Provided by: mergerfs_2.28.1-1build1_amd64 
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
• Runs in userspace (FUSE)
• Configurable behaviors / file placement
• Support for extended attributes (xattrs)
• Support for file attributes (chattr)
• Runtime configurable (via xattrs)
• Safe to run as root
• Opportunistic credential caching
• Works with heterogeneous filesystem types
• Handling of writes to full drives (transparently move file to drive with capacity)
• Handles pool of read-only and read/write drives
• Can turn read-only files into symlinks to underlying file
• Hard link copy-on-write / CoW
• supports POSIX ACLs
How it works
mergerfs logically merges multiple paths together. Think a union of sets. The file/s or directory/s
acted on or presented through mergerfs are based on the policy chosen for that particular action. Read
more about policies below.
A + B = C
/disk1 /disk2 /merged
| | |
+-- /dir1 +-- /dir1 +-- /dir1
| | | | | |
| +-- file1 | +-- file2 | +-- file1
| | +-- file3 | +-- file2
+-- /dir2 | | +-- file3
| | +-- /dir3 |
| +-- file4 | +-- /dir2
| +-- file5 | |
+-- file6 | +-- file4
|
+-- /dir3
| |
| +-- file5
|
+-- file6
mergerfs does not support the copy-on-write (CoW) behavior found in aufs and overlayfs. You can not
mount a read-only filesystem and write to it. However, mergerfs will ignore read-only drives when creat‐
ing new files so you can mix read-write and read-only drives.
OPTIONS
mount options
• allow_other: A libfuse option which allows users besides the one which ran mergerfs to see the filesys‐
tem. This is required for most use-cases.
• minfreespace=SIZE: The minimum space value used for creation policies. Understands 'K', 'M', and 'G'
to represent kilobyte, megabyte, and gigabyte respectively. (default: 4G)
• moveonenospc=BOOL: When enabled if a write fails with ENOSPC or EDQUOT a scan of all drives will be
done looking for the drive with the most free space which is at least the size of the file plus the
amount which failed to write. An attempt to move the file to that drive will occur (keeping all meta‐
data possible) and if successful the original is unlinked and the write retried. (default: false)
• use_ino: Causes mergerfs to supply file/directory inodes rather than libfuse. While not a default it
is recommended it be enabled so that linked files share the same inode value.
• dropcacheonclose=BOOL: When a file is requested to be closed call posix_fadvise on it first to instruct
the kernel that we no longer need the data and it can drop its cache. Recommended when
cache.files=partial|full|auto-full to limit double caching. (default: false)
• symlinkify=BOOL: When enabled and a file is not writable and its mtime or ctime is older than symlinki‐
fy_timeout files will be reported as symlinks to the original files. Please read more below before us‐
ing. (default: false)
• symlinkify_timeout=INT: Time to wait, in seconds, to activate the symlinkify behavior. (default: 3600)
• nullrw=BOOL: Turns reads and writes into no-ops. The request will succeed but do nothing. Useful for
benchmarking mergerfs. (default: false)
• ignorepponrename=BOOL: Ignore path preserving on rename. Typically rename and link act differently de‐
pending 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 dri‐
ve. (default: false)
• security_capability=BOOL: If false return ENOATTR when xattr security.capability is queried. (default:
true)
• xattr=passthrough|noattr|nosys: Runtime control of xattrs. Default is to passthrough xattr requests.
'noattr' will short circuit as if nothing exists. 'nosys' will respond with ENOSYS as if xattrs are
not supported or disabled. (default: passthrough)
• link_cow=BOOL: When enabled if a regular file is opened which has a link count > 1 it will copy the
file to a temporary file and rename over the original. Breaking the link and providing a basic
copy-on-write function similar to cow-shell. (default: false)
• statfs=base|full: Controls how statfs works. 'base' means it will always use all branches in statfs
calculations. 'full' is in effect path preserving and only includes drives where the path exists.
(default: base)
• statfs_ignore=none|ro|nc: 'ro' will cause statfs calculations to ignore available space for branches
mounted or tagged as 'read-only' or 'no create'. 'nc' will ignore available space for branches tagged
as 'no create'. (default: none)
• posix_acl=BOOL: Enable POSIX ACL support (if supported by kernel and underlying filesystem). (default:
false)
• async_read=BOOL: Perform reads asynchronously. If disabled or unavailable the kernel will ensure there
is at most one pending read request per file handle and will attempt to order requests by offset. (de‐
fault: true)
• fuse_msg_size=INT: Set the max number of pages per FUSE message. Only available on Linux >= 4.20 and
ignored otherwise. (min: 1; max: 256; default: 256)
• threads=INT: Number of threads to use in multithreaded mode. When set to zero it will attempt to dis‐
cover and use the number of logical cores. If the lookup fails it will fall back to using 4. If the
thread count is set negative it will look up the number of cores then divide by the absolute value.
ie. threads=-2 on an 8 core machine will result in 8 / 2 = 4 threads. There will always be at least 1
thread. NOTE: higher number of threads increases parallelism but usually decreases throughput. (de‐
fault: 0)
• fsname=STR: Sets the name of the filesystem as seen in mount, df, etc. Defaults to a list of the
source paths concatenated together with the longest common prefix removed.
• func.FUNC=POLICY: Sets the specific FUSE function's policy. See below for the list of value types.
Example: func.getattr=newest
• category.CATEGORY=POLICY: Sets policy of all FUSE functions in the provided category. Example: catego‐
ry.create=mfs
• cache.open=INT: 'open' policy cache timeout in seconds. (default: 0)
• cache.statfs=INT: 'statfs' cache timeout in seconds. (default: 0)
• cache.attr=INT: File attribute cache timeout in seconds. (default: 1)
• cache.entry=INT: File name lookup cache timeout in seconds. (default: 1)
• cache.negative_entry=INT: Negative file name lookup cache timeout in seconds. (default: 0)
• cache.files=libfuse|off|partial|full|auto-full: File page caching mode (default: libfuse)
• cache.symlinks=BOOL: Cache symlinks (if supported by kernel) (default: false)
• cache.readdir=BOOL: Cache readdir (if supported by kernel) (default: false)
• direct_io: deprecated - Bypass page cache. Use cache.files=off instead. (default: false)
• kernel_cache: deprecated - Do not invalidate data cache on file open. Use cache.files=full instead.
(default: false)
• auto_cache: deprecated - Invalidate data cache if file mtime or size change. Use cache.files=auto-full
instead. (default: false)
• async_read: deprecated - Perform reads asynchronously. Use async_read=true instead.
• sync_read: deprecated - Perform reads synchronously. Use async_read=false instead.
NOTE: Options are evaluated in the order listed so if the options are func.rmdir=rand,category.action=ff
the action category setting will override the rmdir setting.
Value Types
• BOOL = 'true' | 'false'
• INT = [0,MAX_INT]
• SIZE = 'NNM'; NN = INT, M = 'K' | 'M' | 'G' | 'T'
• STR = string
• FUNC = FUSE function
• CATEGORY = FUSE function category
• POLICY = mergerfs function policy
branches
The 'branches' (formerly 'srcmounts') argument is a colon (':') delimited list of paths to be pooled to‐
gether. It does not matter if the paths are on the same or different drives nor does it matter the
filesystem. Used and available space will not be duplicated for paths on the same device and any fea‐
tures which aren't supported by the underlying filesystem (such as file attributes or extended attribut‐
es) will return the appropriate errors.
