.\"t .\" Automatically generated by Pandoc 1.19.2.4 .\" .TH "mergerfs" "1" "2018\-10\-31" "mergerfs user manual" "" .hy .SH NAME .PP mergerfs \- a featureful union filesystem .SH SYNOPSIS .PP mergerfs \-o .SH DESCRIPTION .PP \f[B]mergerfs\f[] is a union filesystem geared towards simplifying storage and management of files across numerous commodity storage devices. It is similar to \f[B]mhddfs\f[], \f[B]unionfs\f[], and \f[B]aufs\f[]. .SH FEATURES .IP \[bu] 2 Runs in userspace (FUSE) .IP \[bu] 2 Configurable behaviors .IP \[bu] 2 Support for extended attributes (xattrs) .IP \[bu] 2 Support for file attributes (chattr) .IP \[bu] 2 Runtime configurable (via xattrs) .IP \[bu] 2 Safe to run as root .IP \[bu] 2 Opportunistic credential caching .IP \[bu] 2 Works with heterogeneous filesystem types .IP \[bu] 2 Handling of writes to full drives (transparently move file to drive with capacity) .IP \[bu] 2 Handles pool of readonly and read/write drives .IP \[bu] 2 Turn read\-only files into symlinks to increase read performance .SH How it works .PP 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. .IP .nf \f[C] 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 \f[] .fi .PP mergerfs does \f[B]not\f[] support the copy\-on\-write (CoW) behavior found in \f[B]aufs\f[] and \f[B]overlayfs\f[]. You can \f[B]not\f[] 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. .SH OPTIONS .SS mount options .IP \[bu] 2 \f[B]defaults\f[]: a shortcut for FUSE\[aq]s \f[B]atomic_o_trunc\f[], \f[B]auto_cache\f[], \f[B]big_writes\f[], \f[B]default_permissions\f[], \f[B]splice_move\f[], \f[B]splice_read\f[], and \f[B]splice_write\f[]. These options seem to provide the best performance. .IP \[bu] 2 \f[B]allow_other\f[]: a libfuse option which allows users besides the one which ran mergerfs to see the filesystem. This is required for most use\-cases. .IP \[bu] 2 \f[B]direct_io\f[]: causes FUSE to bypass caching which can increase write speeds at the detriment of reads. Note that not enabling \f[C]direct_io\f[] will cause double caching of files and therefore less memory for caching generally (enable \f[B]dropcacheonclose\f[] to help with this problem). However, \f[C]mmap\f[] does not work when \f[C]direct_io\f[] is enabled. .IP \[bu] 2 \f[B]minfreespace=value\f[]: the minimum space value used for creation policies. Understands \[aq]K\[aq], \[aq]M\[aq], and \[aq]G\[aq] to represent kilobyte, megabyte, and gigabyte respectively. (default: 4G) .IP \[bu] 2 \f[B]moveonenospc=true|false\f[]: when enabled (set to \f[B]true\f[]) if a \f[B]write\f[] fails with \f[B]ENOSPC\f[] or \f[B]EDQUOT\f[] 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) .IP \[bu] 2 \f[B]use_ino\f[]: causes mergerfs to supply file/directory inodes rather than libfuse. While not a default it is generally recommended it be enabled so that hard linked files share the same inode value. .IP \[bu] 2 \f[B]hard_remove\f[]: force libfuse to immedately remove files when unlinked. This will keep the \f[C]\&.fuse_hidden\f[] files from showing up but if software uses an opened but unlinked file in certain ways it could result in errors. .IP \[bu] 2 \f[B]dropcacheonclose=true|false\f[]: when a file is requested to be closed call \f[C]posix_fadvise\f[] on it first to instruct the kernel that we no longer need the data and it can drop its cache. Recommended when \f[B]direct_io\f[] is not enabled to limit double caching. (default: false) .IP \[bu] 2 \f[B]symlinkify=true|false\f[]: when enabled (set to \f[B]true\f[]) and a file is not writable and its mtime or ctime is older than \f[B]symlinkify_timeout\f[] files will be reported as symlinks to the original files. Please read more below before using. (default: false) .IP \[bu] 2 \f[B]symlinkify_timeout=value\f[]: time to wait, in seconds, to activate the \f[B]symlinkify\f[] behavior. (default: 3600) .IP \[bu] 2 \f[B]nullrw=true|false\f[]: turns reads and writes into no\-ops. The request will succeed but do nothing. Useful for benchmarking mergerfs. (default: false) .IP \[bu] 2 \f[B]ignorepponrename=true|false\f[]: ignore path preserving on rename. Typically rename and link act differently depending on the policy of \f[C]create\f[] (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) .IP \[bu] 2 \f[B]security_capability=true|false\f[]: If false return ENOATTR when xattr security.capability is queried. (default: true) .IP \[bu] 2 \f[B]xattr=passthrough|noattr|notsup\f[]: Runtime control of xattrs. Default is to passthrough xattr requests. \[aq]noattr\[aq] will short circuit as if nothing exists. \[aq]notsup\[aq] will respond with ENOTSUP as if xattrs are not supported or disabled. (default: passthrough) .IP \[bu] 2 \f[B]link_cow=true|false\f[]: 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) .IP \[bu] 2 \f[B]threads=num\f[]: number of threads to use in multithreaded mode. When set to zero (the default) 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: number of cores) \f[I]NOTE2:\f[] the option is unavailable when built with system libfuse. .IP \[bu] 2 \f[B]fsname=name\f[]: sets the name of the filesystem as seen in \f[B]mount\f[], \f[B]df\f[], etc. Defaults to a list of the source paths concatenated together with the longest common prefix removed. .IP \[bu] 2 \f[B]func.