FreeBSD Manual Pages

home | help

mergerfs(1) mergerfs(1)

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.

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 func-
tion.

• 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.cre-
ate=mfs /mnt/hdd0:/mnt/hdd1 /media

/etc/fstab
/mnt/hdd0:/mnt/hdd1 /media mergerfs cache.files=partial,dropcacheon-
close=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
(https://github.com/trapexit/mergerfs/wiki/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 poli-
cies. Can be overridden by branch specific option. Understands `K',
`M', and `G' to represent kilobyte, megabyte, and gigabyte respec-
tively. (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 possi-
ble) and if successful the original is unlinked and the write re-
tried. (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 report-
ed 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)

• export-support=BOOL: Sets a low-level FUSE feature intended to indi-
cate the filesystem can support being exported via NFS. (default:
true)

• security_capability=BOOL: If false return ENOATTR when xattr securi-
ty.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 ig-
nore 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. (de-
fault: 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 sym-
link to it.

• readahead=UINT: Set readahead (in kilobytes) for mergerfs and branch-
es 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 un-
available 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 val-
ue. 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 rea-
sonable values based on CPU thread count. NOTE: higher number of
threads increases parallelism but usually decreases throughput. (de-
fault: 0)

• read-thread-count=INT: Alias for threads.

• process-thread-count=INT: Enables separate thread pool to asynchro-
nously 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 sin-
gle 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 de-
tails. (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 be-
low 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 ac-
tion category. (default: epall)

• category.create=POLICY: Sets policy of all FUSE functions in the cre-
ate 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:

• cache.attr=UINT: File attribute cache timeout in seconds. (default:
1)

• cache.entry=UINT: File name lookup cache timeout in seconds. (de-
fault: 1)

• cache.negative_entry=UINT: Negative file name lookup cache timeout in
seconds. (default: 0)

• cache.files.process-names=LIST: A pipe | delimited list of process
comm (https://man7.org/linux/man-pages/man5/proc.5.html) names to en-
able page caching for when cache.files=per-process. (default: "rtor-
rent|qbittorrent-nox")

• cache.writeback=BOOL: Enable kernel writeback caching (default:
false)

• cache.symlinks=BOOL: Cache symlinks (if supported by kernel) (de-
fault: 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 in-
stead. (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 calcula-
tion 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 argu-
ment)

• 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 un-
derlying 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 im-
pacts 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. Ex-
ample: /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 glob-
bing (http://linux.die.net/man/7/glob). 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 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 in-
stalled. 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 un-
derlying file. It is a many to one relationship between names and an
inode. Directories, however, do not have multiple links on most sys-
tems 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 unex-
pectedly. The tool find will abort a directory walk if it sees a di-
rectory inode change. NFS can 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 re-
sort to full file comparisons should it find different inode values.

mergerfs offers multiple ways to calculate the inode in hopes of cover-
ing different usecases.

• passthrough: Passes through the underlying inode value. Mostly in-
tended for testing as using this does not address any of the problems
mentioned above and could confuse file deduplication software as in-
odes 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 in-
ode 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 be-
tween 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 in-
ternal 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
(https://en.wikipedia.org/wiki/Page_(computer_memory)) 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 ab-
straction.

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
(https://github.com/trapexit/mergerfs/issues).

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 new-
path. 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_re-
name_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 multi-
ple oldpaths exist.

symlinkify
Due to the levels of indirection introduced by mergerfs and the under-
lying technology FUSE there can be varying levels of performance degra-
dation. 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. Alterna-
tively create two mounts. One for backup and one for general consump-
tion.

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 per-
formance 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 run-
time 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 it-
self 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 be-
cause 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 securi-
ty 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 read-only file which is empty is
opened for writing.

export-support
In theory this flag should not be exposed to the end user. It is a
low-level FUSE flag which indicates whether or not the kernel can send
certain kinds of messages to it for the purposes of using with NFS.
mergerfs does support these messages but due bugs and quirks found in
the kernel and mergerfs this option is provided just in case it is
needed for debugging.

Given that this flag is set when the FUSE connection is first initiated
it is not possible to change during run time.

