| mergerfs(1) | mergerfs(1) |
mergerfs - a featureful union filesystem
mergerfs -o<options> <srcmounts> <mountpoint>
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.
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 rw and ro drives.
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.
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 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 source mounts 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 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,hard_remove 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.
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.
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 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 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 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 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.
The POSIX filesystem API has a number of functions. creat, stat, chown, etc. In mergerfs these functions are grouped into 3 categories: action, create, and search. Functions and categories can be assigned a policy which dictates how mergerfs behaves. 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 (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.
| 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.
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 | Description |
| all | Search category: acts like ff. Action category: apply 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 path, all) | Search category: acts like epff. Action category: apply to all found. Create category: for mkdir, mknod, and symlink it will apply to all existing paths found. create works like epff. Excludes readonly drives and those with free space less than minfreespace. |
| epff (existing path, first found) | Given the order of the drives, as defined at mount time or configured at runtime, act on the first one found where the relative path already exists. For create category functions it will exclude readonly drives and those with free space less than minfreespace (unless there is no other option). Falls back to ff. |
| eplfs (existing path, least free space) | Of all the drives on which the relative path exists choose the drive with the least free space. For create category functions it will exclude readonly drives and those with free space less than minfreespace. Falls back to lfs. |
| eplus (existing path, least used space) | Of all the drives on which the relative path exists choose the drive with the least used space. For create category functions it will exclude readonly drives and those with free space less than minfreespace. Falls back to lus. |
| epmfs (existing path, most free space) | Of all the drives on which the relative path exists choose the drive with the most free space. For create category functions it will exclude readonly drives and those with free space less than minfreespace. Falls back to mfs. |
| eprand (existing path, random) | Calls epall and then randomizes. Otherwise behaves 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 space) | Pick the drive with the least available free space. For create category functions it will exclude readonly drives and those with free space less than minfreespace. Falls back to mfs. |
| lus (least used space) | Pick the drive with the least used space. For create category functions it will exclude readonly drives and those with free space less than minfreespace. Falls back to mfs. |
| mfs (most free space) | Pick the drive with the most available free space. For create category functions it will exclude readonly drives. Falls back to ff. |
| newest | Pick the file / directory with the largest mtime. For create category functions it will exclude readonly drives and those with free space less than minfreespace (unless there is no other option). |
| rand (random) | Calls all and then randomizes. |
| Category | Policy |
| action | all |
| create | epmfs |
| search | ff |
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, enabling ignorepponrename, 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. Under 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 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 rather complicated logic.
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 basic strategy.
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.
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.
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
$ sudo apt-get -y update $ sudo apt-get -y install git make $ cd mergerfs $ make install-build-pkgs $ # build-essential git g++ debhelper libattr1-dev python automake libtool lsb-release $ make deb $ sudo dpkg -i ../mergerfs_version_arch.deb
$ su - # dnf -y update # dnf -y install git make # cd mergerfs # make install-build-pkgs # # rpm-build libattr-devel gcc-c++ which python automake libtool gettext-devel # make rpm # rpm -i rpmbuild/RPMS/<arch>/mergerfs-<verion>.<arch>.rpm
Have git, g++, make, python, libattr1, automake, libtool installed.
$ cd mergerfs $ make $ sudo make install
<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 (/etc/fstab).
Use xattr -l /mount/point/.mergerfs to see all supported keys. Some are informational and therefore readonly.
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 |
xattr -w user.mergerfs.srcmounts +</mnt/drive3 /mnt/pool/.mergerfs
Input: interger with an optional multiplier suffix. K, M, or G.
Output: value in bytes
Input: true and false
Ouput: true or false
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
[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
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:
[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
MergerFS does not natively support any sort of caching. Most users have no use for such a feature and it would greatly complicate the code. However, there are a few situations where a cache drive could help with a typical mergerfs setup.
The below will mostly address usecase #2. It will also work for #1 assuming the data is regularly accessed and was placed into the system via this method. Otherwise a similar script may need to be written to populate the cache from the backing pool.
Move files from cache to backing pool based only on the last time the file was accessed.
#!/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=- -aq --remove-source-files "${CACHE}/" "${BACKING}/"
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 -aq --remove-source-files "${CACHE}/./${FILE}" "${BACKING}/"
done
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 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. 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.
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 increased.
If direct_io is disabled it is probably a good idea to enable dropcacheonclose to minimize double caching.
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.
Be sure to 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 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 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.
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.
If the issue is that scanning doesn't seem to pick up media then be sure to set func.getattr=newest as mentioned above.
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).
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.
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.
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.
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.
NOTE: as of mergerfs 2.22.0 it includes the most recent version of libfuse 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.
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).
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.
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.
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 an actual 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.
See the previous question's answer.
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 doesn't 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.
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.
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,defaults,allow_other,category.create=ff /mnt/drive1:/mnt/drive2 /mnt/pool
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.
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.
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.
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).
Yes. It will be represented immediately in the pool as the policies perscribe.
there's lots of space left?
First make sure you've read the sections above about policies, path preserving, and the moveonenospc option.
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 an underlying drive and moveonenospc is disabled it will return an ENOSPC (No space left on device) 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 available when making placement decisions in order to minimize this situation.
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.
Yes. While some users have reported problems it appears to always be related to how Samba is setup in relation to permissions.
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.
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 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.
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.
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.
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.
Antonio SJ Musumeci <trapexit@spawn.link>.
| 2018-03-09 | mergerfs user manual |