=\f[]: sets the specific FUSE function\[aq]s policy. See below for the list of value types. Example: \f[B]func.getattr=newest\f[] .IP \[bu] 2 \f[B]category.=\f[]: Sets policy of all FUSE functions in the provided category. Example: \f[B]category.create=mfs\f[] .PP \f[B]NOTE:\f[] Options are evaluated in the order listed so if the options are \f[B]func.rmdir=rand,category.action=ff\f[] the \f[B]action\f[] category setting will override the \f[B]rmdir\f[] setting. .SS branches .PP The \[aq]branches\[aq] (formerly \[aq]srcmounts\[aq]) argument is a colon (\[aq]:\[aq]) 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\[aq]t supported by the underlying filesystem (such as file attributes or extended attributes) will return the appropriate errors. .PP To make it easier to include multiple branches mergerfs supports globbing (http://linux.die.net/man/7/glob). \f[B]The globbing tokens MUST be escaped when using via the shell else the shell itself will apply the glob itself.\f[] .PP Each branch can have a suffix of \f[C]=RW\f[] (read / write), \f[C]=RO\f[] (read only), or \f[C]=NW\f[] (no writes). These suffixes work with globs as well and will apply to each path found. \f[C]RW\f[] is the default behavior and those paths will be eligible for all policy categories. \f[C]RO\f[] will exclude those paths from \f[C]create\f[] and \f[C]action\f[] policies (just as a filesystem being mounted \f[C]ro\f[] would). \f[C]NW\f[] will exclude those paths from \f[C]create\f[] policies (you can\[aq]t create but you can change / delete). .IP .nf \f[C] $\ mergerfs\ \-o\ defaults,allow_other,use_ino\ /mnt/disk\\*:/mnt/cdrom\ /media/drives \f[] .fi .PP The above line will use all mount points in /mnt prefixed with \f[B]disk\f[] and the \f[B]cdrom\f[]. .PP To have the pool mounted at boot or otherwise accessable from related tools use \f[B]/etc/fstab\f[]. .IP .nf \f[C] #\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ /mnt/disk*:/mnt/cdrom\ \ /media/drives\ \ fuse.mergerfs\ \ defaults,allow_other,use_ino\ \ \ 0\ \ \ \ \ \ \ 0 \f[] .fi .PP \f[B]NOTE:\f[] 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. .PP \f[B]NOTE:\f[] for mounting via \f[B]fstab\f[] to work you must have \f[B]mount.fuse\f[] installed. For Ubuntu/Debian it is included in the \f[B]fuse\f[] package. .SS symlinkify .PP 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 \f[C]readlink\f[] policy after the mtime and ctime are older than the timeout. .PP \f[B]WARNING:\f[] 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. .PP \f[B]WARNING:\f[] 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. .SS nullrw .PP 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. .PP By enabling \f[C]nullrw\f[] mergerfs will work as it always does \f[B]except\f[] 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\[aq]t touch the buffer. .PP Example: .IP .nf \f[C] $\ 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 \f[] .fi .PP It\[aq]s important to test with different \f[C]obs\f[] (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\[aq]t optimally size its read and write requests. .SS xattr .PP Runtime extended attribute support can be managed via the \f[C]xattr\f[] 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. .PP \f[C]noattr\f[] 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. .PP \f[C]notsup\f[] will cause mergerfs to return ENOTSUP for any xattr call. The difference with \f[C]noattr\f[] is that the kernel will cache this fact and itself short circuit future calls. This will be more efficient than \f[C]noattr\f[] but will cause mergerfs\[aq] runtime control via the hidden file to stop working. .SH FUNCTIONS / POLICIES / CATEGORIES .PP The POSIX filesystem API has a number of functions. \f[B]creat\f[], \f[B]stat\f[], \f[B]chown\f[], etc. In mergerfs these functions are grouped into 3 categories: \f[B]action\f[], \f[B]create\f[], and \f[B]search\f[]. Functions and categories can be assigned a policy which dictates how \f[B]mergerfs\f[] behaves. Any policy can be assigned to a function or category though some may not be very useful in practice. For instance: \f[B]rand\f[] (random) may be useful for file creation (create) but could lead to very odd behavior if used for \f[C]chmod\f[] (though only if there were more than one copy of the file). .PP 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 \f[B]ro\f[] mount option for this to work. .PP When using policies which are based on a device\[aq]s available space the base path provided is used. Not the full path to the file in question. Meaning that sub mounts won\[aq]t be considered in the space calculations. The reason is that it doesn\[aq]t really work for non\-path preserving policies and can lead to non\-obvious behaviors. .SS Function / Category classifications .PP .TS tab(@); lw(7.9n) lw(62.1n). T{ Category T}@T{ FUSE Functions T} _ T{ action T}@T{ chmod, chown, link, removexattr, rename, rmdir, setxattr, truncate, unlink, utimens T} T{ create T}@T{ create, mkdir, mknod, symlink T} T{ search T}@T{ access, getattr, getxattr, ioctl, listxattr, open, readlink T} T{ N/A T}@T{ fallocate, fgetattr, fsync, ftruncate, ioctl, read, readdir, release, statfs, write T} .TE .PP Due to FUSE limitations \f[B]ioctl\f[] behaves differently if its acting on a directory. It\[aq]ll use the \f[B]getattr\f[] policy to find and open the directory before issuing