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, ftrun-
cate, 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 cal-
culations. The reason is that it doesn't really work for non-path pre-
serving 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
Catego- FUSE Functions
ry
--------------------------------------------------------------------------
action chmod, chown, link, removexattr, rename, rmdir, setxattr, trun-
cate, 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 require-
ments 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 filter-
ing 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 re-
quire 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) de-
pending 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 fundimen-
tal 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 pre-
serving 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 of-
fer 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 For mkdir, mknod, and symlink it will apply to all
path, all) found. create works like epff (but more expensive be-
cause it doesn't stop after finding a valid branch).
epff (existing Given the order of the branches, as defined at mount
path, first time or configured at runtime, act on the first one
found) found where the relative path exists.
eplfs (existing Of all the branches on which the relative path exists
path, least free choose the branch with the least free space.
space)
eplus (existing Of all the branches on which the relative path exists
path, least used choose the branch with the least used space.
space)
epmfs (existing Of all the branches on which the relative path exists
path, most free choose the branch with the most free space.
space)
eppfrd (existing Like pfrd but limited to existing paths.
path, percentage
free random dis-
tribution)
eprand (existing Calls epall and then randomizes. Returns 1.
path, random)
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 Pick the branch with the least available free space.
space)
lus (least used Pick the branch with the least used space.
space)
mfs (most free Pick the branch with the most available free space.
space)
msplfs (most Like eplfs but if it fails to find a branch it will
shared path, try again with the parent directory. Continues this
least free pattern till finding one.
space)
msplus (most Like eplus but if it fails to find a branch it will
shared path, try again with the parent directory. Continues this
least used pattern till finding one.
space)
mspmfs (most Like epmfs but if it fails to find a branch it will
shared path, try again with the parent directory. Continues this
most free space) pattern till finding one.
msppfrd (most Like eppfrd but if it fails to find a branch it will
shared path, try again with the parent directory. Continues this
percentage free pattern till finding one.
random distribu-
tion)
newest Pick the file / directory with the largest mtime.
pfrd (percentage Chooses a branch at random with the likelihood of se-
free random dis- lection based on a branch's available space relative
tribution) 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 reser-
vation 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 con-
tent.

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" : Con-
currently open branch directories using a
thread pool and process them in order of
definition. This keeps memory and CPU us-
age 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 over-
written 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 directo-
ry 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 con-
tacting 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 on-
ly 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 de-
vice / improper link). So if a rename's source and target are on dif-
ferent 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 ap-
plications 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 re-
turned but it will still be possible.

link uses the same strategy but without the removals.

statfs / statvfs
statvfs (http://linux.die.net/man/2/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 be-
havior.

flush-on-close
https://lkml.kernel.org/linux-fsdev-
el/20211024132607.1636952-1-amir73il@gmail.com/T/

By default FUSE would issue a flush before the release of a file de-
scriptor. 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 gener-
ally 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 (https://github.com/trapexit/mergerfs/releases).

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

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 (https://github.com/trapexit/mergerfs).

$ 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 in-
stance. Simply install the new version of mergerfs and follow the in-
structions 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 mergerfs (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)
media on /mnt/mergerfs type mergerfs (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)

A problem with this approach is that the underlying instance will con-
tinue 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 (http://linux.die.net/man/2/listxattr) 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 ac-
cess the values you will need to issue a getxattr (http://lin-
ux.die.net/man/2/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 per-
spective 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 re-
leased 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 noti-
fications to the kernel for all files causing unused files to be re-
leased from memory.

TOOLING
preload.so
EXPERIMENTAL

For some time there has been work to enable passthrough IO in FUSE.
Passthrough IO would allow for near native performance with regards to
reads and writes (at the expense of certain mergerfs features.) Howev-
er, there have been several complications which have kept the feature
from making it into the mainline Linux kernel. Until that feature is
available there are two methods to provide similar functionality. One
method is using the LD_PRELOAD feature of the dynamic linker. The oth-
er leveraging ptrace to intercept syscalls. Each has their disadvan-
tages. At the moment only a preload based tool is available. A ptrace
based tool may be developed later if there is a need.

