 0 0

systemd mount

https://github.com/trapexit/mergerfs/wiki/systemd

[Unit]
Description=mergerfs service

[Service]
Type=simple
KillMode=none
ExecStart=/usr/bin/mergerfs \
  -f \
  -o cache.files=partial \
  -o dropcacheonclose=true \
  -o category.create=mfs \
  /mnt/hdd0:/mnt/hdd1 \
  /media
ExecStop=/bin/fusermount -uz /media
Restart=on-failure

[Install]
WantedBy=default.target

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)
• lazy-umount-mountpoint=BOOL: mergerfs will attempt to "lazy
  umount" the mountpoint before mounting itself. Useful when
  performing live upgrades of 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
  filesystem. (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 branches 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.
• readahead=UINT: Set readahead (in kilobytes) for mergerfs and
  branches if greater than 0. (default: 0)
• 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)
• process-thread-queue-depth=UINT: Sets the number of requests any
  single process thread can have queued up at one time. Meaning the
  total memory usage of the queues is queue depth multiplied by the
  number of process threads plus read thread count. 0 sets the depth
  to the same as the process thread count. (default: 0)
• pin-threads=STR: Selects a strategy to pin threads to CPUs
  (default: unset)
• flush-on-close=never|always|opened-for-write: Flush data cache
  on file close. Mostly for when writeback is enabled or merging
  network filesystems. (default: opened-for-write)
• 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
• func.readdir=seq|cosr|cor|cosr:INT|cor:INT: Sets readdir
  policy. INT value sets the number of threads to use for
  concurrency. (default: seq)
• 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)
• parallel-direct-writes=BOOL: Allow the kernel to dispatch
  multiple, parallel (non-extending) write requests for files opened
  with cache.files=per-process (if the process is not in process-names)
  or cache.files=off. (This requires kernel support, and was added in v6.2)
• 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.
• splice_read: deprecated - Does nothing.
• splice_write: deprecated - Does nothing.
• splice_move: deprecated - Does nothing.
• allow_other: deprecated - mergerfs v2.35.0 and newer sets this FUSE option
  automatically if running as root.
• use_ino: deprecated - mergerfs should always control inode
  calculation so this is enabled all the time.

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.

NOTE: Always look at the documentation for the version of mergerfs
you're using. Not all features are available in older releases. Use
man mergerfs or find the docs as linked in the release.

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 filesystems nor does it matter the filesystem type (within
reason). Used and available space will not be duplicated for paths on
the same filesystem 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 mode

• 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 and syntax 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/hdd\*:/mnt/ssd /media

The above line will use all mount points in /mnt prefixed with hdd and ssd.

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/hdd*:/mnt/ssd    /media          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 for 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 filesystems 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. 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,
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 or generally how the function behaves.

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

Most 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 most policies.

• 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.

If create, mkdir, mknod, or symlink fail with EROFS
or other fundimental errors then mergerfs will mark any branch found
to be read-only as such (IE will set the mode RO) and will rerun the
policy and try again. This is mostly for ext4 filesystems that can
suddenly become read-only when it encounters an error.

Path Preservation

Policies, as described below, are of two basic classifications. 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 branches where the relative
path being accessed already exists.

When using non-path preserving policies paths will be cloned to target
branches 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.

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.

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 branch with the least free space.
eplus (existing path, least used space)	Of all the branches on which the relative path exists choose the branch with the least used space.
epmfs (existing path, most free space)	Of all the branches on which the relative path exists choose the branch 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 branches, as defined at mount time or configured at runtime, act on the first one found.
lfs (least free space)	Pick the branch with the least available free space.
lus (least used space)	Pick the branch with the least used space.
mfs (most free space)	Pick the branch 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

func.readdir

examples: func.readdir=seq, func.readdir=cor:4

readdir has policies to control how it manages reading directory
content.