the \f[B]ioctl\f[]. In other cases where something may be searched (to confirm a directory exists across all source mounts) \f[B]getattr\f[] will also be used. .SS Path Preservation .PP Policies, as described below, are of two core types. \f[C]path\ preserving\f[] and \f[C]non\-path\ preserving\f[]. .PP All policies which start with \f[C]ep\f[] (\f[B]epff\f[], \f[B]eplfs\f[], \f[B]eplus\f[], \f[B]epmfs\f[], \f[B]eprand\f[]) are \f[C]path\ preserving\f[]. \f[C]ep\f[] stands for \f[C]existing\ path\f[]. .PP A path preserving policy will only consider drives where the relative path being accessed already exists. .PP When using non\-path preserving policies paths will be cloned to target drives as necessary. .SS Policy descriptions .PP All \f[B]create\f[] policies will filter out branches which are mounted \f[B]read only\f[] or tagged as \f[B]read only\f[] or \f[B]no write\f[]. All \f[B]action\f[] policies will filter out branches which are mounted or tagged as \f[B]read only\f[]. .PP If all branches are filtered an error will be returned. Typically EROFS or ENOSPC. .PP .TS tab(@); lw(16.6n) lw(53.4n). T{ Policy T}@T{ Description T} _ T{ all T}@T{ Search category: acts like \f[B]ff\f[]. Action category: apply to all found. Create category: for \f[B]mkdir\f[], \f[B]mknod\f[], and \f[B]symlink\f[] it will apply to all branches. \f[B]create\f[] works like \f[B]ff\f[]. It will exclude branches with free space less than \f[B]minfreespace\f[]. T} T{ epall (existing path, all) T}@T{ Search category: acts like \f[B]epff\f[]. Action category: apply to all found. Create category: for \f[B]mkdir\f[], \f[B]mknod\f[], and \f[B]symlink\f[] it will apply to all existing paths found. \f[B]create\f[] works like \f[B]epff\f[]. Excludes branches with free space less than \f[B]minfreespace\f[]. T} T{ epff (existing path, first found) T}@T{ 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 \f[B]create\f[] category functions it will exclude branches with free space less than \f[B]minfreespace\f[]. T} T{ eplfs (existing path, least free space) T}@T{ Of all the drives on which the relative path exists choose the drive with the least free space. For \f[B]create\f[] category functions it will exclude those with free space less than \f[B]minfreespace\f[]. T} T{ eplus (existing path, least used space) T}@T{ Of all the drives on which the relative path exists choose the drive with the least used space. For \f[B]create\f[] category functions it will exclude those with free space less than \f[B]minfreespace\f[]. T} T{ epmfs (existing path, most free space) T}@T{ Of all the drives on which the relative path exists choose the drive with the most free space. For \f[B]create\f[] category functions it will exclude those with free space less than \f[B]minfreespace\f[]. T} T{ eprand (existing path, random) T}@T{ Calls \f[B]epall\f[] and then randomizes. Otherwise behaves the same as \f[B]epall\f[]. T} T{ erofs T}@T{ Exclusively return \f[B]\-1\f[] with \f[B]errno\f[] set to \f[B]EROFS\f[] (Read\-only filesystem). T} T{ ff (first found) T}@T{ Given the order of the drives, as defined at mount time or configured at runtime, act on the first one found. For \f[B]create\f[] category functions it will exclude those with free space less than \f[B]minfreespace\f[]. T} T{ lfs (least free space) T}@T{ Pick the drive with the least available free space. For \f[B]create\f[] category functions it will exclude those with free space less than \f[B]minfreespace\f[]. T} T{ lus (least used space) T}@T{ Pick the drive with the least used space. For \f[B]create\f[] category functions it will exclude those with free space less than \f[B]minfreespace\f[]. T} T{ mfs (most free space) T}@T{ Pick the drive with the most available free space. T} T{ newest T}@T{ Pick the file / directory with the largest mtime. For \f[B]create\f[] category functions it will exclude those with free space less than \f[B]minfreespace\f[]. T} T{ rand (random) T}@T{ Calls \f[B]all\f[] and then randomizes. T} .TE .SS Defaults .PP .TS tab(@); l l. T{ Category T}@T{ Policy T} _ T{ action T}@T{ epall T} T{ create T}@T{ epmfs T} T{ search T}@T{ ff T} .TE .SS rename & link .PP \f[B]NOTE:\f[] If you\[aq]re receiving errors from software when files are moved / renamed then you should consider changing the create policy to one which is \f[B]not\f[] path preserving, enabling \f[C]ignorepponrename\f[], or contacting the author of the offending software and requesting that \f[C]EXDEV\f[] be properly handled. .PP 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\[aq]t be done atomically due to the source and destination paths existing on different mount points it will return \f[B]\-1\f[] with \f[B]errno = EXDEV\f[] (cross device). .PP 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\[aq]s recycling bin feature. .PP As a result a compromise was made in order to get most software to work while still obeying mergerfs\[aq] policies. Below is the rather complicated logic. .IP \[bu] 2 If using a \f[B]create\f[] policy which tries to preserve directory paths (epff,eplfs,eplus,epmfs) .IP \[bu] 2 Using the \f[B]rename\f[] policy get the list of files to rename .IP \[bu] 2 For each file attempt rename: .RS 2 .IP \[bu] 2 If failure with ENOENT run \f[B]create\f[] policy .IP \[bu] 2 If create policy returns the