mergerfs/man/mergerfs.1

.\"t
.\" Automatically generated by Pandoc 1.19.2.4
.\"
.TH "mergerfs" "1" "2020\-04\-20" "mergerfs user manual" ""
.hy
.SH NAME
.PP
mergerfs \- a featureful union filesystem
.SH SYNOPSIS
.PP
mergerfs \-o<options> <branches> <mountpoint>
.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 / file placement
.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 read\-only and read/write drives
.IP \[bu] 2
Can turn read\-only files into symlinks to underlying file
.IP \[bu] 2
Hard link copy\-on\-write / CoW
.IP \[bu] 2
supports POSIX ACLs
.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 read\-write and read\-only drives.
It also does \f[B]not\f[] split data across drives.
It is not RAID0 / striping.
It is simply a union.
.SH BASIC SETUP
.PP
If you don\[aq]t already know that you have a special use case then just
start with one of the following option sets.
.SS You don\[aq]t need \f[C]mmap\f[]
.PP
\f[C]use_ino,cache.files=off,dropcacheonclose=true,allow_other,category.create=mfs\f[]
.SS You do need \f[C]mmap\f[] (used by rtorrent and some other programs)
.PP
\f[C]use_ino,cache.files=partial,dropcacheonclose=true,allow_other,category.create=mfs\f[]
.PP
See the mergerfs wiki for real world
deployments (https://github.com/trapexit/mergerfs/wiki/Real-World-Deployments)
for comparisons / ideas.
.SH OPTIONS
.SS mount options
.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]minfreespace=SIZE\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=BOOL\f[]: When enabled 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 recommended it be enabled so that linked files
share the same inode value.
.IP \[bu] 2
\f[B]dropcacheonclose=BOOL\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]cache.files=partial|full|auto\-full\f[] to limit
double caching.
(default: false)
.IP \[bu] 2
\f[B]symlinkify=BOOL\f[]: When enabled 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=INT\f[]: Time to wait, in seconds, to activate
the \f[B]symlinkify\f[] behavior.
(default: 3600)
.IP \[bu] 2
\f[B]nullrw=BOOL\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=BOOL\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=BOOL\f[]: If false return ENOATTR when xattr
security.capability is queried.
(default: true)
.IP \[bu] 2
\f[B]xattr=passthrough|noattr|nosys\f[]: Runtime control of xattrs.
Default is to passthrough xattr requests.
\[aq]noattr\[aq] will short circuit as if nothing exists.
\[aq]nosys\[aq] will respond with ENOSYS as if xattrs are not supported
or disabled.
(default: passthrough)
.IP \[bu] 2
\f[B]link_cow=BOOL\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]statfs=base|full\f[]: Controls how statfs works.
\[aq]base\[aq] means it will always use all branches in statfs
calculations.
\[aq]full\[aq] is in effect path preserving and only includes drives
where the path exists.
(default: base)
.IP \[bu] 2
\f[B]statfs_ignore=none|ro|nc\f[]: \[aq]ro\[aq] will cause statfs
calculations to ignore available space for branches mounted or tagged as
\[aq]read\-only\[aq] or \[aq]no create\[aq].
\[aq]nc\[aq] will ignore available space for branches tagged as \[aq]no
create\[aq].
(default: none)
.IP \[bu] 2
\f[B]posix_acl=BOOL\f[]: Enable POSIX ACL support (if supported by
kernel and underlying filesystem).
(default: false)
.IP \[bu] 2
\f[B]async_read=BOOL\f[]: Perform reads asynchronously.
If disabled or unavailable the kernel will ensure there is at most one
pending read request per file handle and will attempt to order requests
by offset.
(default: true)
.IP \[bu] 2
\f[B]fuse_msg_size=INT\f[]: Set the max number of pages per FUSE
message.
Only available on Linux >= 4.20 and ignored otherwise.
(min: 1; max: 256; default: 256)
.IP \[bu] 2
\f[B]threads=INT\f[]: Number of threads to use in multithreaded mode.
When set to zero it will attempt to discover and use the number of
logical cores.
If the lookup fails it will fall back to using 4.
If the thread count is set negative it will look up the number of cores
then divide by the absolute value.
ie.
threads=\-2 on an 8 core machine will result in 8 / 2 = 4 threads.
There will always be at least 1 thread.
NOTE: higher number of threads increases parallelism but usually
decreases throughput.
(default: 0)
.IP \[bu] 2
\f[B]fsname=STR\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.FUNC=POLICY\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.CATEGORY=POLICY\f[]: Sets policy of all FUSE functions in
the provided category.
See POLICIES section for defaults.
Example: \f[B]category.create=mfs\f[]
.IP \[bu] 2
\f[B]cache.open=INT\f[]: \[aq]open\[aq] policy cache timeout in seconds.
(default: 0)
.IP \[bu] 2
\f[B]cache.statfs=INT\f[]: \[aq]statfs\[aq] cache timeout in seconds.
(default: 0)
.IP \[bu] 2
\f[B]cache.attr=INT\f[]: File attribute cache timeout in seconds.
(default: 1)
.IP \[bu] 2
\f[B]cache.entry=INT\f[]: File name lookup cache timeout in seconds.
