Provided by: mergerfs_2.21.0-1_amd64 
NAME
mergerfs - a featureful union filesystem
SYNOPSIS
mergerfs -o<options> <srcmounts> <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
• 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 readonly and read/write drives
• Turn read-only files into symlinks to increase read performance
OPTIONS
mount options
• defaults: a shortcut for FUSE's atomic_o_trunc, auto_cache, big_writes, default_permissions,
splice_move, splice_read, and splice_write. These options seem to provide the best performance.
• direct_io: causes FUSE to bypass caching which can increase write speeds at the detriment of reads.
Note that not enabling direct_io will cause double caching of files and therefore less memory for
caching generally. However, mmap does not work when direct_io is enabled.
• minfreespace: the minimum space value used for creation policies. Understands 'K', 'M', and 'G' to
represent kilobyte, megabyte, and gigabyte respectively. (default: 4G)
• moveonenospc: when enabled (set to true) if a write fails with ENOSPC or EDQUOT a scan of all drives
will be done looking for the drive with 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 generally recommended it be enabled so that hard linked files share the same inode value.
• dropcacheonclose: 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 direct_io is not
enabled to limit double caching. (default: false)
• symlinkify: when enabled (set to true) and a file is not writable and its mtime or ctime is older than
symlinkify_timeout files will be reported as symlinks to the original files. Please read more below
before using. (default: false)
• symlinkify_timeout: time to wait, in seconds, to activate the symlinkify behavior. (default: 3600)
• fsname: 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:
category.create=mfs
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.
srcmounts
The srcmounts (source mounts) argument is a colon (':') delimited list of paths to be included in the
pool. 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 source mounts 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 expand it.
$ mergerfs -o defaults,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 defaults,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.
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.
FUNCTIONS / POLICIES / CATEGORIES
The POSIX filesystem API has a number of functions. creat, stat, chown, etc. In mergerfs these func‐
tions are grouped into 3 categories: action, create, and search. Functions and categories can be as‐
signed a policy which dictates how mergerfs behaves. Any policy can be assigned to a function or catego‐
ry 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 (though only if there were more
than one copy of the file).
Policies, when called to create, will ignore drives which are readonly. This allows for readonly and
read/write drives to be mixed together. Note that the drive must be explicitly mounted with the ro mount
option for this to work.
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, listxattr, open, readlink
N/A fallocate, fgetattr, fsync, ftruncate, ioctl, read, readdir,
release, statfs, write
Due to FUSE limitations ioctl behaves differently if its acting on a directory. It'll use the getattr
policy to find and open the directory before issuing the ioctl. In other cases where something may be
searched (to confirm a directory exists across all source mounts) getattr will also be used.
Path Preservation
Policies, as described below, are of two core 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.
As the descriptions explain a path preserving policy will only consider drives where the relative path
being accessed already exists.
When using non-path preserving policies where something is created paths will be copied to target drives
as necessary.
Policy descriptions
Policy Description
──────────────────────────────────────────────────────────────────────────
all Search category: acts like ff. Action category: ap‐
ply to all found. Create category: for mkdir, mknod,
and symlink it will apply to all found. create works
like ff. It will exclude readonly drives and those
with free space less than minfreespace.
epall (existing Search category: acts like epff. Action category:
path, all) apply to all found. Create category: for mkdir, mkn‐
od, and symlink it will apply to all existing paths
found. create works like epff. Excludes readonly
drives and those with free space less than minfree‐
space.
epff (existing Given the order of the drives, as defined at mount
path, first time or configured at runtime, act on the first one
found) found where the relative path already exists. For
create category functions it will exclude readonly
drives and those with free space less than minfree‐
space (unless there is no other option). Falls back
to ff.
eplfs (existing Of all the drives on which the relative path exists
path, least free choose the drive with the least free space. For cre‐
space) ate category functions it will exclude readonly dri‐
ves and those with free space less than minfreespace.
Falls back to lfs.
eplus (existing Of all the drives on which the relative path exists
path, least used choose the drive with the least used space. For cre‐
space) ate category functions it will exclude readonly dri‐
ves and those with free space less than minfreespace.
Falls back to lus.
epmfs (existing Of all the drives on which the relative path exists
path, most free choose the drive with the most free space. For cre‐
space) ate category functions it will exclude readonly dri‐
ves and those with free space less than minfreespace.