same drive as currently evaluating then clone the path .IP \[bu] 2 Re\-attempt rename .RE .IP \[bu] 2 If \f[B]any\f[] of the renames succeed the higher level rename is considered a success .IP \[bu] 2 If \f[B]no\f[] renames succeed the first error encountered will be returned .IP \[bu] 2 On success: .RS 2 .IP \[bu] 2 Remove the target from all drives with no source file .IP \[bu] 2 Remove the source from all drives which failed to rename .RE .IP \[bu] 2 If using a \f[B]create\f[] policy which does \f[B]not\f[] try to preserve directory paths .IP \[bu] 2 Using the \f[B]rename\f[] policy get the list of files to rename .IP \[bu] 2 Using the \f[B]getattr\f[] policy get the target path .IP \[bu] 2 For each file attempt rename: .RS 2 .IP \[bu] 2 If the source drive != target drive: .IP \[bu] 2 Clone target path from target drive to source drive .IP \[bu] 2 Rename .RE .IP \[bu] 2 If \f[B]any\f[] of the renames succeed the higher level rename is considered a success .IP \[bu] 2 If \f[B]no\f[] renames succeed the first error encountered will be returned .IP \[bu] 2 On success: .RS 2 .IP \[bu] 2 Remove the target from all drives with no source file .IP \[bu] 2 Remove the source from all drives which failed to rename .RE .PP The the removals are subject to normal entitlement checks. .PP The above behavior will help minimize the likelihood of EXDEV being returned but it will still be possible. .PP \f[B]link\f[] uses the same basic strategy. .SS readdir .PP readdir (http://linux.die.net/man/3/readdir) is different from all other filesystem functions. While it could have it\[aq]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 \f[B]ls\f[] is a \f[B]readdir\f[] and for each file/directory returned \f[B]getattr\f[] is called. Meaning the policy of \f[B]getattr\f[] is responsible for choosing the file/directory which is the source of the metadata you see in an \f[B]ls\f[]. .SS statvfs .PP 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\[aq]s space. Filesystems mounted further down the tree of the branch will not be included. .SH BUILDING .PP \f[B]NOTE:\f[] Prebuilt packages can be found at: https://github.com/trapexit/mergerfs/releases .PP First get the code from github (http://github.com/trapexit/mergerfs). .IP .nf \f[C] $\ git\ clone\ https://github.com/trapexit/mergerfs.git $\ #\ or $\ wget\ https://github.com/trapexit/mergerfs/releases/download//mergerfs\-.tar.gz \f[] .fi .SS Debian / Ubuntu .IP .nf \f[C] $\ sudo\ apt\-get\ \-y\ update $\ sudo\ apt\-get\ \-y\ install\ git\ make $\ cd\ mergerfs $\ make\ install\-build\-pkgs $\ #\ build\-essential\ git\ g++\ debhelper\ python\ automake\ libtool\ lsb\-release $\ make\ deb $\ sudo\ dpkg\ \-i\ ../mergerfs_version_arch.deb \f[] .fi .SS Fedora .IP .nf \f[C] $\ su\ \- #\ dnf\ \-y\ update #\ dnf\ \-y\ install\ git\ make #\ cd\ mergerfs #\ make\ install\-build\-pkgs #\ #\ rpm\-build\ gcc\-c++\ which\ python\ automake\ libtool\ gettext\-devel #\ make\ rpm #\ rpm\ \-i\ rpmbuild/RPMS//mergerfs\-..rpm \f[] .fi .SS Generically .PP Have git, g++, make, python, automake, libtool installed. .IP .nf \f[C] $\ cd\ mergerfs $\ make $\ sudo\ make\ install \f[] .fi .SS Generically with system libfuse .PP \f[B]NOTE:\f[] Configurable threading and thus \f[C]\-o\ threads=num\f[] option will be unavailable when built with system libfuse. .PP Have git, g++, make, python, pkg\-config installed. Also, install libfuse >= 2.9.7 (but not libfuse\-3.x) and matching libfuse\-dev (or libfuse\-devel). .IP .nf \f[C] $\ cd\ mergerfs $\ make\ INTERNAL_FUSE=0 $\ sudo\ make\ INTERNAL_FUSE=0\ install \f[] .fi .SH RUNTIME .SS .mergerfs pseudo file .IP .nf \f[C] /.mergerfs \f[] .fi .PP There is a pseudo file available at the mount point which allows for the runtime modification of certain \f[B]mergerfs\f[] options. The file will not show up in \f[B]readdir\f[] but can be \f[B]stat\f[]\[aq]ed and manipulated via {list,get,set}xattrs (http://linux.die.net/man/2/listxattr) calls. .PP Any changes made at runtime are \f[B]not\f[] persisted. If you wish for values to persist they must be included as options wherever you configure the mounting of mergerfs (/etc/fstab). .SS Keys .PP Use \f[C]xattr\ \-l\ /mount/point/.mergerfs\f[] to see all supported keys. Some are informational and therefore readonly. .SS user.mergerfs.branches .PP \f[B]NOTE:\f[] formerly \f[C]user.mergerfs.srcmounts\f[] but said key is still supported. .PP Used to query or modify the list of source mounts. When modifying there are several shortcuts to easy manipulation of the list. .PP .TS tab(@); l l. T{ Value T}@T{ Description T} _ T{ [list] T}@T{ set T} T{ +<[list] T}@T{ prepend T} T{ +>[list] T}@T{ append T} T{ \-[list] T}@T{ remove all values provided T} T{ \-< T}@T{ remove first in list T} T{ \-> T}@T{ remove last in list T} .TE .PP \f[C]xattr\ \-w\ user.mergerfs.branches\ +\ \ " \ \ exit\ 1 fi CACHE="${1}" BACKING="${2}" N=${3} find\ "${CACHE}"\ \-type\ f\ \-atime\ +${N}\ \-printf\ \[aq]%P\\n\[aq]\ |\ \\ \ \ rsync\ \-\-files\-from=\-\ \-aq\ \-\-remove\-source\-files\ "${CACHE}/"\ "${BACKING}/" \f[] .fi .SS Percentage full expiring .PP Move the oldest file from the cache to the backing pool. Continue till below percentage threshold. .IP .nf \f[C] #!/bin/bash if\ [\ $#\ !