(default: 1)
.IP \[bu] 2
\f[B]cache.negative_entry=INT\f[]: Negative file name lookup cache
timeout in seconds.
(default: 0)
.IP \[bu] 2
\f[B]cache.files=libfuse|off|partial|full|auto\-full\f[]: File page
caching mode (default: libfuse)
.IP \[bu] 2
\f[B]cache.writeback=BOOL\f[]: Enable kernel writeback caching (default:
false)
.IP \[bu] 2
\f[B]cache.symlinks=BOOL\f[]: Cache symlinks (if supported by kernel)
(default: false)
.IP \[bu] 2
\f[B]cache.readdir=BOOL\f[]: Cache readdir (if supported by kernel)
(default: false)
.IP \[bu] 2
\f[B]direct_io\f[]: deprecated \- Bypass page cache.
Use \f[C]cache.files=off\f[] instead.
(default: false)
.IP \[bu] 2
\f[B]kernel_cache\f[]: deprecated \- Do not invalidate data cache on
file open.
Use \f[C]cache.files=full\f[] instead.
(default: false)
.IP \[bu] 2
\f[B]auto_cache\f[]: deprecated \- Invalidate data cache if file mtime
or size change.
Use \f[C]cache.files=auto\-full\f[] instead.
(default: false)
.IP \[bu] 2
\f[B]async_read\f[]: deprecated \- Perform reads asynchronously.
Use \f[C]async_read=true\f[] instead.
.IP \[bu] 2
\f[B]sync_read\f[]: deprecated \- Perform reads synchronously.
Use \f[C]async_read=false\f[] instead.
.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 Value Types
.IP \[bu] 2
BOOL = \[aq]true\[aq] | \[aq]false\[aq]
.IP \[bu] 2
INT = [0,MAX_INT]
.IP \[bu] 2
SIZE = \[aq]NNM\[aq]; NN = INT, M = \[aq]K\[aq] | \[aq]M\[aq] |
\[aq]G\[aq] | \[aq]T\[aq]
.IP \[bu] 2
STR = string
.IP \[bu] 2
FUNC = FUSE function
.IP \[bu] 2
CATEGORY = FUSE function category
.IP \[bu] 2
POLICY = mergerfs function policy
.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]=NC\f[] (no create).
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]NC\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\ 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 accessible from related
tools use \f[B]/etc/fstab\f[].
.IP
.nf
\f[C]
#\ <file\ system>\ \ \ \ \ \ \ \ <mount\ point>\ \ <type>\ \ \ \ <options>\ \ \ \ \ \ \ \ \ \ \ \ \ <dump>\ \ <pass>
/mnt/disk*:/mnt/cdrom\ \ /media/drives\ \ mergerfs\ \ 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 fuse_msg_size
.PP
FUSE applications communicate with the kernel over a special character
device: \f[C]/dev/fuse\f[].
A large portion of the overhead associated with FUSE is the cost of
going back and forth from user space and kernel space over that device.
Generally speaking the fewer trips needed the better the performance
will be.
Reducing the number of trips can be done a number of ways.
Kernel level caching and increasing message sizes being two significant
ones.
When it comes to reads and writes if the message size is doubled the
number of trips are approximately halved.
.PP
In Linux 4.20 a new feature was added allowing the negotiation of the
max message size.
Since the size is in multiples of
pages (https://en.wikipedia.org/wiki/Page_(computer_memory)) the feature
is called \f[C]max_pages\f[].
There is a maximum \f[C]max_pages\f[] value of 256 (1MiB) and minimum of
1 (4KiB).
The default used by Linux >=4.20, and hardcoded value used before 4.20,
is 32 (128KiB).
In mergerfs its referred to as \f[C]fuse_msg_size\f[] to make it clear
what it impacts and provide some abstraction.
.PP
Since there should be no downsides to increasing \f[C]fuse_msg_size\f[]
/ \f[C]max_pages\f[], outside a minor bump in RAM usage due to larger
message buffers, mergerfs defaults the value to 256.
On kernels before 4.20 the value has no effect.
The reason the value is configurable is to enable experimentation and
benchmarking.
See the BENCHMARKING section for examples.
.SS symlinkify
.PP
Due to the levels of indirection introduced by mergerfs and the
underlying technology FUSE there can be varying levels of performance
degradation.
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 that wouldn\[aq]t exist for an in kernel one.
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
boundaries.
.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
See the BENCHMARKING section for suggestions on how to test.
.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.
The performance problems mostly comes when file caching is enabled.
The kernel will send a \f[C]getxattr\f[] for
\f[C]security.capability\f[] \f[I]before every single write\f[].
It doesn\[aq]t cache the responses to any \f[C]getxattr\f[].
This might be addressed in the future but for now mergerfs can really
only offer the following workarounds.
.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]nosys\f[] will cause mergerfs to return ENOSYS 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 is 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 is made up of a number of functions.
\f[B]creat\f[], \f[B]stat\f[], \f[B]chown\f[], etc.
In mergerfs most of the core functions are grouped into 3 categories:
\f[B]action\f[], \f[B]create\f[], and \f[B]search\f[].
These functions and categories can be assigned a policy which dictates
what file or directory is chosen when performing that behavior.
Any policy can be assigned to a function or category though some may not
be very useful in practice.