Policy	Description
seq	"sequential" : Iterate over branches in the order defined. This is the default and traditional behavior found prior to the readdir policy introduction.
cosr	"concurrent open, sequential read" : Concurrently open branch directories using a thread pool and process them in order of definition. This keeps memory and CPU usage low while also reducing the time spent waiting on branches to respond. Number of threads defaults to the number of logical cores. Can be overwritten via the syntax func.readdir=cosr:N where N is the number of threads.
cor	"concurrent open and read" : Concurrently open branch directories and immediately start reading their contents using a thread pool. This will result in slightly higher memory and CPU usage but reduced latency. Particularly when using higher latency / slower speed network filesystem branches. Unlike seq and cosr the order of files could change due the async nature of the thread pool. Number of threads defaults to the number of logical cores. Can be overwritten via the syntax func.readdir=cor:N where N is the number of threads.

Keep in mind that readdir mostly just provides a list of file names
in a directory and possibly some basic metadata about said files. To
know details about the files, as one would see from commands like
find or ls, it is required to call stat on the file which is
controlled by fuse.getattr.

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 filesystems 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 branch 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 branches with no source file
    • Remove the source from all branches 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 branch != target branch:
      • Clone target path from target branch to source branch
    • 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 branches with no source file
    • Remove the source from all branches 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.

statfs / statvfs

statvfs normalizes the source
filesystems 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 filesystem so multiple sources on the same drive will not result in
double counting its space. Other 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.

flush-on-close

https://lkml.kernel.org/linux-fsdevel/20211024132607.1636952-1-amir73il@gmail.com/T/

By default FUSE would issue a flush before the release of a file
descriptor. This was considered a bit aggressive and a feature added
to give the FUSE server the ability to choose when that happens.

Options:

• always
• never
• opened-for-write

For now it defaults to "opened-for-write" which is less aggressive
than the behavior before this feature was added. It should not be a
problem because the flush is really only relevant when a file is
written to. Given flush is irrelevant for many filesystems in the
future a branch specific flag may be added so only files opened on a
specific branch would be flushed on close.

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.

INSTALL

https://github.com/trapexit/mergerfs/releases

If your distribution's package manager includes mergerfs check if the
version is up to date. If out of date it is recommended to use
the latest release found on the release page. Details for common
distros are below.

Debian

Most Debian installs are of a stable branch and therefore do not have
the most up to date software. While mergerfs is available via apt it
is suggested that uses install the most recent version available from
the releases page.

prebuilt deb

wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs_<ver>.debian-<rel>_<arch>.deb
dpkg -i mergerfs_<ver>.debian-<rel>_<arch>.deb

apt

sudo apt install -y mergerfs

Ubuntu

Most Ubuntu installs are of a stable branch and therefore do not have
the most up to date software. While mergerfs is available via apt it
is suggested that uses install the most recent version available from
the releases page.

prebuilt deb

wget https://github.com/trapexit/mergerfs/releases/download/<version>/mergerfs_<ver>.ubuntu-<rel>_<arch>.deb
dpkg -i mergerfs_<ver>.ubuntu-<rel>_<arch>.deb

apt

sudo apt install -y mergerfs

Raspberry Pi OS

Effectively the same as Debian or Ubuntu.

Fedora

wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-<ver>.fc<rel>.<arch>.rpm
sudo rpm -i mergerfs-<ver>.fc<rel>.<arch>.rpm

CentOS / Rocky

wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-<ver>.el<rel>.<arch>.rpm
sudo rpm -i mergerfs-<ver>.el<rel>.<arch>.rpm

ArchLinux

1. Setup AUR
2. Install mergerfs

Other

Static binaries are provided for situations where native packages are
unavailable.

wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-static-linux_<arch>.tar.gz
sudo tar xvf mergerfs-static-linux_<arch>.tar.gz -C /

BUILD

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 / Rocky / Fedora

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

Generic

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>. Or you can let mergerfs do
it by setting the option lazy-umount-mountpoint=true.

RUNTIME INTERFACES

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

SIGNALS

• USR1: This will cause mergerfs to send invalidation notifications to
  the kernel for all files. This will cause all unused files to be
  released from memory.
• USR2: Trigger a general cleanup of currently unused memory. A more
  thorough version of what happens every ~15 minutes.