Falls back to mfs.
eprand (existing Calls epall and then randomizes. Otherwise behaves
path, random) the same as epall.
erofs Exclusively return -1 with errno set to EROFS
(Read-only filesystem). By setting create functions
to this you can in effect turn the filesystem mostly
readonly.
ff (first found) Given the order of the drives, as defined at mount
time or configured at runtime, act on the first one
found. For create category functions it will exclude
readonly drives and those with free space less than
minfreespace (unless there is no other option).
lfs (least free Pick the drive with the least available free space.
space) For create category functions it will exclude readon‐
ly drives and those with free space less than min‐
freespace. Falls back to mfs.
lus (least used Pick the drive with the least used space. For create
space) category functions it will exclude readonly drives
and those with free space less than minfreespace.
Falls back to mfs.
mfs (most free Pick the drive with the most available free space.
space) For create category functions it will exclude readon‐
ly drives. Falls back to ff.
newest Pick the file / directory with the largest mtime.
For create category functions it will exclude readon‐
ly drives and those with free space less than min‐
freespace (unless there is no other option).
rand (random) Calls all and then randomizes.
Defaults
Category Policy
──────────────────
action all
create epmfs
search ff
rename & link
NOTE: If you're receiving errors from software when files are moved / renamed then you should consider
changing the create policy to one which is not path preserving or contacting the author of the offending
software and requesting that EXDEV be properly handled.
rename (http://man7.org/linux/man-pages/man2/rename.2.html) is a tricky function in a merged system. Un‐
der normal situations rename only works within a single filesystem or device. If a rename can't be done
atomically due to the source and destination paths existing on different mount points it will return -1
with errno = EXDEV (cross device).
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 rather complicated 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 basic strategy.
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.
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.
BUILDING
NOTE: Prebuilt packages can be found at: https://github.com/trapexit/mergerfs/releases
First get the code from github (http://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
$ sudo apt-get install g++ pkg-config git git-buildpackage pandoc debhelper libfuse-dev libattr1-dev python
$ cd mergerfs
$ make deb
$ sudo dpkg -i ../mergerfs_version_arch.deb
Fedora
$ su -
# dnf install rpm-build fuse-devel libattr-devel pandoc gcc-c++ git make which python
# cd mergerfs
# make rpm
# rpm -i rpmbuild/RPMS/<arch>/mergerfs-<verion>.<arch>.rpm
Generically
Have git, python, pkg-config, pandoc, libfuse, libattr1 installed.
$ cd mergerfs
$ make
$ make man
$ sudo make install
RUNTIME
.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.
Even if xattrs are disabled for mergerfs the {list,get,set}xattrs (http://linux.die.net/man/2/listxattr)
calls against this pseudo file will still work.
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 (fstab).
Keys
Use xattr -l /mount/point/.mergerfs to see all supported keys. Some are informational and therefore
readonly.
user.mergerfs.srcmounts
Used to query or modify the list of source mounts. 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
minfreespace
Input: interger with an optional multiplier suffix. K, M, or G.
Output: value in bytes
moveonenospc
Input: true and false
Ouput: true or false
categories / funcs
Input: short policy string as described elsewhere in this document
Output: the policy string except for categories where its funcs have multiple types. In that case it
will be a comma separated list
Example
[trapexit:/tmp/mount] $ xattr -l .mergerfs
user.mergerfs.srcmounts: /tmp/a:/tmp/b
user.mergerfs.minfreespace: 4294967295
user.mergerfs.moveonenospc: false
...