For instance: \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[]
if there were more than one copy of the file.
.PP
Some functions, listed in the category \f[C]N/A\f[] below, can not be
assigned the normal policies.
All functions which work on file handles use the handle which was
acquired by \f[C]open\f[] or \f[C]create\f[].
\f[C]readdir\f[] has no real need for a policy given the purpose is
merely to return a list of entries in a directory.
\f[C]statfs\f[]\[aq]s behavior can be modified via other options.
That said many times the current FUSE kernel driver will not always
provide the file handle when a client calls \f[C]fgetattr\f[],
\f[C]fchown\f[], \f[C]fchmod\f[], \f[C]futimens\f[], \f[C]ftruncate\f[],
etc.
This means it will call the regular, path based, versions.
.PP
When using policies which are based on a branch\[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 (directories), listxattr, open,
readlink
T}
T{
N/A
T}@T{
fchmod, fchown, futimens, ftruncate, fallocate, fgetattr, fsync, ioctl
(files), read, readdir, release, statfs, write, copy_file_range
T}
.TE
.PP
In cases where something may be searched (to confirm a directory exists
across all source mounts) \f[B]getattr\f[] will be used.
.SS Path Preservation
.PP
Policies, as described below, are of two basic 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 Filters
.PP
Policies basically search branches and create a list of files / paths
for functions to work on.
The policy is responsible for filtering and sorting.
The policy type defines the sorting but filtering is mostly uniform as
described below.
.IP \[bu] 2
No \f[B]search\f[] policies filter.
.IP \[bu] 2
All \f[B]action\f[] policies will filter out branches which are mounted
\f[B]read\-only\f[] or tagged as \f[B]RO (read\-only)\f[].
.IP \[bu] 2
All \f[B]create\f[] policies will filter out branches which are mounted
\f[B]read\-only\f[], tagged \f[B]RO (read\-only)\f[] or \f[B]NC (no
create)\f[], or has available space less than \f[C]minfreespace\f[].
.PP
If all branches are filtered an error will be returned.
Typically \f[B]EROFS\f[] or \f[B]ENOSPC\f[] depending on the reasons.
.SS Policy descriptions
.PP
.TS
tab(@);
lw(16.6n) lw(53.4n).
T{
Policy
T}@T{
Description
T}
_
T{
all
T}@T{
Search category: same as \f[B]epall\f[].
Action category: same as \f[B]epall\f[].
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[].
T}
T{
epall (existing path, all)
T}@T{
Search category: same as \f[B]epff\f[] (but more expensive because it
doesn\[aq]t stop after finding a valid branch).
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 found.
\f[B]create\f[] works like \f[B]epff\f[] (but more expensive because it
doesn\[aq]t stop after finding a valid branch).
T}
T{
epff (existing path, first found)
T}@T{
Given the order of the branches, as defined at mount time or configured
at runtime, act on the first one found where the relative path exists.
T}
T{
eplfs (existing path, least free space)
T}@T{
Of all the branches on which the relative path exists choose the drive
with the least free space.
T}
T{
eplus (existing path, least used space)
T}@T{
Of all the branches on which the relative path exists choose the drive
with the least used space.
T}
T{
epmfs (existing path, most free space)
T}@T{
Of all the branches on which the relative path exists choose the drive
with the most free space.
T}
T{
eprand (existing path, random)
T}@T{
Calls \f[B]epall\f[] and then randomizes.
Returns 1.
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{
Search category: same as \f[B]epff\f[].
Action category: same as \f[B]epff\f[].
Create category: Given the order of the drives, as defined at mount time
or configured at runtime, act on the first one found.
T}
T{
lfs (least free space)
T}@T{
Search category: same as \f[B]eplfs\f[].
Action category: same as \f[B]eplfs\f[].
Create category: Pick the drive with the least available free space.
T}
T{
lus (least used space)
T}@T{
Search category: same as \f[B]eplus\f[].
Action category: same as \f[B]eplus\f[].
Create category: Pick the drive with the least used space.
T}
T{
mfs (most free space)
T}@T{
Search category: same as \f[B]epmfs\f[].
Action category: same as \f[B]epmfs\f[].
Create category: Pick the drive with the most available free space.
T}
T{
newest
T}@T{
Pick the file / directory with the largest mtime.
T}
T{
rand (random)
T}@T{
Calls \f[B]all\f[] and then randomizes.
Returns 1.
T}
.TE
.PP
\f[B]NOTE:\f[] If you are using an underlying filesystem that reserves
blocks such as ext2, ext3, or ext4 be aware that mergerfs respects the
reservation by using \f[C]f_bavail\f[] (number of free blocks for
unprivileged users) rather than \f[C]f_bfree\f[] (number of free blocks)
in policy calculations.
\f[B]df\f[] does NOT use \f[C]f_bavail\f[], it uses \f[C]f_bfree\f[], so
direct comparisons between \f[B]df\f[] output and mergerfs\[aq] policies
is not appropriate.
.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 ioctl
.PP
When \f[C]ioctl\f[] is used with an open file then it will use the file
handle which was created at the original \f[C]open\f[] call.
However, when using \f[C]ioctl\f[] with a directory mergerfs will use
the \f[C]open\f[] policy to find the directory to act on.