To make it easier to include multiple branches mergerfs supports globbing (http://lin‐
ux.die.net/man/7/glob). The globbing tokens MUST be escaped when using via the shell else the shell it‐
self will apply the glob itself.
Each branch can have a suffix of =RW (read / write), =RO (read-only), or =NC (no create). These suffixes
work with globs as well and will apply to each path found. RW is the default behavior and those paths
will be eligible for all policy categories. RO will exclude those paths from create and action policies
(just as a filesystem being mounted ro would). NC will exclude those paths from create policies (you
can't create but you can change / delete).
# mergerfs -o allow_other,use_ino /mnt/disk\*:/mnt/cdrom /media/drives
The above line will use all mount points in /mnt prefixed with disk and the cdrom.
To have the pool mounted at boot or otherwise accessable from related tools use /etc/fstab.
# <file system> <mount point> <type> <options> <dump> <pass>
/mnt/disk*:/mnt/cdrom /media/drives fuse.mergerfs allow_other,use_ino 0 0
NOTE: the globbing is done at mount or xattr update time (see below). If a new directory is added match‐
ing the glob after the fact it will not be automatically included.
NOTE: for mounting via fstab to work you must have mount.fuse installed. For Ubuntu/Debian it is includ‐
ed in the fuse package.
fuse_msg_size
FUSE applications communicate with the kernel over a special character device: /dev/fuse. A large por‐
tion 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 appoximately 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 abstraction.
Since there should be no downsides to increasing fuse_msg_size / max_pages, outside a minor bump in RAM
usage due to larger message buffers, mergerfs defaults the value to 256. On kernels before 4.20 the val‐
ue has no effect. The reason the value is configurable is to enable experimentation and benchmarking.
See the nullrw section for benchmarking examples.
symlinkify
Due to the levels of indirection introduced by mergerfs and the underlying technology FUSE there can be
varying levels of performance degredation. This feature will turn non-directories which are not writable
into symlinks to the original file found by the readlink policy after the mtime and ctime are older than
the timeout.
WARNING: The current implementation has a known issue in which if the file is open and being used when
the file is converted to a symlink then the application which has that file open will receive an error
when using it. This is unlikely to occur in practice but is something to keep in mind.
WARNING: Some backup solutions, such as CrashPlan, do not backup the target of a symlink. If using this
feature it will be necessary to point any backup software to the original drives or configure the soft‐
ware to follow symlinks if such an option is available. Alternatively create two mounts. One for backup
and one for general consumption.
nullrw
Due to how FUSE works there is an overhead to all requests made to a FUSE filesystem. 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 perfor‐
mance boundries.
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.
Example:
$ dd if=/dev/zero of=/path/to/mergerfs/mount/benchmark ibs=1M obs=512 count=1024 iflag=dsync,nocache oflag=dsync,nocache conv=fdatasync status=progress
1024+0 records in
2097152+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 15.4067 s, 69.7 MB/s
$ dd if=/dev/zero of=/path/to/mergerfs/mount/benchmark ibs=1M obs=1M count=1024 iflag=dsync,nocache oflag=dsync,nocache conv=fdatasync status=progress
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.219585 s, 4.9 GB/s
$ dd if=/path/to/mergerfs/mount/benchmark of=/dev/null bs=512 count=102400 iflag=dsync,nocache oflag=dsync,nocache conv=fdatasync status=progress
102400+0 records in
102400+0 records out
52428800 bytes (52 MB, 50 MiB) copied, 0.757991 s, 69.2 MB/s
$ dd if=/path/to/mergerfs/mount/benchmark of=/dev/null bs=1M count=1024 iflag=dsync,nocache oflag=dsync,nocache conv=fdatasync status=progress
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.18405 s, 5.8 GB/s
It's important to test with different obs (output block size) values since the relative overhead is
greater with smaller values. As you can see above the size of a read or write can massively impact theo‐
retical performance. If an application performs much worse through mergerfs it could very well be that
it doesn't optimally size its read and write requests. In such cases contact the mergerfs author so it
can be investigated.
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.
noattr will cause mergerfs to short circuit all xattr calls and return ENOATTR where appropriate. merg‐
erfs still gets all the requests but they will not be forwarded on to the underlying filesystems. The
runtime control will still function in this mode.
nosys will cause mergerfs to return ENOSYS for any xattr call. The difference with noattr is that the
kernel will cache this fact and itself short circuit future calls. This will be more efficient than
noattr but will cause mergerfs' runtime control via the hidden file to stop working.
FUNCTIONS / POLICIES / CATEGORIES
The POSIX filesystem API is made up of a number of functions. creat, stat, chown, etc. 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 what file or directory is chosen when performing that
behavior. Any policy can be assigned to a function or category though some may not be very useful in
practice. For instance: rand (random) may be useful for file creation (create) but could lead to very
odd behavior if used for chmod if there were more than one copy of the file.
Some functions, listed in the category N/A below, can not be assigned the normal policies. All functions
which work on file handles use the handle which was acquired by open or create. readdir has no real need
for a policy given the purpose is merely to return a list of entries in a directory. statfs's behavior
can be modified via other options. That said many times the current FUSE kernel driver will not always
provide the file handle when a client calls fgetattr, fchown, fchmod, futimens, ftruncate, etc. This
means it will call the regular, path based, versions.
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 sub mounts won't be considered in the space calcula‐
tions. The reason is that it doesn't really work for non-path preserving policies and can lead to
non-obvious behaviors.
Function / Category classifications
Category FUSE Functions
──────────────────────────────────────────────────────────────────────────
action chmod, chown, link, removexattr, rename, rmdir, setxattr,
truncate, unlink, utimens
create create, mkdir, mknod, symlink
search access, getattr, getxattr, ioctl (directories), listxattr,
open, readlink
N/A fchmod, fchown, futimens, ftruncate, fallocate, fgetattr,
fsync, ioctl (files), read, readdir, release, statfs, write,
copy_file_range
In cases where something may be searched (to confirm a directory exists across all source mounts) getattr
will be used.
Path Preservation
Policies, as described below, are of two basic types. path preserving and non-path preserving.
All policies which start with ep (epff, eplfs, eplus, epmfs, eprand) are path preserving. ep stands for
existing path.
A path preserving policy will only consider drives where the relative path being accessed already exists.
When using non-path preserving policies paths will be cloned to target drives as necessary.
Filters
Policies basically search branches and create a list of files / paths for functions to work on. The pol‐
icy is responsible for filtering and sorting. The policy type defines the sorting but filtering is most‐
ly uniform as described below.
• No search policies filter.
• All action policies will filter out branches which are mounted read-only or tagged as RO (read-only).
• All create policies will filter out branches which are mounted read-only, tagged RO (read-only) or NC
(no create), or has available space less than minfreespace.
If all branches are filtered an error will be returned. Typically EROFS or ENOSPC depending on the rea‐
sons.
Policy descriptions
Policy Description
──────────────────────────────────────────────────────────────────────────
all Search category: same as epall. Action category:
same as epall. Create category: for mkdir, mknod,
and symlink it will apply to all branches. create
works like ff.
epall (existing Search category: same as epff (but more expensive be‐
path, all) cause it doesn't stop after finding a valid branch).