[trapexit:/tmp/mount] $ xattr -p user.mergerfs.category.search .mergerfs
ff
[trapexit:/tmp/mount] $ xattr -w user.mergerfs.category.search newest .mergerfs
[trapexit:/tmp/mount] $ xattr -p user.mergerfs.category.search .mergerfs
newest
[trapexit:/tmp/mount] $ xattr -w user.mergerfs.srcmounts +/tmp/c .mergerfs
[trapexit:/tmp/mount] $ xattr -p user.mergerfs.srcmounts .mergerfs
/tmp/a:/tmp/b:/tmp/c
[trapexit:/tmp/mount] $ xattr -w user.mergerfs.srcmounts =/tmp/c .mergerfs
[trapexit:/tmp/mount] $ xattr -p user.mergerfs.srcmounts .mergerfs
/tmp/c
[trapexit:/tmp/mount] $ xattr -w user.mergerfs.srcmounts '+</tmp/a:/tmp/b' .mergerfs
[trapexit:/tmp/mount] $ xattr -p user.mergerfs.srcmounts .mergerfs
/tmp/a:/tmp/b:/tmp/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:/tmp/mount] $ ls
A B C
[trapexit:/tmp/mount] $ xattr -p user.mergerfs.fullpath A
/mnt/a/full/path/to/A
[trapexit:/tmp/mount] $ xattr -p user.mergerfs.basepath A
/mnt/a
[trapexit:/tmp/mount] $ xattr -p user.mergerfs.relpath A
/full/path/to/A
[trapexit:/tmp/mount] $ 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.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
• 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
TIPS / NOTES
• The recommended options are defaults,allow_other,direct_io,use_ino.
• Run mergerfs as root 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 direct_io if you expect applications (such as rtorrent) to mmap (http://lin‐
ux.die.net/man/2/mmap) files. It is not currently supported in FUSE w/ direct_io enabled.
• Since POSIX gives you only error or success on calls its difficult to determine the proper behavior
when applying the behavior to multiple targets. mergerfs will return an error only if all attempts of
an action fail. Any success will lead to a success returned. This means however that some odd situa‐
tions 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 Kodi
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 un‐
link'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 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 far more expensive and for many ap‐
plications 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. That alone is far more expensive than ff but
would also possibly spin up sleeping drives.
If you always want the directory information from the one with the most recent mtime then use the newest
policy for getattr.
cached memory appears greater than it should be
Use the direct_io option as described above. Due to what mergerfs is doing there ends up being two
caches of a file under normal usage. One from the underlying filesystem and one from mergerfs. Enabling
direct_io removes the mergerfs cache. This saves on memory but means the kernel needs to communicate
with mergerfs more often and can therefore result in slower speeds.
Since enabling direct_io disables mmap this is not an ideal situation however write speeds should be in‐
creased.
If direct_io is disabled it is probably a good idea to enable dropcacheonclose to minimize double
caching.
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 turn off direct_io. rtorrent and some other applications use mmap (http://lin‐
ux.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 will 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 applica‐
tions 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.
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.
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
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. Add
no_splice_write,no_splice_move,no_splice_read to mergerfs' options. Should be placed after defaults if
it is used since it will turn them on. This however is not guaranteed to work.
FAQ
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.
If you wish to get similar behavior to mhddfs from mergerfs then set category.create=ff.
Why use mergerfs over aufs?
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 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
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. It will be represented immediately in the pool as the policies perscribe.
Why do I get an out of space error even though the system says
there's lots of space left?
First make sure you've read the sections above about policies, path preserving, and the moveonenospc op‐
tion.
Remember that mergerfs is simply presenting a logical merging of the contents of the pooled drives. The
reported free space is the aggregate space available not the contiguous space available. MergerFS does
not split files across drives. If the writing of a file fills a drive and moveonenospc is disabled it
will return an ENOSPC error.
If moveonenospc is enabled but there exists no drives with enough space for the file and the data to be
written (or the drive happened to fill up as the file was being moved) it will error indicating there
isn't enough space.
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. In the future it might be worth considering the number of inodes avail‐
able when making placement decisions in order to minimize this situation.
Can mergerfs mounts be exported over NFS?
Yes. 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.
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.
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 many other situations. Rather than attempting to simulate POSIX ACL be‐
havior 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.
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.
SUPPORT
Issues with the software
• github.com: https://github.com/trapexit/mergerfs/issues
• email: trapexit@spawn.link
• twitter: https://twitter.com/_trapexit
Support development
• Gratipay: https://gratipay.com/~trapexit
• BitCoin: 12CdMhEPQVmjz3SSynkAEuD5q9JmhTDCZA
LINKS
• http://github.com/trapexit/mergerfs
• http://github.com/trapexit/mergerfs-tools
• http://github.com/trapexit/scorch
• http://github.com/trapexit/backup-and-recovery-howtos
AUTHORS
Antonio SJ Musumeci <trapexit@spawn.link>.
mergerfs user manual 2017-02-18 mergerfs(1)