.SS unlink
.PP
In FUSE there is an opaque "file handle" which is created by
\f[C]open\f[], \f[C]create\f[], or \f[C]opendir\f[], passed to the
kernel, and then is passed back to the FUSE userland application by the
kernel.
Unfortunately, the FUSE kernel driver does not always send the file
handle when it theoretically could/should.
This complicates certain behaviors / workflows particularly in the high
level API.
As a result mergerfs is currently doing a few hacky things.
.PP
libfuse2 and libfuse3, when using the high level API, will rename names
to \f[C]\&.fuse_hiddenXXXXXX\f[] if the file is open when unlinked or
renamed over.
It does this so the file is still available when a request referencing
the now missing file is made.
This file however keeps a \f[C]rmdir\f[] from succeeding and can be
picked up by software reading directories.
.PP
The change mergerfs has done is that if a file is open when an unlink or
rename happens it will open the file and keep it open till closed by all
those who opened it prior.
When a request comes in referencing that file and it doesn\[aq]t include
a file handle it will instead use the file handle created at
unlink/rename time.
.PP
This won\[aq]t result in technically proper behavior but close enough
for many usecases.
.PP
The plan is to rewrite mergerfs to use the low level API so these
invasive libfuse changes are no longer necessary.
.SS rename & link
.PP
\f[B]NOTE:\f[] If you\[aq]re receiving errors from software when files
are moved / renamed / linked 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
\f[C]rename\f[] and \f[C]link\f[] are tricky functions in a union
filesystem.
\f[C]rename\f[] 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).
So if a \f[C]rename\f[]\[aq]s source and target are on different drives
within the pool it creates an issue.
.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 compliant 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 basic 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 strategy but without the removals.
.SS readdir
.PP
readdir (http://linux.die.net/man/3/readdir) is different from all other
filesystem functions.
While it could have its 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 statfs / 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 its space.
Filesystems mounted further down the tree of the branch will not be
included when checking the mount\[aq]s stats.
.PP
The options \f[C]statfs\f[] and \f[C]statfs_ignore\f[] can be used to
modify \f[C]statfs\f[] behavior.
.SH BUILD / UPDATE
.PP
\f[B]NOTE:\f[] Prebuilt packages can be found at:
https://github.com/trapexit/mergerfs/releases
.PP
First get the code from github (https://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/<ver>/mergerfs\-<ver>.tar.gz
\f[]
.fi
.SS Debian / Ubuntu
.IP
.nf
\f[C]
$\ cd\ mergerfs
$\ sudo\ tools/install\-build\-pkgs
$\ make\ deb
$\ sudo\ dpkg\ \-i\ ../mergerfs_version_arch.deb
\f[]
.fi
.SS RHEL / CentOS /Fedora
.IP
.nf
\f[C]
$\ su\ \-
#\ cd\ mergerfs
#\ tools/install\-build\-pkgs
#\ make\ rpm
#\ rpm\ \-i\ rpmbuild/RPMS/<arch>/mergerfs\-<verion>.<arch>.rpm
\f[]
.fi
.SS Generically
.PP
Have git, g++, make, python installed.
.IP
.nf
\f[C]
$\ cd\ mergerfs
$\ make
$\ sudo\ make\ install
\f[]
.fi
.SS Build options
.IP
.nf
\f[C]
$\ make\ help
usage:\ make

make\ USE_XATTR=0\ \ \ \ \ \ \-\ build\ program\ without\ xattrs\ functionality
make\ STATIC=1\ \ \ \ \ \ \ \ \ \-\ build\ static\ binary
make\ LTO=1\ \ \ \ \ \ \ \ \ \ \ \ \-\ build\ with\ link\ time\ optimization
\f[]
.fi
.SH RUNTIME CONFIG
.SS .mergerfs pseudo file
.IP
.nf
\f[C]
<mountpoint>/.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\ /mountpoint/.mergerfs\f[] to see all supported
keys.
Some are informational and therefore read\-only.
\f[C]setxattr\f[] will return EINVAL on read\-only keys.
.SS Values
.PP
Same as the command line.
.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 branches.
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\ +</mnt/drive3\ /mnt/pool/.mergerfs\f[]
.PP
The \f[C]=NC\f[], \f[C]=RO\f[], \f[C]=RW\f[] syntax works just as on the
command line.
.SS Example
.IP
.nf
\f[C]
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-l\ .mergerfs
user.mergerfs.branches:\ /mnt/a=RW:/mnt/b=RW
user.mergerfs.minfreespace:\ 4294967295
user.mergerfs.moveonenospc:\ false
\&...