Action category: apply to all found. Create catego‐
ry: for mkdir, mknod, and symlink it will apply to
all found. create works like epff (but more expen‐
sive because 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 drive with the least free space.
space)
eplus (existing Of all the branches on which the relative path exists
path, least used choose the drive with the least used space.
space)
epmfs (existing Of all the branches on which the relative path exists
path, most free choose the drive with the most free space.
space)
eprand (existing Calls epall and then randomizes.
path, random)
erofs Exclusively return -1 with errno set to EROFS
(read-only filesystem).
ff (first found) Search category: same as epff. Action category: same
as epff. Create category: Given the order of the
drives, as defined at mount time or configured at
runtime, act on the first one found.
lfs (least free Search category: same as eplfs. Action category:
space) same as eplfs. Create category: Pick the drive with
the least available free space.
lus (least used Search category: same as eplus. Action category:
space) same as eplus. Create category: Pick the drive with
the least used space.
mfs (most free Search category: same as epmfs. Action category:
space) same as epmfs. Create category: Pick the drive with
the most available free space.
newest Pick the file / directory with the largest mtime.
rand (random) Calls all and then randomizes.
Defaults
Category Policy
──────────────────
action epall
create epmfs
search ff
ioctl
When ioctl is used with an open file then it will use the file handle which was created at the original
open call. However, when using ioctl with a directory mergerfs will use the open policy to find the di‐
rectory to act on.
unlink
In FUSE there is an opaque "file handle" which is created by open, create, or opendir, passed to the ker‐
nel, and then is passed back to the FUSE userland application by the kernel. Unfortunately, the FUSE
kernel driver does not always send the file handle when it theoretically could/should. This complicates
certain behaviors / workflows particularly in the high level API. As a result mergerfs is currently do‐
ing a few hacky things.
libfuse2 and libfuse3, when using the high level API, will rename names to .fuse_hiddenXXXXXX if the file
is open when unlinked or renamed over. It does this so the file is still available when a request refer‐
encing the now missing file is made. This file however keeps a rmdir from succeeding and can be picked
up by software reading directories.
The change mergerfs has done is that if a file is open when an unlink or rename happens it will open the
file and keep it open till closed by all those who opened it prior. When a request comes in referencing
that file and it doesn't include a file handle it will instead use the file handle created at unlink/re‐
name time.
This won't result in technically proper behavior but close enough for many usecases.
The plan is to rewrite mergerfs to use the low level API so these invasive libfuse changes are no longer
necessary.
rename & link
NOTE: If you're receiving errors from software when files are moved / renamed / linked then you should
consider changing the create policy to one which is not path preserving, enabling ignorepponrename, or
contacting the author of the offending software and requesting that EXDEV be properly handled.
rename and link are tricky functions in a union filesystem. rename only works within a single filesystem
or device. If a rename can't be done atomically due to the source and destination paths existing on dif‐
ferent mount points it will return -1 with errno = EXDEV (cross device). So if a rename's source and
target are on different drives within the pool it creates an issue.
Originally mergerfs would return EXDEV whenever a rename was requested which was cross directory in any
way. This made the code simple and was technically complient 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 run create policy
• If create policy returns the same drive as currently evaluating then clone the path
• Re-attempt rename
• If any of the renames succeed the higher level rename is considered a success
• If no renames succeed the first error encountered will be returned
• On success:
• Remove the target from all drives with no source file
• Remove the source from all drives which failed to rename
• If using a create policy which does not try to preserve directory paths
• Using the rename policy get the list of files to rename
• Using the getattr policy get the target path
• For each file attempt rename:
• If the source drive != target drive:
• Clone target path from target drive to source drive
• Rename
• If any of the renames succeed the higher level rename is considered a success
• If no renames succeed the first error encountered will be returned
• On success:
• Remove the target from all drives with no source file
• Remove the source from all drives which failed to rename
The the removals are subject to normal entitlement checks.
The above behavior will help minimize the likelihood of EXDEV being returned but it will still be possi‐
ble.
link uses the same strategy but without the removals.
readdir
readdir (http://linux.die.net/man/3/readdir) is different from all other filesystem functions. While it
could have it's own set of policies to tweak its behavior at this time it provides a simple union of
files and directories found. Remember that any action or information queried about these files and di‐
rectories come from the respective function. For instance: an ls is a readdir and for each file/directo‐
ry returned getattr is called. Meaning the policy of getattr is responsible for choosing the file/direc‐
tory which is the source of the metadata you see in an ls.
statfs / statvfs
statvfs (http://linux.die.net/man/2/statvfs) normalizes the source drives based on the fragment size and
sums the number of adjusted blocks and inodes. This means you will see the combined space of all
sources. Total, used, and free. The sources however are dedupped based on the drive so multiple sources
on the same drive will not result in double counting it's space. Filesystems mounted further down the
tree of the branch will not be included when checking the mount's stats.
The options statfs and statfs_ignore can be used to modify statfs behavior.
BUILDING
NOTE: Prebuilt packages can be found at: https://github.com/trapexit/mergerfs/releases
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
Fedora
$ su -
# cd mergerfs
# tools/install-build-pkgs
# make rpm
# rpm -i rpmbuild/RPMS/<arch>/mergerfs-<verion>.<arch>.rpm
Generically
Have git, g++, make, python installed.
$ cd mergerfs
$ make
$ sudo make install
Build options
$ make help
usage: make
make USE_XATTR=0 - build program without xattrs functionality
make STATIC=1 - build static binary
make LTO=1 - build with link time optimization
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 xattr -l /mountpoint/.mergerfs to see all supported keys. Some are informational and therefore
read-only. setxattr will return EINVAL on read-only keys.
Values
Same as the command line.
user.mergerfs.branches
NOTE: formerly user.mergerfs.srcmounts but said key is still supported.
Used to query or modify the list of branches. When modifying there are several shortcuts to easy manipu‐
lation 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] $ xattr -l .mergerfs
user.mergerfs.branches: /mnt/a=RW:/mnt/b=RW
user.mergerfs.minfreespace: 4294967295
user.mergerfs.moveonenospc: false
...