IOCTLS

Found in fuse_ioctl.cpp:

typedef char IOCTL_BUF[4096];
#define IOCTL_APP_TYPE  0xDF
#define IOCTL_FILE_INFO            _IOWR(IOCTL_APP_TYPE,0,IOCTL_BUF)
#define IOCTL_GC                   _IO(IOCTL_APP_TYPE,1)
#define IOCTL_GC1                  _IO(IOCTL_APP_TYPE,2)
#define IOCTL_INVALIDATE_ALL_NODES _IO(IOCTL_APP_TYPE,3)

• IOCTL_FILE_INFO: Same as the "file / directory xattrs" mentioned
  above. Use a buffer size of 4096 bytes. Pass in a string of
  "basepath", "relpath", "fullpath", or "allpaths". Receive details in
  same buffer.
• IOCTL_GC: Triggers a thorough garbage collection of excess
  memory. Same as SIGUSR2.
• IOCTL_GC1: Triggers a simple garbage collection of excess
  memory. Same as what happens every 15 minutes normally.
• IOCTL_INVALIDATE_ALL_NODES: Same as SIGUSR1. Send invalidation
  notifications to the kernel for all files causing unused files to be
  released from memory.

TOOLING

preload.so

EXPERIMENTAL

This preloadable library overrides the creation and opening of files
in order to simulate passthrough file IO. It catches the
open/creat/fopen calls, lets mergerfs do the call, queries mergerfs
for the branch the file exists on, and reopens the file on the underlying
filesystem. Meaning that you will get native read/write performance.

This will only work on dynamically linked software. Anything
statically compiled will not work. Many GoLang and Rust apps are
statically compiled.

The library will not interfere with non-mergerfs filesystems.

While the library was written to account for a number of edgecases
there could be some yet accounted for so please report any oddities.

Thank you to
nohajc for
prototyping the idea.

general usage

LD_PRELOAD=/usr/lib/mergerfs/preload.so touch /mnt/mergerfs/filename

Docker usage

Assume /mnt/fs0 and /mnt/fs1 are pooled with mergerfs at
/mnt/mergerfs.

Remember that you must bind into the container the original host paths
to the same locations otherwise the preload module will not be able to
find the files.

docker run \
  -e LD_PRELOAD=/usr/lib/mergerfs/preload.so \
  -v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \
  -v /mnt:/mnt \
  ubuntu:latest \
  bash

Misc

• 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 filesystems by moving them from the most filled to the least filled
  • mergerfs.consolidate: move files within a single mergerfs directory to the filesystem 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 (equivalent to
  cache.files=partial) only for processes whose 'comm' name matches
  one of the values defined in cache.files.process-names. If the
  name does not match the file open is equivalent to
  cache.files=off.

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).

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 filesystem 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 filesystem 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
filesystem could help with a typical mergerfs setup.

1. Fast network, slow filesystems, many readers: You've a 10+Gbps network
   with many readers and your regular filesystems can't keep up.
2. Fast network, slow filesystems, small'ish bursty writes: You have a
   10+Gbps network and wish to transfer amounts of data less than your
   cache filesystem 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
filesystems (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 devices
   and one which has both the fast devices (SSD,NVME,etc.) and slow
   devices.
2. The 'cache' pool should have the cache filesystems 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 filesystem(s) are far smaller than the
   backing filesystems. 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 set the
   slow filesystems mode to NC though that'd mean if the cache
   filesystems 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 filesystem if not larger. This way in the worst case the
   whole of the cache filesystem(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
filesystem itself. Not the pool with the cache filesystem. You
could have data loss if the source is the cache pool.

mergerfs.time-based-mover

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

mergerfs.percent-full-mover

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. Device 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

• disable security_capability and/or xattr
• increase cache timeouts cache.attr, cache.entry, cache.negative_entry
• enable (or disable) page caching (cache.files)
• enable parallel-direct-writes
• enable cache.writeback
• 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 devices
• use LVM and LVM cache to place a SSD in front of your HDDs
• increase readahead: readahead=1024

If you come across a setting that significantly impacts performance
please contact trapexit so he may investigate further. Please test
both against your normal setup, a singular branch, and with
nullrw=true

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 device. 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. 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, device, 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 literal and thorough. 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.
• Run mergerfs as root. mergerfs is designed and intended to be run
  as root and may exibit incorrect behavior if run otherwise..
• 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 filesystem 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 filesystems.

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 filesystem 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|per-processe 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 device,
filesystem, 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.