[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.category.search\ .mergerfs
ff

[trapexit:/mnt/mergerfs]\ $\ xattr\ \-w\ user.mergerfs.category.search\ newest\ .mergerfs
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.category.search\ .mergerfs
newest

[trapexit:/mnt/mergerfs]\ $\ xattr\ \-w\ user.mergerfs.branches\ +/mnt/c\ .mergerfs
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.branches\ .mergerfs
/mnt/a:/mnt/b:/mnt/c

[trapexit:/mnt/mergerfs]\ $\ xattr\ \-w\ user.mergerfs.branches\ =/mnt/c\ .mergerfs
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.branches\ .mergerfs
/mnt/c

[trapexit:/mnt/mergerfs]\ $\ xattr\ \-w\ user.mergerfs.branches\ \[aq]+</mnt/a:/mnt/b\[aq]\ .mergerfs
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.branches\ .mergerfs
/mnt/a:/mnt/b:/mnt/c
\f[]
.fi
.SS file / directory xattrs
.PP
While they won\[aq]t show up when using
listxattr (http://linux.die.net/man/2/listxattr) \f[B]mergerfs\f[]
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:
.IP \[bu] 2
\f[B]user.mergerfs.basepath\f[]: the base mount point for the file given
the current getattr policy
.IP \[bu] 2
\f[B]user.mergerfs.relpath\f[]: the relative path of the file from the
perspective of the mount point
.IP \[bu] 2
\f[B]user.mergerfs.fullpath\f[]: the full path of the original file
given the getattr policy
.IP \[bu] 2
\f[B]user.mergerfs.allpaths\f[]: a NUL (\[aq]\[aq]) separated list of
full paths to all files found
.IP
.nf
\f[C]
[trapexit:/mnt/mergerfs]\ $\ ls
A\ B\ C
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.fullpath\ A
/mnt/a/full/path/to/A
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.basepath\ A
/mnt/a
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.relpath\ A
/full/path/to/A
[trapexit:/mnt/mergerfs]\ $\ xattr\ \-p\ user.mergerfs.allpaths\ A\ |\ tr\ \[aq]\\0\[aq]\ \[aq]\\n\[aq]
/mnt/a/full/path/to/A
/mnt/b/full/path/to/A
\f[]
.fi
.SH UPGRADE
.PP
mergerfs can be upgraded live by mounting on top of the previous
version.
Simply install the new version of mergerfs and follow the instructions
below.
.PP
Add \f[C]nonempty\f[] to your mergerfs option list and call mergerfs
again or if using \f[C]/etc/fstab\f[] call for it to mount again.
Existing open files and such will continue to work fine though they
won\[aq]t see runtime changes since any such change would be the new
mount.
If you plan on changing settings with the new mount you should / could
apply those before mounting the new version.
.IP
.nf
\f[C]
$\ sudo\ mount\ /mnt/mergerfs
$\ mount\ |\ grep\ mergerfs
media\ on\ /mnt/mergerfs\ type\ fuse.mergerfs\ (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)
media\ on\ /mnt/mergerfs\ type\ fuse.mergerfs\ (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)
\f[]
.fi
.SH TOOLING
.IP \[bu] 2
https://github.com/trapexit/mergerfs\-tools
.IP \[bu] 2
mergerfs.ctl: A tool to make it easier to query and configure mergerfs
at runtime
.IP \[bu] 2
mergerfs.fsck: Provides permissions and ownership auditing and the
ability to fix them
.IP \[bu] 2
mergerfs.dedup: Will help identify and optionally remove duplicate files
.IP \[bu] 2
mergerfs.dup: Ensure there are at least N copies of a file across the
pool
.IP \[bu] 2
mergerfs.balance: Rebalance files across drives by moving them from the
most filled to the least filled
.IP \[bu] 2
mergerfs.consolidate: move files within a single mergerfs directory to
the drive with most free space
.IP \[bu] 2
mergerfs.mktrash: Creates FreeDesktop.org Trash specification compatible
directories on a mergerfs mount
.IP \[bu] 2
https://github.com/trapexit/scorch
.IP \[bu] 2
scorch: A tool to help discover silent corruption of files and keep
track of files
.IP \[bu] 2
https://github.com/trapexit/bbf
.IP \[bu] 2
bbf (bad block finder): a tool to scan for and \[aq]fix\[aq] hard drive
bad blocks and find the files using those blocks
.SH CACHING
.SS page caching
.PP
https://en.wikipedia.org/wiki/Page_cache
.PP
tl;dr: * cache.files=off: Disables page caching.
Underlying files cached, mergerfs files are not.
* cache.files=partial: Enables page caching.
Underlying files cached, mergerfs files cached while open.
* cache.files=full: Enables page caching.
Underlying files cached, mergerfs files cached across opens.
* cache.files=auto\-full: Enables page caching.
Underlying files cached, mergerfs files cached across opens if mtime and
size are unchanged since previous open.
* cache.files=libfuse: follow traditional libfuse \f[C]direct_io\f[],
\f[C]kernel_cache\f[], and \f[C]auto_cache\f[] arguments.
.PP
FUSE, which mergerfs uses, offers a number of page caching modes.
mergerfs tries to simplify their use via the \f[C]cache.files\f[]
option.
It can and should replace usage of \f[C]direct_io\f[],
\f[C]kernel_cache\f[], and \f[C]auto_cache\f[].
.PP
Due to mergerfs using FUSE and therefore being a userland process
proxying existing filesystems the kernel will double cache the content
being read and written through mergerfs.
Once from the underlying filesystem and once from mergerfs (it sees them
as two separate entities).
Using \f[C]cache.files=off\f[] will keep the double caching from
happening by disabling caching of mergerfs but this has the side effect
that \f[I]all\f[] read and write calls will be passed to mergerfs which
may be slower than enabling caching, you lose shared \f[C]mmap\f[]
support which can affect apps such as rtorrent, and no read\-ahead will
take place.