/usr/lib/mergerfs/preload.so

This preloadable library (https://man7.org/linux/man-
pages/man8/ld.so.8.html#ENVIRONMENT) overrides the creation and opening
of files in order to simulate passthrough file IO. It catches the
open/creat/fopen calls, has mergerfs do the call, queries mergerfs for
the branch the file exists on, reopens the file on the underlying
filesystem and returns that instead. Meaning that you will get native
read/write performance because mergerfs is no longer part of the work-
flow. Keep in mind that this also means certain mergerfs features that
work by interrupting the read/write workflow, such as moveonenospc,
will no longer work.

Also understand that this will only work on dynamically linked soft-
ware. Anything statically compiled will not work. Many GoLang and
Rust apps are statically compiled.

The library will not interfere with non-mergerfs filesystems. The li-
brary is written to always fallback to returning the mergerfs opened
file on error.

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 (https://github.com/nohajc/mergerfs-io-passthrough)
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 /media.

All mergerfs branch paths must be bind mounted into the container at
the same path as found on the host so the preload library can see them.

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

or more explicitly

docker run \
-e LD_PRELOAD=/usr/lib/mergerfs/preload.so \
-v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \
-v /media:/data \
-v /mnt/fs0:/mnt/fs0 \
-v /mnt/fs1:/mnt/fs1 \
ubuntu:latest \
bash

systemd unit
Use the Environment option to set the LD_PRELOAD variable.

• https://www.freedesktop.org/software/systemd/man/latest/systemd.ser-
vice.html#Command%20lines

• https://serverfault.com/questions/413397/how-to-set-environment-vari-
able-in-systemd-service

[Service]
Environment=LD_PRELOAD=/usr/lib/mergerfs/preload.so

Misc
• https://github.com/trapexit/mergerfs-tools

• mergerfs.ctl: A tool to make it easier to query and configure merg-
erfs 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 un-
changed since previous open.

• cache.files=libfuse: follow traditional libfuse direct_io, ker-
nel_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 au-
to_cache.

Due to mergerfs using FUSE and therefore being a userland process prox-
ying existing filesystems the kernel will double cache the content be-
ing read and written through mergerfs. Once from the underlying
filesystem and once from mergerfs (it sees them as two separate enti-
ties). Using cache.files=off will keep the double caching from happen-
ing 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 mini-
mize 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 limi-
tations on the what underlying filesystems would be supported and com-
plicates the code.

kernel documentation: https://www.kernel.org/doc/Documentation/filesys-
tems/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 attribut-
es. 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 filesys-
tems are being manipulated at the same time as it could lead to odd be-
havior or data corruption. The options for setting these are cache.en-
try and cache.negative_entry for the entry cache and cache.attr for the
attributes cache. cache.negative_entry refers to the timeout for nega-
tive 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 im-
prove the throughput for apps which write to files inefficiently. The
amount the kernel can aggregate is limited by the size of a FUSE mes-
sage. 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 be-
cause the kernel may request file data from mergerfs to populate the
write cache. The O_APPEND change means that if a file is changed out-
side 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 new-
er).

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 perfor-
mance 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.merg-
erfs.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 net-
work 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, us-
ing 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 de-
vices.

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 merg-
erfs.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 it-
self. 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 it-
self. 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 con-
figuration. 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 opti-
mal 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 un-
derstand 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 dis-
abling 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 re-
move 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 / filesys-
tem 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 sce-
nario. 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 perfor-
mance. 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 per-
forms 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 ca-
sual tests little gain was found. More tests will need to be done be-
fore this feature would become available. If you have an app that ap-
pears 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 merg-
erfs.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 rtor-
rent) to use mmap (http://linux.die.net/man/2/mmap) files. Shared
mmap is not currently supported in FUSE w/ page caching disabled.
Enabling dropcacheonclose is recommended when cache.files=par-
tial|full|auto-full.

• Kodi (http://kodi.tv), Plex (http://plex.tv), Subsonic (http://sub-
sonic.org), etc. can use directory mtime (http://lin-
ux.die.net/man/2/stat) to more efficiently determine whether to scan
for new content rather than simply performing a full scan. If using
the default getattr policy of ff 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.getat-
tr=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 behav-
iors. 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 (http://linux.die.net/man/1/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 di-
rectory 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 generally does not like out of band changes. Take a look at the
section on NFS in the #remote-filesystems for more details.