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.category.search .mergerfs
ff
[trapexit:/mnt/mergerfs] $ xattr -w user.mergerfs.category.search newest .mergerfs
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.category.search .mergerfs
newest
[trapexit:/mnt/mergerfs] $ xattr -w user.mergerfs.branches +/mnt/c .mergerfs
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.branches .mergerfs
/mnt/a:/mnt/b:/mnt/c
[trapexit:/mnt/mergerfs] $ xattr -w user.mergerfs.branches =/mnt/c .mergerfs
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.branches .mergerfs
/mnt/c
[trapexit:/mnt/mergerfs] $ xattr -w user.mergerfs.branches '+</mnt/a:/mnt/b' .mergerfs
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.branches .mergerfs
/mnt/a:/mnt/b:/mnt/c
file / directory xattrs
While they won't show up when using listxattr (http://linux.die.net/man/2/listxattr) mergerfs offers a
number of special xattrs to query information about the files served. To access the values you will need
to issue a getxattr (http://linux.die.net/man/2/getxattr) for one of the following:
• user.mergerfs.basepath: the base mount point for the file given the current getattr policy
• user.mergerfs.relpath: the relative path of the file from the perspective of the mount point
• user.mergerfs.fullpath: the full path of the original file given the getattr policy
• user.mergerfs.allpaths: a NUL ('') separated list of full paths to all files found
[trapexit:/mnt/mergerfs] $ ls
A B C
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.fullpath A
/mnt/a/full/path/to/A
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.basepath A
/mnt/a
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.relpath A
/full/path/to/A
[trapexit:/mnt/mergerfs] $ xattr -p user.mergerfs.allpaths A | tr '\0' '\n'
/mnt/a/full/path/to/A
/mnt/b/full/path/to/A
TOOLING
• https://github.com/trapexit/mergerfs-tools
• mergerfs.ctl: A tool to make it easier to query and configure mergerfs at runtime
• mergerfs.fsck: Provides permissions and ownership auditing and the ability to fix them
• mergerfs.dedup: Will help identify and optionally remove duplicate files
• mergerfs.dup: Ensure there are at least N copies of a file across the pool
• mergerfs.balance: Rebalance files across drives by moving them from the most filled to the least filled
• mergerfs.mktrash: Creates FreeDesktop.org Trash specification compatible directories on a mergerfs
mount
• 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
tl;dr: * cache.files=off: Disables page caching. Underlying files cached, mergerfs files are not. *
cache.files=partial: Enables page caching. Underlying files cached, mergerfs files cached while open. *
cache.files=full: Enables page caching. Underlying files cached, mergerfs files cached across opens. *
cache.files=auto-full: Enables page caching. Underlying files cached, mergerfs files cached across opens
if mtime and size are unchanged since previous open. * cache.files=libfuse: follow traditional libfuse
direct_io, 'kernel_cache, andauto_cache` arguments.
FUSE, which mergerfs uses, offers a number of page caching modes. mergerfs tries to simplify their use
via the cache.files option. It can and should replace usage of direct_io, kernel_cache, and auto_cache.
Due to mergerfs using FUSE and therefore being a userland process proxying existing filesystems the ker‐
nel will double cache the content being read and written through mergerfs. Once from the underlying
filesystem and once from mergerfs (it sees them as two separate entities). Using cache.files=off will
keep the double caching from happening by disabling caching of mergerfs but this has the side effect that
all read and write calls will be passed to mergerfs which may be slower than enabling caching, you lose
shared mmap support which can affect apps such as rtorrent, and no read-ahead will take place. The ker‐
nel 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 drop‐
cacheonclose 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 no‐
cache project.
If most files are read once through and closed (like media) it is best to enable dropcacheonclose regard‐
less of caching mode in order to minimize buffer bloat.
It is difficult to balance memory usage, cache bloat & duplication, and performance. Ideally mergerfs
would be able to disable caching for the files it reads/writes but allow page caching for itself. That
would limit the FUSE overhead. However, there isn't a good way to achieve this. It would need to open
all files with O_DIRECT which places limitations on the what underlying filesystems would be supported
and complicates the code.
kernel documenation: https://www.kernel.org/doc/Documentation/filesystems/fuse-io.txt
entry & attribute caching
Given the relatively high cost of FUSE due to the kernel <-> userspace round trips there are kernel side
caches for file entries and attributes. The entry cache limits the lookup calls to mergerfs which ask if
a file exists. The attribute cache limits the need to make getattr calls to mergerfs which provide file
attributes (mode, size, type, etc.). As with the page cache these should not be used if the underlying
filesystems are being manipulated at the same time as it could lead to odd behavior or data corruption.
The options for setting these are cache.entry and cache.negative_entry for the entry cache and cache.attr
for the attributes cache. cache.negative_entry refers to the timeout for negative responses to lookups
(non-existant files).
policy caching
Policies are run every time a function (with a policy as mentioned above) is called. These policies can
be expensive depending on mergerfs' setup and client usage patterns. Generally we wouldn't want to cache
policy results because it may result in stale responses if the underlying drives are used directly.
The open policy cache will cache the result of an open policy for a particular input for cache.open sec‐
onds or until the file is unlinked. Each file close (release) will randomly chose to clean up the cache
of expired entries.
This cache is really only useful in cases where you have a large number of branches and open is called on
the same files repeatedly (like Transmission which opens and closes a file on every read/write presumably
to keep file handle usage low).
statfs caching
Of the syscalls used by mergerfs in policies the statfs / statvfs call is perhaps the most expensive.
It's used to find out the available space of a drive and whether it is mounted read-only. Depending on
the setup and usage pattern these queries can be relatively costly. When cache.statfs is enabled all
calls to statfs by a policy will be cached for the number of seconds its set to.
Example: If the create policy is mfs and the timeout is 60 then for that 60 seconds the same drive will
be returned as the target for creates because the available space won't be updated for that time.
symlink caching
As of version 4.20 Linux supports symlink caching. Significant performance increases can be had in work‐
loads which use a lot of symlinks. Setting cache.symlinks=true will result in requesting symlink caching
from the kernel only if supported. As a result its safe to enable it on systems prior to 4.20. That
said it is disabled by default for now. You can see if caching is enabled by querying the xattr
user.mergerfs.cache.symlinks but given it must be requested at startup you can not change it at runtime.
readdir caching
As of version 4.20 Linux supports readdir caching. This can have a significant impact on directory tra‐
versal. 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 con‐
figuarable at runtime via xattr user.mergerfs.cache.readdir.
writeback caching
writeback caching is a technique for improving write speeds by batching writes at a faster device and
then bulk writing to the slower device. With FUSE the kernel will wait for a number of writes to be made
and then send it to the filesystem as one request. mergerfs currently uses a modified and vendored lib‐
fuse 2.9.7 which does not support writeback caching. Adding said feature should not be difficult but
benchmarking needs to be done to see if what effect it will have.
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 tradi‐
tional HDDs for instance.
MergerFS does not natively support any sort of tiered caching. Most users have no use for such a feature
and its inclusion would complicate the code. However, there are a few situations where a cache drive
could help with a typical mergerfs setup.
1. Fast network, slow drives, many readers: You've a 10+Gbps network with many readers and your regular
drives can't keep up.
2. Fast network, slow drives, small'ish bursty writes: You have a 10+Gbps network and wish to transfer
amounts of data less than your cache drive but wish to do so quickly.
With #1 its arguable if you should be using mergerfs at all. RAID would probably be the better solution.
If you're going to use mergerfs there are other tactics that may help: spreading the data across drives
(see the mergerfs.dup tool) and setting func.open=rand, using symlinkify, or using dm-cache or a similar
technology to add tiered cache to the underlying device.
With #2 one could use dm-cache as well but there is another solution which requires only mergerfs and a
cronjob.
1. Create 2 mergerfs pools. One which includes just the slow drives and one which has both the fast dri‐
ves (SSD,NVME,etc.) and slow drives.
2. The 'cache' pool should have the cache drives listed first.
3. The best create policies to use for the 'cache' pool would probably be ff, epff, lfs, or eplfs. The
latter two under the assumption that the cache drive(s) are far smaller than the backing drives. If
using path preserving policies remember that you'll need to manually create the core directories of
those paths you wish to be cached. Be sure the permissions are in sync. Use mergerfs.fsck to check /
correct them. You could also tag the slow drives as =NC though that'd mean if the cache drives fill
you'd get "out of space" errors.
4. Enable moveonenospc and set minfreespace appropriately. Perhaps setting minfreespace to the size of
the largest cache drive.
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".