The kernel will still cache the underlying filesystem data but that only
helps so much given mergerfs will still process all requests.
.PP
If you do enable file page caching,
\f[C]cache.files=partial|full|auto\-full\f[], you should also enable
\f[C]dropcacheonclose\f[] which will cause mergerfs to instruct the
kernel to flush the underlying file\[aq]s page cache when the file is
closed.
This behavior is the same as the rsync fadvise / drop cache patch and
Feh\[aq]s nocache project.
.PP
If most files are read once through and closed (like media) it is best
to enable \f[C]dropcacheonclose\f[] regardless of caching mode in order
to minimize buffer bloat.
.PP
It is difficult to balance memory usage, cache bloat & duplication, and
performance.
Ideally mergerfs would be able to disable caching for the files it
reads/writes but allow page caching for itself.
That would limit the FUSE overhead.
However, there isn\[aq]t a good way to achieve this.
It would need to open all files with O_DIRECT which places limitations
on the what underlying filesystems would be supported and complicates
the code.
.PP
kernel documentation:
https://www.kernel.org/doc/Documentation/filesystems/fuse\-io.txt
.SS entry & attribute caching
.PP
Given the relatively high cost of FUSE due to the kernel <\-> userspace
round trips there are kernel side caches for file entries and
attributes.
The entry cache limits the \f[C]lookup\f[] calls to mergerfs which ask
if a file exists.
The attribute cache limits the need to make \f[C]getattr\f[] calls to
mergerfs which provide file attributes (mode, size, type, etc.).
As with the page cache these should not be used if the underlying
filesystems are being manipulated at the same time as it could lead to
odd behavior or data corruption.
The options for setting these are \f[C]cache.entry\f[] and
\f[C]cache.negative_entry\f[] for the entry cache and
\f[C]cache.attr\f[] for the attributes cache.
\f[C]cache.negative_entry\f[] refers to the timeout for negative
responses to lookups (non\-existent files).
.SS writeback caching
.PP
When \f[C]cache.files\f[] is enabled the default is for it to perform
writethrough caching.
This behavior won\[aq]t help improve performance as each write still
goes one for one through the filesystem.
By enabling the FUSE writeback cache small writes may be aggregated by
the kernel and then sent to mergerfs as one larger request.
This can greatly improve the throughput for apps which write to files
inefficiently.
The amount the kernel can aggregate is limited by the size of a FUSE
message.
Read the \f[C]fuse_msg_size\f[] section for more details.
.PP
There is a small side effect as a result of enabling wrtieback caching.
Underlying files won\[aq]t ever be opened with O_APPEND or O_WRONLY.
The former because the kernel then manages append mode and the latter
because the kernel may request file data from mergerfs to populate the
write cache.
The O_APPEND change means that if a file is changed outside of mergerfs
it could lead to corruption as the kernel won\[aq]t know the end of the
file has changed.
That said any time you use caching you should keep from using the same
file outside of mergerfs at the same time.
.PP
Note that if an application is properly sizing writes then writeback
caching will have little or no effect.
It will only help with writes of sizes below the FUSE message size (128K
on older kernels, 1M on newer).
.SS policy caching
.PP
Policies are run every time a function (with a policy as mentioned
above) is called.
These policies can be expensive depending on mergerfs\[aq] setup and
client usage patterns.
Generally we wouldn\[aq]t want to cache policy results because it may
result in stale responses if the underlying drives are used directly.
.PP
The \f[C]open\f[] policy cache will cache the result of an \f[C]open\f[]
policy for a particular input for \f[C]cache.open\f[] seconds or until
the file is unlinked.
Each file close (release) will randomly chose to clean up the cache of
expired entries.
.PP
This cache is really only useful in cases where you have a large number
of branches and \f[C]open\f[] is called on the same files repeatedly
(like \f[B]Transmission\f[] which opens and closes a file on every
read/write presumably to keep file handle usage low).
.SS statfs caching
.PP
Of the syscalls used by mergerfs in policies the \f[C]statfs\f[] /
\f[C]statvfs\f[] call is perhaps the most expensive.
It\[aq]s used to find out the available space of a drive and whether it
is mounted read\-only.
Depending on the setup and usage pattern these queries can be relatively
costly.
When \f[C]cache.statfs\f[] is enabled all calls to \f[C]statfs\f[] by a
policy will be cached for the number of seconds its set to.
.PP
Example: If the create policy is \f[C]mfs\f[] and the timeout is 60 then
for that 60 seconds the same drive will be returned as the target for
creates because the available space won\[aq]t be updated for that time.
.SS symlink caching
.PP
As of version 4.20 Linux supports symlink caching.
Significant performance increases can be had in workloads which use a
lot of symlinks.
Setting \f[C]cache.symlinks=true\f[] will result in requesting symlink
caching from the kernel only if supported.
As a result its safe to enable it on systems prior to 4.20.
That said it is disabled by default for now.
You can see if caching is enabled by querying the xattr
\f[C]user.mergerfs.cache.symlinks\f[] but given it must be requested at
startup you can not change it at runtime.
.SS readdir caching
.PP
As of version 4.20 Linux supports readdir caching.