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
(http://linux.die.net/man/2/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 dropcacheon-
close=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 imple-
mentation when page caching is disabled. To fix this place the data
elsewhere (preferable) or enable cache.files (with dropcacheon-
close=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 er-
rors 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 simi-
lar it tries rename and on EXDEV falls back to a manual copying of data
between the two locations and unlinking the source. In these older
versions of GVFS-fuse if it received EXDEV it would translate that into
EIO. This would cause mv or most any application attempting to move
files around on that SMB share to fail with a IO error.

GVFS-fuse v1.22.0 (https://bugzilla.gnome.org/show_bug.cgi?id=734568)
and above fixed this issue but a large number of systems use the older
release. On Ubuntu the version can be checked by issuing apt-cache
showpkg gvfs-fuse. Most distros released in 2015 seem to have the up-
dated 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 un-
fortunately 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 implemen-
tation MAY also support "top directory trashes" which many probably do.

To create a $topdir/.Trash directory as defined in the standard use the
mergerfs-tools (https://github.com/trapexit/mergerfs-tools) tool merg-
erfs.mktrash.

Supplemental user groups
Due to the overhead of getgroups/setgroups (http://lin-
ux.die.net/man/2/setgroups) mergerfs utilizes a cache. This cache is
opportunistic and per thread. Each thread will query the supplemental
groups for a user when that particular thread needs to change creden-
tials 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 with-
out 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 compati-
ble with older systems which may not have C++11 compilers. There is
enough storage for 256 users' supplemental groups. Each user is al-
lowed 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 sup-
plemental 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 lim-
ited 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.

Remote Filesystems
Many users ask about compatibility with remote filesystems. This sec-
tion is to describe any known issues or quirks when using mergerfs with
common remote filesystems.

Keep in mind that, like with caching, it is not a good idea to change
the contents of the remote filesystem out-of-band
(https://en.wikipedia.org/wiki/Out-of-band). Meaning that you really
shouldn't change the contents of the underlying filesystems or mergerfs
on the server hosting the remote filesystem. Doing so can lead to
weird behavior, inconsistency, errors, and even data corruption should
multiple programs try to write or read the same data at the same time.
This isn't to say you can't do it or that data corruption is likely but
it could happen. It is better to always use the remote filesystem.
Even on the machine serving it.

NFS
NFS (https://en.wikipedia.org/wiki/Network_File_System) is a common re-
mote filesystem on Unix/POSIX systems. Due to how NFS works there are
some settings which need to be set in order for mergerfs to work with
it.

It should be noted that NFS and FUSE (the technology mergerfs uses) do
not work perfectly with one another due to certain design choices in
FUSE (and mergerfs.) Due to these issues it is generally recommended to
use SMB when possible till situations change. That said mergerfs
should generally work as an export of NFS and issues discovered should
still be reported.

To ensure compatibility between mergerfs and NFS use the following set-
tings.

mergerfs settings: * noforget * inodecalc=path-hash

NFS export settings: * fsid=UUID * no_root_squash

noforget is needed because NFS uses the name_to_handle_at and
open_by_handle_at functions which allow a program to keep a reference
to a file without technically having it open in the typical sense. The
problem is that FUSE has no way to know that NFS has a handle that it
will later use to open the file again. As a result it is possible for
the kernel to tell mergerfs to forget about the node and should NFS
ever ask for that node's details in the future it would have nothing to
respond with. Keeping nodes around forever is not ideal but at the mo-
ment the only way to manage the situation.

inodecalc=path-hash is needed because NFS is sensitive to out-of-band
changes. FUSE doesn't care if a file's inode value changes but NFS,
being stateful, does. So if you used the default inode calculation al-
gorithm then it is possible that if you changed a file or updated a di-
rectory the file mergerfs will use will be on a different branch and
therefore the inode would change. This isn't an ideal solution and
others are being considered but it works for most situations.

fsid=UUID is needed because FUSE filesystems don't have different
st_dev values which can cause issues when exporting. The easiest thing
to do is set each mergerfs export fsid to some random value. An easy
way to generate a random value is to use the command line tool uuid or
uuidgen or through a website such as uuidgenerator.net
(https://www.uuidgenerator.net/).

no_root_squash is not strictly necessary but can lead to confusing per-
mission and ownership issues if root squashing is enabled.