=\ 3\ ];\ then \ \ echo\ "usage:\ $0\ \ \ " \ \ exit\ 1 fi CACHE="${1}" BACKING="${2}" PERCENTAGE=${3} set\ \-o\ errexit while\ [\ $(df\ \-\-output=pcent\ "${CACHE}"\ |\ grep\ \-v\ Use\ |\ cut\ \-d\[aq]%\[aq]\ \-f1)\ \-gt\ ${PERCENTAGE}\ ] do \ \ \ \ FILE=$(find\ "${CACHE}"\ \-type\ f\ \-printf\ \[aq]%A\@\ %P\\n\[aq]\ |\ \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ sort\ |\ \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ head\ \-n\ 1\ |\ \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ cut\ \-d\[aq]\ \[aq]\ \-f2\-) \ \ \ \ test\ \-n\ "${FILE}" \ \ \ \ rsync\ \-aq\ \-\-remove\-source\-files\ "${CACHE}/./${FILE}"\ "${BACKING}/" done \f[] .fi .SH TIPS / NOTES .IP \[bu] 2 The recommended base options are \f[B]defaults,allow_other,direct_io,use_ino\f[]. (\f[B]use_ino\f[] will only work when used with mergerfs 2.18.0 and above.) .IP \[bu] 2 Run mergerfs as \f[C]root\f[] unless you\[aq]re merging paths which are owned by the same user otherwise strange permission issues may arise. .IP \[bu] 2 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. .IP \[bu] 2 If you don\[aq]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 \f[C]mergerfs.fsck\f[] to audit the drive for out of sync permissions. .IP \[bu] 2 Do \f[B]not\f[] use \f[C]direct_io\f[] if you expect applications (such as rtorrent) to mmap (http://linux.die.net/man/2/mmap) files. It is not currently supported in FUSE w/ \f[C]direct_io\f[] enabled. Enabling \f[C]dropcacheonclose\f[] is recommended when \f[C]direct_io\f[] is disabled. .IP \[bu] 2 Since POSIX gives you only error or success on calls its difficult to determine the proper behavior when applying the behavior to multiple targets. \f[B]mergerfs\f[] 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. .IP \[bu] 2 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 \f[B]getattr\f[] policy of \f[B]ff\f[] its possible those programs will miss an update on account of it returning the first directory found\[aq]s \f[B]stat\f[] info and its a later directory on another mount which had the \f[B]mtime\f[] recently updated. To fix this you will want to set \f[B]func.getattr=newest\f[]. Remember though that this is just \f[B]stat\f[]. If the file is later \f[B]open\f[]\[aq]ed or \f[B]unlink\f[]\[aq]ed and the policy is different for those then a completely different file or directory could be acted on. .IP \[bu] 2 Some policies mixed with some functions may result in strange behaviors. Not that some of these behaviors and race conditions couldn\[aq]t happen outside \f[B]mergerfs\f[] 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. .IP \[bu] 2 For consistency its generally best to set \f[B]category\f[] wide policies rather than individual \f[B]func\f[]\[aq]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. .SH KNOWN ISSUES / BUGS .SS directory mtime is not being updated .PP Remember that the default policy for \f[C]getattr\f[] is \f[C]ff\f[]. The information for the first directory found will be returned. If it wasn\[aq]t the directory which had been updated then it will appear outdated. .PP 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 \f[C]ff\f[] but would also possibly spin up sleeping drives. .PP If you always want the directory information from the one with the most recent mtime then use the \f[C]newest\f[] policy for \f[C]getattr\f[]. .SS cached memory appears greater than it should be .PP Use the \f[C]direct_io\f[] 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 \f[C]direct_io\f[] 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. .PP Since enabling \f[C]direct_io\f[] disables \f[C]mmap\f[] this is not an ideal situation however write speeds should be increased. .PP If \f[C]direct_io\f[] is disabled it is probably a good idea to enable \f[C]dropcacheonclose\f[] to minimize double caching. .SS NFS clients don\[aq]t work .PP Some NFS clients appear to fail when a mergerfs mount is exported. Kodi in particular seems to have issues. .PP Try enabling the \f[C]use_ino\f[] option. Some have reported that it fixes the issue. .SS rtorrent fails with ENODEV (No such device) .PP Be sure to turn off \f[C]direct_io\f[]. 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 \f[C]direct_io\f[]. There will be a performance penalty on writes with \f[C]direct_io\f[] off as well as the problem of double caching but it\[aq]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 \f[C]direct_io\f[] enabled and one without it. .SS Plex doesn\[aq]t work with mergerfs .PP It does. If you\[aq]re trying to put Plex\[aq]s config / metadata on mergerfs you have to leave \f[C]direct_io\f[] off because Plex is using sqlite which apparently needs mmap. mmap doesn\[aq]t work with \f[C]direct_io\f[]. .PP If the issue is that scanning doesn\[aq]t seem to pick up media then be sure to set \f[C]func.getattr=newest\f[] as mentioned above. .SS mmap performance is really bad .PP 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). .SS When a program tries to move or rename a file it fails .PP Please read the section above regarding rename & link (#rename--link). .PP The problem is that many applications do not properly handle \f[C]EXDEV\f[] errors which \f[C]rename\f[] and \f[C]link\f[] 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: \f[C]\-o\ category.create=mfs\f[] .PP Ideally the offending software would be fixed and it is recommended that if you run