This can have a significant impact on directory traversal.
Especially when combined with entry (\f[C]cache.entry\f[]) and attribute
(\f[C]cache.attr\f[]) caching.
Setting \f[C]cache.readdir=true\f[] will result in requesting readdir
caching from the kernel on each \f[C]opendir\f[].
If the kernel doesn\[aq]t support readdir caching setting the option to
\f[C]true\f[] has no effect.
This option is configurable at runtime via xattr
\f[C]user.mergerfs.cache.readdir\f[].
.SS tiered caching
.PP
Some storage technologies support what some call "tiered" caching.
The placing of usually smaller, faster storage as a transparent cache to
larger, slower storage.
NVMe, SSD, Optane in front of traditional HDDs for instance.
.PP
MergerFS does not natively support any sort of tiered caching.
Most users have no use for such a feature and its inclusion would
complicate the code.
However, there are a few situations where a cache drive could help with
a typical mergerfs setup.
.IP "1." 3
Fast network, slow drives, many readers: You\[aq]ve a 10+Gbps network
with many readers and your regular drives can\[aq]t keep up.
.IP "2." 3
Fast network, slow drives, small\[aq]ish bursty writes: You have a
10+Gbps network and wish to transfer amounts of data less than your
cache drive but wish to do so quickly.
.PP
With #1 its arguable if you should be using mergerfs at all.
RAID would probably be the better solution.
If you\[aq]re going to use mergerfs there are other tactics that may
help: spreading the data across drives (see the mergerfs.dup tool) and
setting \f[C]func.open=rand\f[], using \f[C]symlinkify\f[], or using
dm\-cache or a similar technology to add tiered cache to the underlying
device.
.PP
With #2 one could use dm\-cache as well but there is another solution
which requires only mergerfs and a cronjob.
.IP "1." 3
Create 2 mergerfs pools.
One which includes just the slow drives and one which has both the fast
drives (SSD,NVME,etc.) and slow drives.
.IP "2." 3
The \[aq]cache\[aq] pool should have the cache drives listed first.
.IP "3." 3
The best \f[C]create\f[] policies to use for the \[aq]cache\[aq] pool
would probably be \f[C]ff\f[], \f[C]epff\f[], \f[C]lfs\f[], or
\f[C]eplfs\f[].
The latter two under the assumption that the cache drive(s) are far
smaller than the backing drives.
If using path preserving policies remember that you\[aq]ll need to
manually create the core directories of those paths you wish to be
cached.
Be sure the permissions are in sync.
Use \f[C]mergerfs.fsck\f[] to check / correct them.
You could also tag the slow drives as \f[C]=NC\f[] though that\[aq]d
mean if the cache drives fill you\[aq]d get "out of space" errors.
.IP "4." 3
Enable \f[C]moveonenospc\f[] and set \f[C]minfreespace\f[]
appropriately.
To make sure there is enough room on the "slow" pool you might want to
set \f[C]minfreespace\f[] to at least as large as the size of the
largest cache drive if not larger.
This way in the worst case the whole of the cache drive(s) can be moved
to the other drives.
.IP "5." 3
Set your programs to use the cache pool.
.IP "6." 3
Save one of the below scripts or create you\[aq]re own.
.IP "7." 3
Use \f[C]cron\f[] (as root) to schedule the command at whatever
frequency is appropriate for your workflow.
.SS time based expiring
.PP
Move files from cache to backing pool based only on the last time the
file was accessed.
Replace \f[C]\-atime\f[] with \f[C]\-amin\f[] if you want minutes rather
than days.
May want to use the \f[C]fadvise\f[] / \f[C]\-\-drop\-cache\f[] version
of rsync or run rsync with the tool "nocache".
.IP
.nf
\f[C]
#!/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\ \[aq]%P\\n\[aq]\ |\ \\
\ \ rsync\ \-\-files\-from=\-\ \-axqHAXWES\ \-\-preallocate\ \-\-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\ <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\[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\ "${FILE}"
\ \ \ \ rsync\ \-axqHAXWES\ \-\-preallocate\ \-\-remove\-source\-files\ "${CACHE}/./${FILE}"\ "${BACKING}/"
done
\f[]
.fi
.SH PERFORMANCE
.PP
mergerfs is at its core just a proxy and therefore its theoretical max
performance is that of the underlying devices.
However, given it is a FUSE filesystem working from userspace there is
an increase in overhead relative to kernel based solutions.
That said the performance can match the theoretical max but it depends
greatly on the system\[aq]s configuration.
Especially when adding network filesystems into the mix there are many
variables which can impact performance.
Drive speeds and latency, network speeds and latency, general
concurrency, read/write sizes, etc.
Unfortunately, given the number of variables it has been difficult to
find a single set of settings which provide optimal performance.
If you\[aq]re having performance issues please look over the suggestions
below (including the benchmarking section.)