SMB / CIFS
SMB (https://en.wikipedia.org/wiki/Server_Message_Block) is a protocol
most used by Microsoft Windows systems to share file shares, printers,
etc. However, due to the popularity for Windows, it is also supported
on many other platforms including Linux. The most popular way of sup-
porting SMB on Linux is via the software Samba.

Samba (https://en.wikipedia.org/wiki/Samba_(software)), and other ways
of serving Linux filesystems, via SMB should work fine with mergerfs.
The services do not tend to use the same technologies which NFS uses
and therefore don't have the same issues. There should not be an spe-
cial settings required to use mergerfs with Samba. However, CIFSD
(https://en.wikipedia.org/wiki/CIFSD) and other programs have not been
extensively tested. If you use mergerfs with CIFSD or other SMB
servers please submit your experiences so these docs can be updated.

SSHFS
SSHFS (https://en.wikipedia.org/wiki/SSHFS) is a FUSE filesystem lever-
aging SSH as the connection and transport layer. While often simpler
to setup when compared to NFS or Samba the performance can be lacking
and the project is very much in maintenance mode.

There are no known issues using sshfs with mergerfs. You may want to
use the following arguments to improve performance but your millage may
vary.

• -o Ciphers=arcfour

• -o Compression=no

More info can be found here (https://ideatrash.net/2016/08/odds-and-
ends-optimizing-sshfs-moving.html).

Other
There are other remote filesystems but none popularly used to serve
mergerfs. If you use something not listed above feel free to reach out
and I will add it to the list.

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
(https://www.openmediavault.org) 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 ker-
nel bugs (https://github.com/trapexit/mergerfs/wiki/Kernel-Is-
sues-&-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 dri-
ves/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 ques-
tion 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 writ-
ten 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 pre-
dictably 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 preser-
vation can help but will require some manual attention. Colocating af-
ter 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 consol-
idate 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 preserv-
ing policy?

The default create policy is epmfs. That is a path preserving algo-
rithm. 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 di-
rectory 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 and this policy is the
safest policy as it will not change the general layout of the existing
filesystems. 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 calculat-
ed.

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 sepa-
rate 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 filesys-
tem 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.

Does FICLONE or FICLONERANGE work?
Unfortunately not. FUSE, the technology mergerfs is based on, does not
support the clone_file_range feature needed for it to work. mergerfs
won't even know such a request is made. The kernel will simply return
an error back to the application making the request.

Should FUSE gain the ability mergerfs will be updated to support it.

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 --de-
vice=/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 (https://hotio.dev/containers/mergerfs),
with Docker you should probably be mounting with bind-propagation set
to slave.

Does mergerfs support CoW / copy-on-write / writes to read-only filesys-
tems?
Not in the sense of a filesystem like BTRFS or ZFS nor in the overlayfs
or aufs sense. It does offer a cow-shell (http://manpages.ubun-
tu.com/manpages/bionic/man1/cow-shell.1.html) 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 over-
layfs. 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, merg-
erfs 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 nei-
ther will mergerfs. However, because mergerfs is creating a union of
paths it may be able to read some files and directories on one filesys-
tem 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 (https://github.com/trapexit/mergerfs-
tools) 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 exe-
cutable 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 or SMB (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 disadvan-
tages 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 nega-
tives 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 filesys-
tem to mount using the mount command but in vendoring the libfuse li-
brary 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 per-
missions.

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.

What should mergerfs NOT be used for?
• databases: Even if the database stored data in separate files (merg-
erfs wouldn't offer much otherwise) the higher latency of the indi-
rection will kill performance. If it is a lightly used SQLITE data-
base 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 avail-
ability 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). Espe-
cially 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, moveo-
nenospc, 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 theo-
retical space available.

Due to path preservation, branch tagging, read-only status, and min-
freespace 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 avail-
able 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>

I notice massive slowdowns of writes when enabling cache.files.
When file caching is enabled in any form (cache.files!=off) it will is-
sue getxattr requests for security.capability prior to every single
write. This will usually result in a performance degradation, espe-
cially when using a network filesystem (such as NFS or SMB.) Unfortu-
nately 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 getxat-
tr. Not just security.capability. This will not be cached by the
kernel either but mergerfs' runtime config system will still func-
tion.