into this problem you contact the software\[aq]s author and request proper handling of \f[C]EXDEV\f[] errors. .SS Samba: Moving files / directories fails .PP Workaround: Copy the file/directory and then remove the original rather than move. .PP This isn\[aq]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 \f[B]STATUS_NOT_SAME_DEVICE\f[] which comes from the \f[B]EXDEV\f[] which is returned by \f[B]rename\f[] when the call is crossing mount points. When a program gets an \f[B]EXDEV\f[] it needs to explicitly take an alternate action to accomplish it\[aq]s goal. In the case of \f[B]mv\f[] or similar it tries \f[B]rename\f[] and on \f[B]EXDEV\f[] 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 \f[B]EXDEV\f[] it would translate that into \f[B]EIO\f[]. This would cause \f[B]mv\f[] or most any application attempting to move files around on that SMB share to fail with a IO error. .PP 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 \f[C]apt\-cache\ showpkg\ gvfs\-fuse\f[]. 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. .PP In Apple\[aq]s MacOSX 10.9 they replaced Samba (client and server) with their own product. It appears their new client does not handle \f[B]EXDEV\f[] either and responds similar to older release of gvfs on Linux. .SS Trashing files occasionally fails .PP This is the same issue as with Samba. \f[C]rename\f[] returns \f[C]EXDEV\f[] (in our case that will really only happen with path preserving policies like \f[C]epmfs\f[]) and the software doesn\[aq]t handle the situtation well. This is unfortunately a common failure of software which moves files around. The standard indicates that an implementation \f[C]MAY\f[] choose to support non\-user home directory trashing of files (which is a \f[C]MUST\f[]). The implementation \f[C]MAY\f[] also support "top directory trashes" which many probably do. .PP To create a \f[C]$topdir/.Trash\f[] directory as defined in the standard use the mergerfs\-tools (https://github.com/trapexit/mergerfs-tools) tool \f[C]mergerfs.mktrash\f[]. .SS tar: Directory renamed before its status could be extracted .PP Make sure to use the \f[C]use_ino\f[] option. .SS Supplemental user groups .PP 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\[aq]s (at least the standard version) thread pool dynamically grows and shrinks it\[aq]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. .PP 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\[aq] 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\[aq]s cache at random. So long as there aren\[aq]t more than 256 active users this should be fine. If either value is too low for your needs you will have to modify \f[C]gidcache.hpp\f[] to increase the values. Note that doing so will increase the memory needed by each thread. .SS mergerfs or libfuse crashing .PP \f[B]NOTE:\f[] 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... .PP If suddenly the mergerfs mount point disappears and \f[C]Transport\ endpoint\ is\ not\ connected\f[] is returned when attempting to perform actions within the mount directory \f[B]and\f[] the version of libfuse (use \f[C]mergerfs\ \-v\f[] to find the version) is older than \f[C]2.9.4\f[] its likely due to a bug in libfuse. Affected versions of libfuse can be found in Debian Wheezy, Ubuntu Precise and others. .PP 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. .SS mergerfs appears to be crashing or exiting .PP There seems to be an issue with Linux version \f[C]4.9.0\f[] 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). .SS mergerfs under heavy load and memory preasure leads to kernel panic .PP https://lkml.org/lkml/2016/9/14/527 .IP .nf \f[C] [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:[]\ \ []\ 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]\ \ []\ __list_lru_walk_one.isra.3+0x8f/0x130 [25192.584137]\ \ []\ ?\ memcg_drain_all_list_lrus+0x190/0x190 [25192.586165]\ \ []\ list_lru_walk_one+0x23/0x30 [25192.588145]\ \ []\ scan_shadow_nodes+0x34/0x50 [25192.590074]\ \ []\ shrink_slab.part.40+0x1ed/0x3d0 [25192.591985]\ \ []\ shrink_zone+0x2ca/0x2e0 [25192.593863]\ \ []\ kswapd+0x51e/0x990 [25192.595737]\ \ []\ ?\ mem_cgroup_shrink_node_zone+0x1c0/0x1c0 [25192.597613]\ \ []\ kthread+0xd8/0xf0 [25192.599495]\ \ []\ ?\ kthread_create_on_node+0x1e0/0x1e0 [25192.601335]\ \ []\ ret_from_fork+0x3f/0x70 [25192.603193]\ \ []\ ?\ kthread_create_on_node+0x1e0/0x1e0 \f[] .fi .PP There is a bug in the kernel. A work around appears to be turning off \f[C]splice\f[]. Add \f[C]no_splice_write,no_splice_move,no_splice_read\f[] to mergerfs\[aq] options. Should be placed after \f[C]defaults\f[] if it is used since it will turn them on. This however is not guaranteed to work. .SS rm: fts_read failed: No such file or directory .PP Not \f[I]really\f[] 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 \f[C]rm\ \-rf\f[] or \f[C]find\f[] when scanning directories and they may try to remove them and they might have disappeared already. There is nothing \f[I]wrong\f[] 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. .PP Using the \f[B]hard_remove\f[] 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. .SH FAQ .SS How well does mergerfs scale? Is it "production ready?" .PP Users have reported running mergerfs on everything from a Raspberry Pi to dual socket Xeon systems with >20 cores. I\[aq]m aware of at least a few companies which use mergerfs in production. Open Media Vault (https://www.openmediavault.org) includes mergerfs is it\[aq]s sole solution for pooling drives. .SS Can mergerfs be used with drives which already have data / are in use? .PP Yes. MergerFS is a proxy and does \f[B]NOT\f[] interfere with the normal form or function of the drives / mounts / paths it manages. .PP MergerFS is \f[B]not\f[] a traditional filesystem. MergerFS is \f[B]not\f[] RAID. It does \f[B]not\f[] manipulate the data that passes through it. It does \f[B]not\f[] shard data across drives. It merely shards some \f[B]behavior\f[] and aggregates others. .SS Can mergerfs be removed without affecting the data? .PP See the previous question\[aq]s answer. .SS Do hard links work? .PP Yes. You need to use \f[C]use_ino\f[] to support proper reporting of inodes. Read the section "rename & link" for caveats. .SS Does mergerfs support CoW / copy\-on\-write? .PP 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 hardlink 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. .SS Why can\[aq]t I see my files / directories? .PP It\[aq]s almost always a permissions issue. Unlike mhddfs, which runs as root and attempts to access content as such, mergerfs always changes it\[aq]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. .PP 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\[aq]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 \f[C]chmod\ \-R\ 644\f[] when they should have \f[C]chmod\ \-R\ u=rwX,go=rX\f[]. .PP 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. .SS Why is only one drive being used? .PP Are you using a path preserving policy? The default policy for file creation is \f[C]epmfs\f[]. That means only the drives with the path preexisting will be considered when creating a file. If you don\[aq]t care about where files and directories are created you likely shouldn\[aq]t be using a path preserving policy and instead something like \f[C]mfs\f[]. .PP This can be especially apparent when filling an empty pool from an external source. If you do want path preservation you\[aq]ll need to perform the manual act of creating paths on the drives you want the data to land on before transfering your data. .SS Why was libfuse embedded into mergerfs? .PP A significant number of users use mergerfs on distros with very old versions of libfuse which have serious bugs. Requiring updated versions of libfuse on those distros isn\[aq]t pratical (no package offered, user inexperience, etc.). The only practical way to provide a stable runtime on those systems was to "vendor" the library into the project. .SS Why use mergerfs over mhddfs? .PP mhddfs is no longer maintained and has some known stability and security issues (see below). MergerFS provides a superset of mhddfs\[aq] features and should offer the same or maybe better performance. .PP Below is an example of mhddfs and mergerfs setup to work similarly. .PP \f[C]mhddfs\ \-o\ mlimit=4G,allow_other\ /mnt/drive1,/mnt/drive2\ /mnt/pool\f[] .PP \f[C]mergerfs\ \-o\ minfreespace=4G,defaults,allow_other,category.create=ff\ /mnt/drive1:/mnt/drive2\ /mnt/pool\f[] .SS Why use mergerfs over aufs? .PP aufs is mostly abandoned and no longer available in many distros. .PP 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. .SS Why use mergerfs over unionfs? .PP UnionFS is more like aufs then mergerfs in that it offers overlay / CoW features. If you\[aq]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. .SS Why use mergerfs over LVM/ZFS/BTRFS/RAID0 drive concatenation / striping? .PP 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. .PP 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. .SS Why use mergerfs over ZFS? .PP 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 \f[C]write\ once,\ read\ many\f[] 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). .SS Can drives be written to directly? Outside of mergerfs while pooled? .PP Yes. It will be represented immediately in the pool as the policies perscribe. .SS Why do I get an "out of space" error even though the system says there\[aq]s lots of space left? .PP First make sure you\[aq]ve read the sections above about policies, path preserving, and the \f[B]moveonenospc\f[] option. .PP 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 \f[B]not\f[] the contiguous space available. MergerFS does not split files across drives. If the writing of a file fills an underlying drive and \f[B]moveonenospc\f[] is disabled it will return an ENOSPC (No space left on device) error. .PP If \f[B]moveonenospc\f[] 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\[aq]t enough space. .PP It is also possible that the filesystem selected has run out of inodes. Use \f[C]df\ \-i\f[] 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. .SS Can mergerfs mounts be exported over NFS? .PP 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 \f[C]use_ino\f[] option often fixes that problem. .SS Can mergerfs mounts be exported over Samba / SMB? .PP Yes. While some users have reported problems it appears to always be related to how Samba is setup in relation to permissions. .SS How are inodes calculated? .PP mergerfs\-inode = (original\-inode | (device\-id << 32)) .PP While \f[C]ino_t\f[] is 64 bits only a few