#!/bin/bash
if [ $# != 3 ]; then
echo "usage: $0 <cache-drive> <backing-pool> <days-old>"
exit 1
fi
CACHE="${1}"
BACKING="${2}"
N=${3}
find "${CACHE}" -type f -atime +${N} -printf '%P\n' | \
rsync --files-from=- -axqHAXWES --preallocate --remove-source-files "${CACHE}/" "${BACKING}/"
percentage full expiring
Move the oldest file from the cache to the backing pool. Continue till below percentage threshold.
#!/bin/bash
if [ $# != 3 ]; then
echo "usage: $0 <cache-drive> <backing-pool> <percentage>"
exit 1
fi
CACHE="${1}"
BACKING="${2}"
PERCENTAGE=${3}
set -o errexit
while [ $(df --output=pcent "${CACHE}" | grep -v Use | cut -d'%' -f1) -gt ${PERCENTAGE} ]
do
FILE=$(find "${CACHE}" -type f -printf '%A@ %P\n' | \
sort | \
head -n 1 | \
cut -d' ' -f2-)
test -n "${FILE}"
rsync -axqHAXWES --preallocate --remove-source-files "${CACHE}/./${FILE}" "${BACKING}/"
done
TIPS / NOTES
• use_ino will only work when used with mergerfs 2.18.0 and above.
• Run mergerfs as root (with allow_other) unless you're merging paths which are owned by the same user
otherwise strange permission issues may arise.
• https://github.com/trapexit/backup-and-recovery-howtos : A set of guides / howtos on creating a data
storage system, backing it up, maintaining it, and recovering from failure.
• If you don't see some directories and files you expect in a merged point or policies seem to skip dri‐
ves be sure the user has permission to all the underlying directories. Use mergerfs.fsck to audit the
drive for out of sync permissions.
• Do not use cache.files=off or direct_io if you expect applications (such as rtorrent) to mmap
(http://linux.die.net/man/2/mmap) files. Shared mmap is not currently supported in FUSE w/ direct_io
enabled. Enabling dropcacheonclose is recommended when cache.files=partial|full|auto-full or di‐
rect_io=false.
• Since POSIX functions give only a singular error or success its difficult to determine the proper be‐
havior when applying the function to multiple targets. mergerfs will return an error only if all at‐
tempts of an action fail. Any success will lead to a success returned. This means however that some
odd situations may arise.
• Kodi (http://kodi.tv), Plex (http://plex.tv), Subsonic (http://subsonic.org), etc. can use directory
mtime (http://linux.die.net/man/2/stat) to more efficiently determine whether to scan for new content
rather than simply performing a full scan. If using the default getattr policy of ff its possible
those programs will miss an update on account of it returning the first directory found's stat info and
its a later directory on another mount which had the mtime recently updated. To fix this you will want
to set func.getattr=newest. Remember though that this is just stat. If the file is later open'ed or
unlink'ed and the policy is different for those then a completely different file or directory could be
acted on.
• Some policies mixed with some functions may result in strange behaviors. Not that some of these behav‐
iors 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
directory mtime is not being updated
Remember that the default policy for getattr is ff. The information for the first directory found will
be returned. If it wasn't the directory which had been updated then it will appear outdated.
The reason this is the default is because any other policy would be more expensive and for many applica‐
tions it is unnecessary. To always return the directory with the most recent mtime or a faked value
based on all found would require a scan of all drives.
If you always want the directory information from the one with the most recent mtime then use the newest
policy for getattr.
mv /mnt/pool/foo /mnt/disk1/foo removes foo
This is not a bug.
Run in verbose mode to better undertand what's happening:
$ mv -v /mnt/pool/foo /mnt/disk1/foo
copied '/mnt/pool/foo' -> '/mnt/disk1/foo'
removed '/mnt/pool/foo'
$ ls /mnt/pool/foo
ls: cannot access '/mnt/pool/foo': No such file or directory
mv, when working across devices, is copying the source to target and then removing the source. Since the
source is the target in this case, depending on the unlink policy, it will remove the just copied file
and other files across the branches.
If you want to move files to one drive just copy them there and use mergerfs.dedup to clean up the old
paths or manually remove them from the branches directly.
cached memory appears greater than it should be
Use cache.files=off or direct_io=true. See the section on page caching.
NFS clients returning ESTALE / Stale file handle
Be sure to use noforget and use_ino arguments.
NFS clients don't work
Some NFS clients appear to fail when a mergerfs mount is exported. Kodi in particular seems to have is‐
sues.
Try enabling the use_ino option. Some have reported that it fixes the issue.
rtorrent fails with ENODEV (No such device)
Be sure to set cache.files=partial|full|auto-full or turn off direct_io. rtorrent and some other appli‐
cations use mmap (http://linux.die.net/man/2/mmap) to read and write to files and offer no failback to
traditional methods. FUSE does not currently support mmap while using direct_io. There may be a perfor‐
mance 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 merg‐
erfs twice. Once with direct_io enabled and one without it. Be sure to set dropcacheonclose=true if not
using direct_io.
rtorrent fails with files >= 4GiB
This is a kernel bug with mmap and FUSE on 32bit platforms. A fix should become available for all LTS
releases.
https://marc.info/?l=linux-fsdevel&m=155550785230874&w=2
Plex doesn't work with mergerfs
It does. If you're trying to put Plex's config / metadata on mergerfs you have to leave direct_io off
because Plex is using sqlite which apparently needs mmap. mmap doesn't work with direct_io. To fix this
place the data elsewhere or disable direct_io (with dropcacheonclose=true).
If the issue is that scanning doesn't seem to pick up media then be sure to set func.getattr=newest as
mentioned above.
mmap performance is really bad
There is a bug (https://lkml.org/lkml/2016/3/16/260) in caching which affects overall performance of mmap
through FUSE in Linux 4.x kernels. It is fixed in 4.4.10 and 4.5.4 (https://lkml.org/lkml/2016/5/11/59).
When a program tries to move or rename a file it fails
Please read the section above regarding rename & link (#rename--link).
The problem is that many applications do not properly handle EXDEV errors which rename and link may re‐
turn even though they are perfectly valid situations which do not indicate actual drive or OS errors.
The error will only be returned by mergerfs if using a path preserving policy as described in the policy
section above. If you do not care about path preservation simply change the mergerfs policy to the
non-path preserving version. For example: -o category.create=mfs
Ideally the offending software would be fixed and it is recommended that if you run into this problem you
contact the software's author and request proper handling of EXDEV errors.
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 it's goal. In the case of mv
or similar it tries rename and on EXDEV falls back to a manual copying of data between the two locations
and unlinking the source. In these older versions of GVFS-fuse if it received EXDEV it would translate
that into EIO. This would cause mv or most any application attempting to move files around on that SMB
share to fail with a IO error.
GVFS-fuse v1.22.0 (https://bugzilla.gnome.org/show_bug.cgi?id=734568) and above fixed this issue but a
large number of systems use the older release. On Ubuntu the version can be checked by issuing
apt-cache showpkg gvfs-fuse. Most distros released in 2015 seem to have the updated release and will
work fine but older systems may not. Upgrading gvfs-fuse or the distro in general will address the prob‐
lem.
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 situtation well. This is
unfortunately a common failure of software which moves files around. The standard indicates that an im‐
plementation MAY choose to support non-user home directory trashing of files (which is a MUST). The im‐
plementation MAY also support "top directory trashes" which many probably do.
