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 creating 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 filesystem. 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 metadata 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 symlinkify_timeout files will be reported as symlinks to the original files.
Please read more below before using. (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 depending on the policy of create (read below). Enabling this will cause
rename and link to always use the non-path preserving behavior. This means files, when
renamed or linked, will stay on the same drive. (default: false)
• security_capability=BOOL: If false return ENOATTR when xattr security.capability is
queried. (default: true)
• xattr=passthrough|noattr|nosys: Runtime control of xattrs. Default is to passthrough
xattr requests. 'noattr' will short circuit as if nothing exists. 'nosys' will respond
with ENOSYS as if xattrs are not supported or disabled. (default: passthrough)
• link_cow=BOOL: When enabled if a regular file is opened which has a link count > 1 it
will copy the file to a temporary file and rename over the original. Breaking the link
and providing a basic copy-on-write function similar to cow-shell. (default: false)
• statfs=base|full: Controls how statfs works. 'base' means it will always use all
branches in statfs calculations. 'full' is in effect path preserving and only includes
drives where the path exists. (default: base)
• statfs_ignore=none|ro|nc: 'ro' will cause statfs calculations to ignore available space
for branches mounted or tagged as 'read-only' or 'no create'. 'nc' will ignore
available space for branches tagged as 'no create'. (default: none)
• posix_acl=BOOL: Enable POSIX ACL support (if supported by kernel and underlying
filesystem). (default: false)
• async_read=BOOL: Perform reads asynchronously. If disabled or unavailable the kernel
will ensure there is at most one pending read request per file handle and will attempt
to order requests by offset. (default: true)
• fuse_msg_size=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 discover 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. (default: 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: category.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 together. 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 features which aren't supported by the underlying
filesystem (such as file attributes or extended attributes) will return the appropriate
errors.
To make it easier to include multiple branches mergerfs supports globbing
(http://linux.die.net/man/7/glob). The globbing tokens MUST be escaped when using via the
shell else the shell itself will apply the glob itself.
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 matching the glob after the fact it will not be automatically included.
NOTE: for mounting via fstab to work you must have mount.fuse installed. For
Ubuntu/Debian it is included in the fuse package.
fuse_msg_size
FUSE applications communicate with the kernel over a special character device: /dev/fuse.
A large portion of the overhead associated with FUSE is the cost of going back and forth
from user space and kernel space over that device. Generally speaking the fewer trips
needed the better the performance will be. Reducing the number of trips can be done a
number of ways. Kernel level caching and increasing message sizes being two significant
ones. When it comes to reads and writes if the message size is doubled the number of
trips are 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 value 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 software to follow symlinks if such an option is available.
Alternatively create two mounts. One for backup and one for general consumption.
nullrw
Due to how FUSE works there is an overhead to all requests made to a FUSE filesystem.
Meaning that even a simple passthrough will have some slowdown. However, generally the
overhead is minimal in comparison to the cost of the underlying I/O. By disabling the
underlying I/O we can test the theoretical performance 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 theoretical 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. mergerfs still gets all the requests but they will not be forwarded on to
the underlying filesystems. The runtime control will still function in this mode.
nosys will cause mergerfs to return ENOSYS for any xattr call. The difference with noattr
is that the kernel will cache this fact and itself short circuit future calls. This 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 calculations. 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 policy is responsible for filtering and sorting. The policy type defines
the sorting but filtering is mostly 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 reasons.
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
path, all) because it doesn't stop after finding a valid
branch). Action category: apply to all found.
Create category: for mkdir, mknod, and symlink it
will apply to all found. create works like epff (but
more expensive because it doesn't stop after finding
a valid branch).
epff (existing 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 directory to act on.
unlink
In FUSE there is an opaque "file handle" which is created by open, create, or opendir,
passed to the kernel, 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 doing 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 referencing 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/rename 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 different 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 applications fail to handle EXDEV at all and treat it as a
normal error or otherwise handle it poorly. Such apps include: gvfsd-fuse v1.20.3 and
prior, Finder / CIFS/SMB client in Apple OSX 10.9+, NZBGet, Samba's recycling bin feature.
As a result a compromise was made in order to get most software to work while still
obeying mergerfs' policies. Below is the basic logic.
• If using a create policy which tries to preserve directory paths
(epff,eplfs,eplus,epmfs)
• Using the rename policy get the list of files to rename
• For each file attempt rename:
• If failure with ENOENT run create policy
• If create policy returns the same drive as currently evaluating then clone the path
• Re-attempt rename
• If any of the renames succeed the higher level rename is considered a success
• If no renames succeed the first error encountered will be returned
• On success:
• Remove the target from all drives with no source file
• Remove the source from all drives which failed to rename
• If using a create policy which does not try to preserve directory paths
• Using the rename policy get the list of files to rename
• Using the getattr policy get the target path
• For each file attempt rename:
• If the source drive != target drive:
• Clone target path from target drive to source drive
• Rename
• If any of the renames succeed the higher level rename is considered a success
• If no renames succeed the first error encountered will be returned
• On success:
• Remove the target from all drives with no source file
• Remove the source from all drives which failed to rename
The the removals are subject to normal entitlement checks.