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 filesystems. The author of mergerfs had it
running for over 300 days managing 16+ devices 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 filesystems which already have data / are in use?

Yes. mergerfs is really just a proxy and does NOT interfere with
the normal form or function of the filesystems / mounts / paths it
manages. It is just another userland application that is acting as a
man-in-the-middle. It can't do anything that any other random piece of
software can't do.

mergerfs is not a traditional filesystem that takes control over
the underlying block device. mergerfs is not RAID. It does not
manipulate the data that passes through it. It does not shard data
across filesystems. It merely shards some behavior and aggregates
others.

Can drives/filesystems be removed from the pool at will?

Yes. See previous question's answer.

Can mergerfs be removed without affecting the data?

Yes. See the previous question's answer.

Can drives/filesystems be moved to another pool?

Yes. See the previous question's answer.

How do I migrate data into or out of the pool when adding/removing drives/filesystems?

You don't need to. See the previous question's answer.

How do I remove a drive/filesystem but keep the data in the pool?

Nothing special needs to be done. Remove the branch from mergerfs'
config and copy (rsync) the data from the removed filesystem into the
pool. Effectively the same as if it were you transfering data from one
filesystem to another.

If you wish to continue using the pool while performing the transfer
simply create another, temporary pool without the filesystem in
question and then copy the data. It would probably be a good idea to
set the branch to RO prior to doing this to ensure no new content is
written to the filesystem while performing the copy.

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 filesystems. 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 device regularly and want the data
to predictably be on that device 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.

epmfs is the default category.create policy because ep policies
are not going to change the general layout of the branches. It won't
place files/dirs on branches that don't already have the relative
branch. So it keeps the system in a known state. It's much easier to
stop using epmfs or redistribute files around the filesystem than it
is to consolidate them back.

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:

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

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

Did you start with empty filesystems? Did you explicitly configure a
category.create policy? Are you using an existing path / path preserving policy?

The default create policy is epmfs. That is a path preserving
algorithm. With such a policy for mkdir and create with a set of
empty filesystems it will select only 1 filesystem when the first
directory is created. Anything, files or directories, created in that
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 filesystems 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 filesystems 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 filesystem.

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 filesystem but not another resulting in an
incomplete set.

Whenever you run into a split permission issue (seeing some but not
all files) try using
mergerfs.fsck 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 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
• 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 splice support removed?

After a lot of testing over the years splicing always appeared to be
at best provide equivalent performance and in cases worse
performance. Splice is not supported on other platforms forcing a
traditional read/write fallback to be provided. The splice code was
removed to simplify the codebase.

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 filesystems
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 filesystems (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.

Why use mergerfs over StableBit's DrivePool?

DrivePool works only on Windows so not as common an alternative as
other Linux solutions. If you want to use Windows then DrivePool is a
good option. Functionally the two projects work a bit
differently. DrivePool always writes to the filesystem with the most
free space and later rebalances. mergerfs does not offer rebalance but
chooses a branch at file/directory create time. DrivePool's
rebalancing can be done differently in any directory and has file
pattern matching to further customize the behavior. mergerfs, not
having rebalancing does not have these features, but similar features
are planned for mergerfs v3. DrivePool has builtin file duplication
which mergerfs does not natively support (but can be done via an
external script.)

There are a lot of misc differences between the two projects but most
features in DrivePool can be replicated with external tools in
combination with mergerfs.

Additionally DrivePool is a closed source commercial product vs
mergerfs a ISC licensed OSS project.

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 filesystems 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 filesystem it's 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.

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
filesystems).

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 the issue
you are encountering 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 --version
• 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
• discord: https://discord.gg/MpAr69V
• reddit: https://www.reddit.com/r/mergerfs

Donations

https://github.com/trapexit/support

Development and support of a project like mergerfs requires a
significant amount of time and effort. The software is released under
the very liberal ISC license and is therefore free to use for personal
or commercial uses.

If you are a personal user and find mergerfs and its support valuable
and would like to support the project financially it would be very
much appreciated.

If you are using mergerfs commercially please consider sponsoring the
project to ensure it continues to be maintained and receive
updates. If custom features are needed feel free to contact me
directly.

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