To create a $topdir/.Trash directory as defined in the standard use the mergerfs-tools
(https://github.com/trapexit/mergerfs-tools) tool mergerfs.mktrash.
tar: Directory renamed before its status could be extracted
Make sure to use the use_ino option.
Supplemental user groups
Due to the overhead of getgroups/setgroups (http://linux.die.net/man/2/setgroups) mergerfs utilizes a
cache. This cache is opportunistic and per thread. Each thread will query the supplemental groups for a
user when that particular thread needs to change credentials and will keep that data for the lifetime of
the thread. This means that if a user is added to a group it may not be picked up without the restart of
mergerfs. However, since the high level FUSE API's (at least the standard version) thread pool dynami‐
cally grows and shrinks it's possible that over time a thread will be killed and later a new thread with
no cache will start and query the new data.
The gid cache uses fixed storage to simplify the design and be compatible with older systems which may
not have C++11 compilers. There is enough storage for 256 users' supplemental groups. Each user is al‐
lowed upto 32 supplemental groups. Linux >= 2.6.3 allows upto 65535 groups per user but most other *nixs
allow far less. NFS allowing only 16. The system does handle overflow gracefully. If the user has more
than 32 supplemental groups only the first 32 will be used. If more than 256 users are using the system
when an uncached user is found it will evict an existing user's cache at random. So long as there aren't
more than 256 active users this should be fine. If either value is too low for your needs you will have
to modify gidcache.hpp to increase the values. Note that doing so will increase the memory needed by
each thread.
mergerfs or libfuse crashing
NOTE: as of mergerfs 2.22.0 it includes the most recent version of libfuse (or requires libfuse-2.9.7) so
any crash should be reported. For older releases continue reading...
If suddenly the mergerfs mount point disappears and Transport endpoint is not connected is returned when
attempting to perform actions within the mount directory and the version of libfuse (use mergerfs -v to
find the version) is older than 2.9.4 its likely due to a bug in libfuse. Affected versions of libfuse
can be found in Debian Wheezy, Ubuntu Precise and others.
In order to fix this please install newer versions of libfuse. If using a Debian based distro (De‐
bian,Ubuntu,Mint) you can likely just install newer versions of libfuse (https://packages.debian.org/un‐
stable/libfuse2) and fuse (https://packages.debian.org/unstable/fuse) from the repo of a newer release.
mergerfs appears to be crashing or exiting
There seems to be an issue with Linux version 4.9.0 and above in which an invalid message appears to be
transmitted to libfuse (used by mergerfs) causing it to exit. No messages will be printed in any logs as
its not a proper crash. Debugging of the issue is still ongoing and can be followed via the fuse-devel
thread (https://sourceforge.net/p/fuse/mailman/message/35662577).
mergerfs under heavy load and memory preasure leads to kernel panic
https://lkml.org/lkml/2016/9/14/527
[25192.515454] kernel BUG at /build/linux-a2WvEb/linux-4.4.0/mm/workingset.c:346!
[25192.517521] invalid opcode: 0000 [#1] SMP
[25192.519602] Modules linked in: netconsole ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 configfs binfmt_misc veth bridge stp llc nf_conntrack_ipv6 nf_defrag_ipv6 xt_conntrack ip6table_filter ip6_tables xt_multiport iptable_filter ipt_MASQUERADE nf_nat_masquerade_ipv4 xt_comment xt_nat iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack xt_CHECKSUM xt_tcpudp iptable_mangle ip_tables x_tables intel_rapl x86_pkg_temp_thermal intel_powerclamp eeepc_wmi asus_wmi coretemp sparse_keymap kvm_intel ppdev kvm irqbypass mei_me 8250_fintek input_leds serio_raw parport_pc tpm_infineon mei shpchp mac_hid parport lpc_ich autofs4 drbg ansi_cprng dm_crypt algif_skcipher af_alg btrfs raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid0 multipath linear raid10 raid1 i915 crct10dif_pclmul crc32_pclmul aesni_intel i2c_algo_bit aes_x86_64 drm_kms_helper lrw gf128mul glue_helper ablk_helper syscopyarea cryptd sysfillrect sysimgblt fb_sys_fops drm ahci r8169 libahci mii wmi fjes video [last unloaded: netconsole]
[25192.540910] CPU: 2 PID: 63 Comm: kswapd0 Not tainted 4.4.0-36-generic #55-Ubuntu
[25192.543411] Hardware name: System manufacturer System Product Name/P8H67-M PRO, BIOS 3904 04/27/2013
[25192.545840] task: ffff88040cae6040 ti: ffff880407488000 task.ti: ffff880407488000
[25192.548277] RIP: 0010:[<ffffffff811ba501>] [<ffffffff811ba501>] shadow_lru_isolate+0x181/0x190
[25192.550706] RSP: 0018:ffff88040748bbe0 EFLAGS: 00010002
[25192.553127] RAX: 0000000000001c81 RBX: ffff8802f91ee928 RCX: ffff8802f91eeb38
[25192.555544] RDX: ffff8802f91ee938 RSI: ffff8802f91ee928 RDI: ffff8804099ba2c0
[25192.557914] RBP: ffff88040748bc08 R08: 000000000001a7b6 R09: 000000000000003f
[25192.560237] R10: 000000000001a750 R11: 0000000000000000 R12: ffff8804099ba2c0
[25192.562512] R13: ffff8803157e9680 R14: ffff8803157e9668 R15: ffff8804099ba2c8
[25192.564724] FS: 0000000000000000(0000) GS:ffff88041f280000(0000) knlGS:0000000000000000
[25192.566990] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[25192.569201] CR2: 00007ffabb690000 CR3: 0000000001e0a000 CR4: 00000000000406e0
[25192.571419] Stack:
[25192.573550] ffff8804099ba2c0 ffff88039e4f86f0 ffff8802f91ee928 ffff8804099ba2c8
[25192.575695] ffff88040748bd08 ffff88040748bc58 ffffffff811b99bf 0000000000000052
[25192.577814] 0000000000000000 ffffffff811ba380 000000000000008a 0000000000000080
[25192.579947] Call Trace:
[25192.582022] [<ffffffff811b99bf>] __list_lru_walk_one.isra.3+0x8f/0x130
[25192.584137] [<ffffffff811ba380>] ? memcg_drain_all_list_lrus+0x190/0x190
[25192.586165] [<ffffffff811b9a83>] list_lru_walk_one+0x23/0x30
[25192.588145] [<ffffffff811ba544>] scan_shadow_nodes+0x34/0x50
[25192.590074] [<ffffffff811a0e9d>] shrink_slab.part.40+0x1ed/0x3d0
[25192.591985] [<ffffffff811a53da>] shrink_zone+0x2ca/0x2e0
[25192.593863] [<ffffffff811a64ce>] kswapd+0x51e/0x990
[25192.595737] [<ffffffff811a5fb0>] ? mem_cgroup_shrink_node_zone+0x1c0/0x1c0
[25192.597613] [<ffffffff810a0808>] kthread+0xd8/0xf0
[25192.599495] [<ffffffff810a0730>] ? kthread_create_on_node+0x1e0/0x1e0
[25192.601335] [<ffffffff8182e34f>] ret_from_fork+0x3f/0x70
[25192.603193] [<ffffffff810a0730>] ? kthread_create_on_node+0x1e0/0x1e0
There is a bug in the kernel. A work around appears to be turning off splice. Don't add the splice_*
arguments or add no_splice_write,no_splice_move,no_splice_read. This, however, is not guaranteed to
work.
rm: fts_read failed: No such file or directory
NOTE: This is only relevant to mergerfs versions at or below v2.25.x and should not occur in more recent
versions. See the notes on unlink.