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 (https://github.com/trapexit/mhddfs) manages running as root by
calling getuid() (https://github.com/trapex-
it/mhddfs/blob/cae96e6251dd91e2bdc24800b4a18a74044f6672/src/main.c#L319)
and if it returns 0 then it will chown (http://lin-
ux.die.net/man/1/chown) the file. Not only is that a race condition
but it doesn't handle other situations. Rather than attempting to sim-
ulate POSIX ACL behavior the proper way to manage this is to use se-
teuid (http://linux.die.net/man/2/seteuid) and setegid (http://lin-
ux.die.net/man/2/setegid), in effect becoming the user making the orig-
inal 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 creden-
tials. 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 cre-
dentials 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 read-
lock allowing multiple threads to share the credentials. Once a re-
quest 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 solu-
tion but should work reasonably well assuming there are few users.

mergerfs versus X
mhddfs
mhddfs had not been maintained for some time and has some known stabil-
ity and security issues. mergerfs provides a superset of mhddfs' fea-
tures and should offer the same or 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

aufs
aufs is mostly abandoned and no longer available in most Linux 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 has.

unionfs-fuse
unionfs-fuse is more like aufs than mergerfs in that it offers overlay
/ copy-on-write (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
read/write filesystems over read-only ones.

overlayfs
overlayfs is similar to aufs and unionfs-fuse in that it also is pri-
marily used to layer a read/write filesystem over one or more read-only
filesystems. It does not have the ability to spread files/directories
across numerous filesystems.

RAID0, JBOD, 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 but all other filesys-
tems and their data will continue to be accessible.

The main practical difference with mergerfs is the fact you don't actu-
ally have contiguous space as large as if you used those other tech-
nologies. Meaning you can't create a 2TB file on a pool of 2 1TB
filesystems.

When combined with something like SnapRaid (http://www.snapraid.it)
and/or an offsite backup solution you can have the flexibility of JBOD
without the single point of failure.

UnRAID
UnRAID is a full OS and its storage layer, as I understand, is propri-
etary and closed source. Users who have experience with both have of-
ten said they prefer the flexibility offered by mergerfs and for some
the fact it is 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.

For semi-static data mergerfs + SnapRaid (http://www.snapraid.it) pro-
vides a similar solution.

ZFS
mergerfs is very different from ZFS. mergerfs is intended to provide
flexible pooling of arbitrary filesystems (local or remote), of arbi-
trary sizes, and arbitrary filesystems. For write once, read many use-
cases such as bulk media storage. Where data integrity and backup is
managed in other ways. In those usecases ZFS can introduce a number of
costs and limitations as described here (http://louwrentius.com/the-
hidden-cost-of-using-zfs-for-your-home-nas.html), here (https://markm-
cb.com/2020/01/07/five-years-of-btrfs/), and here (https://utcc.utoron-
to.ca/~cks/space/blog/solaris/ZFSWhyNoRealReshaping).

StableBit's DrivePool
DrivePool works only on Windows so not as common an alternative as oth-
er 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 fur-
ther 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 combina-
tion with mergerfs.

Additionally DrivePool is a closed source commercial product vs merg-
erfs a ISC licensed OSS project.

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 diffi-
cult or impossible to diagnose. Also please read the above documenta-
tion as it provides details on many previously encountered ques-
tions/issues.

Please make sure you are using the latest release
(https://github.com/trapexit/mergerfs/releases) or have tried it in
comparison. Old versions, which are often included in distros like De-
bian 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.
(mailto:support@spawn.link)

Information to include in bug reports
• Information about the broader problem along with any attempted solu-
tions. (https://xyproblem.info)

• 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 is-
sue): df -h

• All information about the relevant paths and files: permissions, own-
ership, 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 pro-
grams: 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 signifi-
cant 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 (mail-
to:support@spawn.link).

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

mergerfs user manual mergerfs(1)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=mergerfs&sektion=1&manpath=FreeBSD+Ports+14.3.quarterly>

home | help

Header And Logo

Peripheral Links

Site Navigation

FreeBSD Manual Pages