The above behavior will help minimize the likelihood of EXDEV being returned but it will
still be possible.
link uses the same strategy but without the removals.
readdir
readdir (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 directories come from the respective
function. For instance: an ls is a readdir and for each file/directory returned getattr
is called. Meaning the policy of getattr is responsible for choosing the file/directory
which is the source of the metadata you see in an ls.
statfs / statvfs
statvfs (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 manipulation of the list.
Value Description
──────────────────────────────────────
[list] set
+<[list] prepend
+>[list] append
-[list] remove all values provided
-< remove first in list
-> remove last in list
xattr -w user.mergerfs.branches +</mnt/drive3 /mnt/pool/.mergerfs
The =NC, =RO, =RW syntax works just as on the command line.
Example
[trapexit:/mnt/mergerfs] $ 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 kernel will double cache the content being read and written through
mergerfs. Once from the underlying filesystem and once from mergerfs (it sees them as two
separate entities). Using cache.files=off will keep the double caching from happening by
disabling caching of mergerfs but this has the side effect that all read and write calls
will be passed to mergerfs which may be slower than enabling caching, you lose shared mmap
support which can affect apps such as rtorrent, and no read-ahead will take place. The
kernel will still cache the underlying filesystem data but that only helps so much given
mergerfs will still process all requests.
If you do enable file page caching, cache.files=partial|full|auto-full, you should also
enable dropcacheonclose which will cause mergerfs to instruct the kernel to flush the
underlying file's page cache when the file is closed. This behavior is the same as the
rsync fadvise / drop cache patch and Feh's nocache project.
If most files are read once through and closed (like media) it is best to enable
dropcacheonclose regardless of caching mode in order to minimize buffer bloat.
It is difficult to balance memory usage, cache bloat & duplication, and performance.
Ideally mergerfs would be able to disable caching for the files it reads/writes but allow
page caching for itself. That would limit the FUSE overhead. However, there isn't a good
way to achieve this. It would need to open all files with O_DIRECT which places
limitations on the what underlying filesystems would be supported and complicates the
code.
kernel 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 seconds or until the file is unlinked. Each file close (release) will randomly
chose to clean up the cache of expired entries.
This cache is really only useful in cases where you have a large number of branches and
open is called on the same files repeatedly (like Transmission which opens and closes a
file on every read/write presumably to keep file handle usage low).
statfs caching
Of the syscalls used by mergerfs in policies the statfs / statvfs call is perhaps the most
expensive. It's used to find out the available space of a drive and whether it is mounted
read-only. Depending on the setup and usage pattern these queries can be relatively
costly. When cache.statfs is enabled all calls to statfs by a policy will be cached for
the number of seconds its set to.
Example: If the create policy is mfs and the timeout is 60 then for that 60 seconds the
same drive will be returned as the target for creates because the available space won't be
updated for that time.
symlink caching
As of version 4.20 Linux supports symlink caching. Significant performance increases can
be had in workloads which use a lot of symlinks. Setting cache.symlinks=true will result
in requesting symlink caching from the kernel only if supported. As a result its safe to
enable it on systems prior to 4.20. That said it is disabled by default for now. You can
see if caching is enabled by querying the xattr user.mergerfs.cache.symlinks but given it
must be requested at startup you can not change it at runtime.
readdir caching
As of version 4.20 Linux supports readdir caching. This can have a significant impact on
directory traversal. Especially when combined with entry (cache.entry) and attribute
(cache.attr) caching. Setting cache.readdir=true will result in requesting readdir
caching from the kernel on each opendir. If the kernel doesn't support readdir caching
setting the option to true has no effect. This option is configuarable 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 libfuse 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 traditional HDDs for instance.
MergerFS does not natively support any sort of tiered caching. Most users have no use for
such a feature and its inclusion would complicate the code. However, there are a few
situations where a cache drive could help with a typical mergerfs setup.
1. Fast network, slow drives, many readers: You've a 10+Gbps network with many readers and
your regular drives can't keep up.
2. Fast network, slow drives, small'ish bursty writes: You have a 10+Gbps network and wish
to transfer amounts of data less than your cache drive but wish to do so quickly.
With #1 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 drives (SSD,NVME,etc.) and slow drives.
2. The 'cache' pool should have the cache drives listed first.
3. The best create policies to use for the 'cache' pool would probably be ff, epff, lfs,
or eplfs. The latter two under the assumption that the cache drive(s) are far smaller
than the backing drives. If using path preserving policies remember that you'll need
to manually create the core directories of those paths you wish to be cached. Be sure
the permissions are in sync. Use mergerfs.fsck to check / correct them. You could
also tag the slow drives as =NC though that'd mean if the cache drives fill you'd get
"out of space" errors.