Not really a bug. The FUSE library will move files when asked to delete them as a way to deal with cer‐
tain edge cases and then later delete that file when its clear the file is no longer needed. This howev‐
er can lead to two issues. One is that these hidden files are noticed by rm -rf or find when scanning
directories and they may try to remove them and they might have disappeared already. There is nothing
wrong about this happening but it can be annoying. The second issue is that a directory might not be
able to removed on account of the hidden file being still there.
Using the hard_remove option will make it so these temporary files are not used and files are deleted im‐
medately. That has a side effect however. Files which are unlinked and then they are still used (in
certain forms) will result in an error (ENOENT).
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 is it's sole solution for pooling drives.
Can mergerfs be used with drives which already have data / are in
use?
Yes. MergerFS is a proxy and does NOT interfere with the normal form or function of the drives / mounts
/ paths it manages.
MergerFS is not a traditional filesystem. MergerFS is not RAID. It does not manipulate the data that
passes through it. It does not shard data across drives. It merely shards some behavior and aggregates
others.
Can mergerfs be removed without affecting the data?
See the previous question's answer.
Do hard links work?
Yes. You need to use use_ino to support proper reporting of inodes.
What mergerfs does not do is fake hard links across branches. Read the section "rename & link" for how
it works.
Does mergerfs support CoW / copy-on-write?
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.ubuntu.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 hardlink‐
ing duplicate files but wish to treat each name as if it were a unique and separate file.
Why can't I see my files / directories?
It's almost always a permissions issue. Unlike mhddfs, which runs as root and attempts to access content
as such, mergerfs always changes it's credentials to that of the caller. This means that if the user
does not have access to a file or directory than neither will mergerfs. However, because mergerfs is
creating a union of paths it may be able to read some files and directories on one drive but not another
resulting in an incomplete set.
Whenever you run into a split permission issue (seeing some but not all files) try using mergerfs.fsck
(https://github.com/trapexit/mergerfs-tools) tool to check for and fix the mismatch. If you aren't see‐
ing anything at all be sure that the basic permissions are correct. The user and group values are cor‐
rect and that directories have their executable bit set. A common mistake by users new to Linux is to
chmod -R 644 when they should have chmod -R u=rwX,go=rX.
If using a network filesystem such as NFS, SMB, CIFS (Samba) be sure to pay close attention to anything
regarding permissioning and users. Root squashing and user translation for instance has bitten a few
mergerfs users. Some of these also affect the use of mergerfs from container platforms such as Docker.
Why is only one drive being used?
Are you using a path preserving policy? The default policy for file creation is epmfs. That means only
the drives with the path preexisting will be considered when creating a file. If you don't care about
where files and directories are created you likely shouldn't be using a path preserving policy and in‐
stead something like mfs.
This can be especially apparent when filling an empty pool from an external source. If you do want path
preservation you'll need to perform the manual act of creating paths on the drives you want the data to
land on before transfering your data. Setting func.mkdir=epall can simplify managing path perservation
for create.
Why was libfuse embedded into mergerfs?
1. A significant number of users use mergerfs on distros with old versions of libfuse which have serious
bugs. Requiring updated versions of libfuse on those distros isn't pratical (no package offered, user
inexperience, etc.). The only practical way to provide a stable runtime on those systems was to "ven‐
dor" / embed the library into the project.
2. mergerfs was written to use the high level API. There are a number of limitations in the HLAPI that
make certain features difficult or impossible to implement. While some of these features could be
patched into newer versions of libfuse without breaking the public API some of them would require
hacky code to provide backwards compatibility. While it may still be worth working with upstream to
address these issues in future versions, since the library needs to be vendored for stability and com‐
patibility reasons it is preferable / easier to modify the API. Longer term the plan is to rewrite
mergerfs to use the low level API.
Why did support for system libfuse get removed?
See above first.
If/when mergerfs is rewritten to use the low-level API then it'll be plausible to support system libfuse
but till then its simply too much work to manage the differences across the versions.
Why use mergerfs over mhddfs?
mhddfs is no longer maintained and has some known stability and security issues (see below). MergerFS
provides a superset of mhddfs' features and should offer the same or maybe better performance.
Below is an example of mhddfs and mergerfs setup to work similarly.
mhddfs -o mlimit=4G,allow_other /mnt/drive1,/mnt/drive2 /mnt/pool
mergerfs -o minfreespace=4G,allow_other,category.create=ff /mnt/drive1:/mnt/drive2 /mnt/pool
Why use mergerfs over aufs?
aufs is mostly abandoned and no longer available in many distros.
While aufs can offer better peak performance mergerfs provides more configurability and is generally eas‐
ier to use. mergerfs however does not offer the overlay / copy-on-write (CoW) features which aufs and
overlayfs have.
Why use mergerfs over unionfs?
UnionFS is more like aufs then mergerfs in that it offers overlay / CoW features. If you're just looking
to create a union of drives and want flexibility in file/directory placement then mergerfs offers that
whereas unionfs is more for overlaying RW filesystems over RO ones.
Why use mergerfs over LVM/ZFS/BTRFS/RAID0 drive concatenation /
striping?
With simple JBOD / drive concatenation / stripping / RAID0 a single drive failure will result in full
pool failure. mergerfs performs a similar behavior without the possibility of catastrophic failure and
the difficulties in recovery. Drives may fail however all other data will continue to be accessable.
When combined with something like SnapRaid (http://www.snapraid.it) and/or an offsite backup solution you
can have the flexibilty of JBOD without the single point of failure.
Why use mergerfs over ZFS?
MergerFS is not intended to be a replacement for ZFS. MergerFS is intended to provide flexible pooling
of arbitrary drives (local or remote), of arbitrary sizes, and arbitrary filesystems. For
write once, read many usecases such as bulk media storage. Where data integrity and backup is managed in
other ways. In that situation ZFS can introduce major maintance and cost burdens as described here
(http://louwrentius.com/the-hidden-cost-of-using-zfs-for-your-home-nas.html).
Can drives be written to directly? Outside of mergerfs while pooled?
Yes, however its not recommended to use the same file from within the pool and from without at the same
time. Especially if using caching of any kind (cache.files, cache.entry, cache.attr, cache.negative_en‐
try, cache.symlinks, cache.readdir, etc.).
Why do I get an out of space / no space left on device / ENOSPC
error even though there appears to be lots of space available?
First make sure you've read the sections above about policies, path preservation, branch filtering, and
the options minfreespace, moveonenospc, statfs, and statfs_ignore.
mergerfs is simply presenting a union of the content within multiple branches. The reported free space
is an aggregate of space available within the pool (behavior modified by statfs and statfs_ignore). It
does not represent a contiguous space. In the same way that read-only filesystems, those with quotas, or
reserved space report the full theoretical space available.
Due to path preservation, branch tagging, read-only status, and minfreespace settings it is perfectly
valid that ENOSPC / "out of space" / "no space left on device" be returned. It is doing what was asked
of it: filtering possible branches due to those settings. Only one error can be returned and if one of
the reasons for filtering a branch was minfreespace then it will be returned as such. moveonenospc is
only relevant to writing a file which is too large for the drive its currently on.