filesystems use more than 32. Similarly, while \f[C]dev_t\f[] is also 64 bits it was traditionally 16 bits. Bitwise or\[aq]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. .PP 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 \f[C]nodeid\f[] 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 \f[C]stat\f[] family of calls or \f[C]readdir\f[]. .PP From FUSE docs regarding \f[C]use_ino\f[]: .IP .nf \f[C] 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"). \f[] .fi .SS I notice massive slowdowns of writes over NFS .PP Due to how NFS works and interacts with FUSE when not using \f[C]direct_io\f[] its possible that a getxattr for \f[C]security.capability\f[] will be issued prior to any write. This will usually result in a massive slowdown for writes. Using \f[C]direct_io\f[] will keep this from happening (and generally good to enable unless you need the features it disables) but the \f[C]security_capability\f[] option can also help by short circuiting the call and returning \f[C]ENOATTR\f[]. .PP You could also set \f[C]xattr\f[] to \f[C]noattr\f[] or \f[C]notsup\f[] to short circuit or stop all xattr requests. .SS What are these .fuse_hidden files? .PP When not using \f[C]hard_remove\f[] 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. .PP The files are cleaned up once the file is finally closed. Only if mergerfs crashes or is killed would they be left around. .SS It\[aq]s mentioned that there are some security issues with mhddfs. What are they? How does mergerfs address them? .PP mhddfs (https://github.com/trapexit/mhddfs) manages running as \f[B]root\f[] by calling getuid() (https://github.com/trapexit/mhddfs/blob/cae96e6251dd91e2bdc24800b4a18a74044f6672/src/main.c#L319) and if it returns \f[B]0\f[] then it will chown (http://linux.die.net/man/1/chown) the file. Not only is that a race condition but it doesn\[aq]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. .PP 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 \f[B]Samba\f[], mergerfs uses \f[B]syscall(SYS_setreuid,...)\f[] to set the callers credentials for that thread only. Jumping back to \f[B]root\f[] as necessary should escalated privileges be needed (for instance: to clone paths between drives). .PP 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\[aq]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\[aq]t starve. This isn\[aq]t the best solution but should work reasonably well assuming there are few users. .SH PERFORMANCE TWEAKING .IP \[bu] 2 try adding (or removing) \f[C]direct_io\f[] .IP \[bu] 2 try adding (or removing) \f[C]auto_cache\f[] / \f[C]noauto_cache\f[] (included in \f[C]defaults\f[]) .IP \[bu] 2 try adding (or removing) \f[C]kernel_cache\f[] (don\[aq]t use the underlying filesystems directly if enabling \f[C]kernel_cache\f[]) .IP \[bu] 2 try adding (or removing) \f[C]splice_move\f[], \f[C]splice_read\f[], and \f[C]splice_write\f[] (all three included in \f[C]defaults\f[]) .IP \[bu] 2 try changing the number of worker threads .IP \[bu] 2 try disabling \f[C]security_capability\f[] or \f[C]xattr\f[] .IP \[bu] 2 test theoretical performance using \f[C]nullrw\f[] or mounting a ram disk .IP \[bu] 2 use \f[C]symlinkify\f[] if your data is largely static and you need native speed reads .IP \[bu] 2 use lvm and lvm cache to place a SSD in front of your HDDs (howto coming) .SH SUPPORT .PP 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 \f[B]please, please\f[] 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. .SS Information to include in bug reports .IP \[bu] 2 Version of mergerfs: \f[C]mergerfs\ \-V\f[] .IP \[bu] 2 mergerfs settings: from \f[C]/etc/fstab\f[] or command line execution .IP \[bu] 2 Version of Linux: \f[C]uname\ \-a\f[] .IP \[bu] 2 Versions of any additional software being used .IP \[bu] 2 List of drives, their filesystems, and sizes (before and after issue): \f[C]df\ \-h\f[] .IP \[bu] 2 A \f[C]strace\f[] of the app having problems: .IP \[bu] 2 \f[C]strace\ \-f\ \-o\ /tmp/app.strace.txt\ \f[] .IP \[bu] 2 A \f[C]strace\f[] of mergerfs while the program is trying to do whatever it\[aq]s failing to do: .IP \[bu] 2 \f[C]strace\ \-f\ \-p\ \ \-o\ /tmp/mergerfs.strace.txt\f[] .IP \[bu] 2 \f[B]Precise\f[] directions on replicating the issue. Do not leave \f[B]anything\f[] out. .IP \[bu] 2 Try to recreate the problem in the simplist way using standard programs. .SS Contact / Issue submission .IP \[bu] 2 github.com: https://github.com/trapexit/mergerfs/issues .IP \[bu] 2 email: trapexit\@spawn.link .IP \[bu] 2 twitter: https://twitter.com/_trapexit .SS Support development .PP 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. .IP \[bu] 2 Bitcoin (BTC): 12CdMhEPQVmjz3SSynkAEuD5q9JmhTDCZA .IP \[bu] 2 Bitcoin Cash (BCH): 1AjPqZZhu7GVEs6JFPjHmtsvmDL4euzMzp .IP \[bu] 2 Ethereum (ETH): 0x09A166B11fCC127324C7fc5f1B572255b3046E94 .IP \[bu] 2 Litecoin (LTC): LXAsq6yc6zYU3EbcqyWtHBrH1Ypx4GjUjm .IP \[bu] 2 Ripple (XRP): rNACR2hqGjpbHuCKwmJ4pDpd2zRfuRATcE .IP \[bu] 2 PayPal: trapexit\@spawn.link .IP \[bu] 2 Patreon: https://www.patreon.com/trapexit .SH LINKS .IP \[bu] 2 http://github.com/trapexit/mergerfs .IP \[bu] 2 http://github.com/trapexit/mergerfs\-tools .IP \[bu] 2 http://github.com/trapexit/scorch .IP \[bu] 2 http://github.com/trapexit/backup\-and\-recovery\-howtos .SH AUTHORS Antonio SJ Musumeci .