4. Enable moveonenospc and set minfreespace appropriately. 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 drives 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 direct_io=false.
• Since POSIX functions give only a singular error or success its difficult to determine
the proper behavior when applying the function 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 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 behaviors and race conditions couldn't happen outside mergerfs but that they
are far more likely to occur on account of the attempt to merge together multiple
sources of data which could be out of sync due to the different policies.
• For consistency its generally best to set category wide policies rather than individual
func's. This will help limit the confusion of tools such as rsync
(http://linux.die.net/man/1/rsync). However, the flexibility is there if needed.
KNOWN ISSUES / BUGS
directory mtime is not being updated
Remember that the default policy for getattr is ff. The information for the first
directory found will be returned. If it wasn't the directory which had been updated then
it will appear outdated.
The reason this is the default is because any other policy would be more expensive and for
many applications it is unnecessary. To always return the directory with the most recent
mtime or a faked value based on all found would require a scan of all drives.
If you always want the directory information from the one with the most recent mtime then
use the newest policy for getattr.
mv /mnt/pool/foo /mnt/disk1/foo removes foo
This is not a bug.
Run in verbose mode to better 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 issues.
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 applications 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 performance penalty on writes with direct_io off as
well as the problem of double caching but it's the only way to get such applications to
work. If the performance loss is too high for other apps you can mount mergerfs twice.
Once with direct_io enabled and one without it. Be sure to set dropcacheonclose=true if
not using direct_io.
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 return even though they are perfectly valid situations which do not indicate
actual drive or OS errors. The error will only be returned by mergerfs if using a path
preserving policy as described in the policy section above. If you do not care about path
preservation simply change the mergerfs policy to the non-path preserving version. For
example: -o category.create=mfs
Ideally the offending software would be fixed and it is recommended that if you run into
this problem you contact the software's author and request proper handling of EXDEV
errors.
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 problem.
In Apple's MacOSX 10.9 they replaced Samba (client and server) with their own product. It
appears their new client does not handle EXDEV either and responds similar to older
release of gvfs on Linux.
Trashing files occasionally fails
This is the same issue as with Samba. rename returns EXDEV (in our case that will really
only happen with path preserving policies like epmfs) and the software doesn't handle the
situtation well. This is unfortunately a common failure of software which moves files
around. The standard indicates that an implementation MAY choose to support non-user home
directory trashing of files (which is a MUST). The implementation MAY also support "top
directory trashes" which many probably do.
To create a $topdir/.Trash directory as defined in the standard use the mergerfs-tools
(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 dynamically grows and shrinks
it's possible that over time a thread will be killed and later a new thread with no cache
will start and query the new data.
The gid cache uses fixed storage to simplify the design and be compatible with older
systems which may not have C++11 compilers. There is enough storage for 256 users'
supplemental groups. Each user is allowed 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 (Debian,Ubuntu,Mint) you can likely just install newer versions of libfuse
(https://packages.debian.org/unstable/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 certain edge cases and then later delete that file when its clear the file is no
longer needed. This however 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 immedately. 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 hardlinking 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 seeing anything at all be sure that the basic permissions are
correct. The user and group values are correct and that directories have their executable
bit set. A common mistake by users new to Linux is to chmod -R 644 when they should have
chmod -R u=rwX,go=rX.
If using a network filesystem such as NFS, SMB, CIFS (Samba) be sure to pay close
attention to anything regarding permissioning and users. Root squashing and user
translation for instance has bitten a few mergerfs users. Some of these also affect the
use of mergerfs from container platforms such as Docker.
Why 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 instead 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 "vendor" / embed the library into the
project.
2. mergerfs was written to use the high level API. There are a number of limitations in
the HLAPI that make certain features difficult or impossible to implement. While some
of these features could be patched into newer versions of libfuse without breaking the
public API some of them would require hacky code to provide backwards compatibility.
While it may still be worth working with upstream to address these issues in future
versions, since the library needs to be vendored for stability and compatibility
reasons it is preferable / easier to modify the API. Longer term the plan is to
rewrite mergerfs to use the low level API.
Why did support for system libfuse get removed?
See above first.
If/when mergerfs is rewritten to use the low-level API then it'll be plausible to support
system libfuse but till then 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 easier to use. mergerfs however does not offer the overlay / copy-on-write
(CoW) features which aufs and overlayfs have.
Why use mergerfs over unionfs?
UnionFS is more like aufs 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_entry, 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 mergerfs 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 benefits.
While atypical, yes, inodes can be reused and not refer to the same file. The internal id
used to reference a file in FUSE is different from the inode value presented. The former
is the nodeid and is actually a tuple of (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 generally 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 unlinked. This is to simplify "use after unlink" usecases. There is a
possibility these files end up being 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 solution 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 also 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 software 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>.