It is also possible that the filesystem selected has run out of inodes. Use df -i to list the total and
available inodes per filesystem.
If you don't care about path preservation then simply change the create policy to one which isn't. mfs
is probably what most are looking for. The reason its 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 merg‐
erfs 3.
Can mergerfs mounts be exported over NFS?
Yes. Due to current usage of libfuse by mergerfs and how NFS interacts with it it is necessary to add
noforget to mergerfs options to keep from getting "stale file handle" errors.
Some clients (Kodi) have issues in which the contents of the NFS mount will not be presented but users
have found that enabling the use_ino option often fixes that problem.
Can mergerfs mounts be exported over Samba / SMB?
Yes. While some users have reported problems it appears to always be related to how Samba is setup in
relation to permissions.
How are inodes calculated?
mergerfs-inode = (original-inode | (device-id << 32))
While ino_t is 64 bits only a few filesystems use more than 32. Similarly, while dev_t is also 64 bits
it was traditionally 16 bits. Bitwise or'ing them together should work most of the time. While totally
unique inodes are preferred the overhead which would be needed does not seem to outweighted by the bene‐
fits.
While atypical, yes, inodes can be reused and not refer to the same file. The internal id used to refer‐
ence a file in FUSE is different from the inode value presented. The former is the nodeid and is actual‐
ly a tuple of (nodeid,generation). That tuple is not user facing. The inode is merely metadata passed
through the kernel and found using the stat family of calls or readdir.
From FUSE docs regarding use_ino:
Honor the st_ino field in the functions getattr() and
fill_dir(). This value is used to fill in the st_ino field
in the stat(2), lstat(2), fstat(2) functions and the d_ino
field in the readdir(2) function. The filesystem does not
have to guarantee uniqueness, however some applications
rely on this value being unique for the whole filesystem.
Note that this does *not* affect the inode that libfuse
and the kernel use internally (also called the "nodeid").
I notice massive slowdowns of writes over NFS
Due to how NFS works and interacts with FUSE when not using cache.files=off or direct_io its possible
that a getxattr for security.capability will be issued prior to any write. This will usually result in a
massive slowdown for writes. Using cache.files=off or direct_io will keep this from happening (and gen‐
erally good to enable unless you need the features it disables) but the security_capability option can
also help by short circuiting the call and returning ENOATTR.
You could also set xattr to noattr or nosys to short circuit or stop all xattr requests.
What are these .fuse_hidden files?
NOTE: mergerfs >= 2.26.0 will not have these temporary files. See the notes on unlink.
When not using hard_remove libfuse will create .fuse_hiddenXXXXXXXX files when an opened file is un‐
linked. This is to simplify "use after unlink" usecases. There is a possibility these files end up be‐
ing picked up by software scanning directories and not ignoring hidden files. This is rarely a problem
but a solution is in the works.
The files are cleaned up once the file is finally closed. Only if mergerfs crashes or is killed would
they be left around. They are safe to remove as they are already unlinked files.
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/trapexit/mhddfs/blob/cae96e6251dd91e2bdc24800b4a18a74044f6672/src/main.c#L319) and if
it returns 0 then it will chown (http://linux.die.net/man/1/chown) the file. Not only is that a race
condition but it doesn't handle other situations. Rather than attempting to simulate POSIX ACL behavior
the proper way to manage this is to use seteuid (http://linux.die.net/man/2/seteuid) and setegid
(http://linux.die.net/man/2/setegid), in effect becoming the user making the original call, and perform
the action as them. This is what mergerfs does and why mergerfs should always run as root.
In Linux setreuid syscalls apply only to the thread. GLIBC hides this away by using realtime signals to
inform all threads to change credentials. Taking after Samba, mergerfs uses syscall(SYS_setreuid,...) to
set the callers credentials for that thread only. Jumping back to root as necessary should escalated
privileges be needed (for instance: to clone paths between drives).
For non-Linux systems mergerfs uses a read-write lock and changes credentials only when necessary. If
multiple threads are to be user X then only the first one will need to change the processes credentials.
So long as the other threads need to be user X they will take a readlock allowing multiple threads to
share the credentials. Once a request comes in to run as user Y that thread will attempt a write lock
and change to Y's credentials when it can. If the ability to give writers priority is supported then
that flag will be used so threads trying to change credentials don't starve. This isn't the best solu‐
tion but should work reasonably well assuming there are few users.
PERFORMANCE TWEAKING
NOTE: be sure to read about these features before changing them
• enable (or disable) splice_move, splice_read, and splice_write
• increase cache timeouts cache.attr, cache.entry, cache.negative_entry
• enable (or disable) page caching (cache.files)
• enable cache.open
• enable cache.statfs
• enable cache.symlinks
• enable cache.readdir
• change the number of worker threads
• disable security_capability and/or xattr
• disable posix_acl
• disable async_read
• test theoretical performance using nullrw or mounting a ram disk
• use symlinkify if your data is largely static
• use tiered cache drives
• use lvm and lvm cache to place a SSD in front of your HDDs (howto coming)
SUPPORT
Filesystems are very complex and difficult to debug. mergerfs, while being just a proxy of sorts, is al‐
so very difficult to debug given the large number of possible settings it can have itself and the massive
number of environments it can run in. When reporting on a suspected issue please, please include as much
of the below information as possible otherwise it will be difficult or impossible to diagnose. Also
please make sure to read all of the above documentation as it includes nearly every known system or user
issue previously encountered.
Information to include in bug reports
• Version of mergerfs: mergerfs -V
• mergerfs settings: from /etc/fstab or command line execution
• Version of Linux: uname -a
• Versions of any additional software being used
• List of drives, their filesystems, and sizes (before and after issue): df -h
• A strace of the app having problems:
• strace -f -o /tmp/app.strace.txt <cmd>
• A strace of mergerfs while the program is trying to do whatever it's failing to do:
• strace -f -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 simplist way using standard programs.
Contact / Issue submission
• github.com: https://github.com/trapexit/mergerfs/issues
• email: trapexit@spawn.link
• twitter: https://twitter.com/_trapexit
• reddit: https://www.reddit.com/user/trapexit
• discord: https://discord.gg/MpAr69V
Support development
This software is free to use and released under a very liberal license. That said if you like this soft‐
ware and would like to support its development donations are welcome.
• PayPal: https://paypal.me/trapexit
• Patreon: https://www.patreon.com/trapexit
• SubscribeStar: https://www.subscribestar.com/trapexit
• Bitcoin (BTC): 12CdMhEPQVmjz3SSynkAEuD5q9JmhTDCZA
• Bitcoin Cash (BCH): 1AjPqZZhu7GVEs6JFPjHmtsvmDL4euzMzp
• Ethereum (ETH): 0x09A166B11fCC127324C7fc5f1B572255b3046E94
• Litecoin (LTC): LXAsq6yc6zYU3EbcqyWtHBrH1Ypx4GjUjm
LINKS
• https://spawn.link
• https://github.com/trapexit/mergerfs
• https://github.com/trapexit/mergerfs-tools
• https://github.com/trapexit/scorch
• https://github.com/trapexit/bbf
• https://github.com/trapexit/backup-and-recovery-howtos
AUTHORS
Antonio SJ Musumeci <trapexit@spawn.link>.
mergerfs user manual 2019-06-03 mergerfs(1)