.PP
NOTE: be sure to read about these features before changing them to
understand what behaviors it may impact
.IP \[bu] 2
enable (or disable) \f[C]splice_move\f[], \f[C]splice_read\f[], and
\f[C]splice_write\f[]
.IP \[bu] 2
disable \f[C]security_capability\f[] and/or \f[C]xattr\f[]
.IP \[bu] 2
increase cache timeouts \f[C]cache.attr\f[], \f[C]cache.entry\f[],
\f[C]cache.negative_entry\f[]
.IP \[bu] 2
enable (or disable) page caching (\f[C]cache.files\f[])
.IP \[bu] 2
enable \f[C]cache.writeback\f[]
.IP \[bu] 2
enable \f[C]cache.open\f[]
.IP \[bu] 2
enable \f[C]cache.statfs\f[]
.IP \[bu] 2
enable \f[C]cache.symlinks\f[]
.IP \[bu] 2
enable \f[C]cache.readdir\f[]
.IP \[bu] 2
change the number of worker threads
.IP \[bu] 2
disable \f[C]posix_acl\f[]
.IP \[bu] 2
disable \f[C]async_read\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 read\-only
.IP \[bu] 2
use tiered cache drives
.IP \[bu] 2
use LVM and LVM cache to place a SSD in front of your HDDs
.PP
If you come across a setting that significantly impacts performance
please contact trapexit so he may investigate further.
.SH BENCHMARKING
.PP
Filesystems are complicated.
They do many things and many of those are interconnected.
Additionally, the OS, drivers, hardware, etc.
all can impact performance.
Therefore, when benchmarking, it is \f[B]necessary\f[] that the test
focus as narrowly as possible.
.PP
For most throughput is the key benchmark.
To test throughput \f[C]dd\f[] is useful but \f[B]must\f[] be used with
the correct settings in order to ensure the filesystem or device is
actually being tested.
The OS can and will cache data.
Without forcing synchronous reads and writes and/or disabling caching
the values returned will not be representative of the device\[aq]s true
performance.
.PP
When benchmarking through mergerfs ensure you only use 1 branch to
remove any possibility of the policies complicating the situation.
Benchmark the underlying filesystem first and then mount mergerfs over
it and test again.
If you\[aq]re experience speeds below your expectation you will need to
narrow down precisely which component is leading to the slowdown.
Preferably test the following in the order listed (but not combined).
.IP "1." 3
Enable \f[C]nullrw\f[] mode with \f[C]nullrw=true\f[].
This will effectively make reads and writes no\-ops.
Removing the underlying device / filesystem from the equation.
This will give us the top theoretical speeds.
.IP "2." 3
Mount mergerfs over \f[C]tmpfs\f[].
\f[C]tmpfs\f[] is a RAM disk.
Extremely high speed and very low latency.
This is a more realistic best case scenario.
Example: \f[C]mount\ \-t\ tmpfs\ \-o\ size=2G\ tmpfs\ /tmp/tmpfs\f[]
.IP "3." 3
Mount mergerfs over a local drive.
NVMe, SSD, HDD, etc.
If you have more than one I\[aq]d suggest testing each of them as drives
and/or controllers (their drivers) could impact performance.
.IP "4." 3
Finally, if you intend to use mergerfs with a network filesystem, either
as the source of data or to combine with another through mergerfs, test
each of those alone as above.
.PP
Once you find the component which has the performance issue you can do
further testing with different options to see if they impact
performance.
For reads and writes the most relevant would be: \f[C]cache.files\f[],
\f[C]async_read\f[], \f[C]splice_move\f[], \f[C]splice_read\f[],
\f[C]splice_write\f[].
Less likely but relevant when using NFS or with certain filesystems
would be \f[C]security_capability\f[], \f[C]xattr\f[], and
\f[C]posix_acl\f[].
If you find a specific system, drive, filesystem, controller, etc.
that performs poorly contact trapexit so he may investigate further.
.PP
Sometimes the problem is really the application accessing or writing
data through mergerfs.
Some software use small buffer sizes which can lead to more requests and
therefore greater overhead.
You can test this out yourself by replace \f[C]bs=1M\f[] in the examples
below with \f[C]ibs\f[] or \f[C]obs\f[] and using a size of \f[C]512\f[]
instead of \f[C]1M\f[].
In one example test using \f[C]nullrw\f[] the write speed dropped from
4.9GB/s to 69.7MB/s when moving from \f[C]1M\f[] to \f[C]512\f[].
Similar results were had when testing reads.
Small writes overhead may be improved by leveraging a write cache but in
casual tests little gain was found.
More tests will need to be done before this feature would become
available.
If you have an app that appears slow with mergerfs it could be due to
this.
Contact trapexit so he may investigate further.
.SS write benchmark
.IP
.nf
\f[C]
$\ dd\ if=/dev/zero\ of=/mnt/mergerfs/1GB.file\ bs=1M\ count=1024\ oflag=nocache\ conv=fdatasync\ status=progress
\f[]
.fi
.SS read benchmark
.IP
.nf
\f[C]
$\ dd\ if=/mnt/mergerfs/1GB.file\ of=/dev/null\ bs=1M\ count=1024\ iflag=nocache\ conv=fdatasync\ status=progress
\f[]
.fi
.SH TIPS / NOTES
.IP \[bu] 2
\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[] (with \f[B]allow_other\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]cache.files=off\f[] (or \f[C]direct_io\f[]) if
you expect applications (such as rtorrent) to
mmap (http://linux.die.net/man/2/mmap) files.
Shared mmap is not currently supported in FUSE w/ \f[C]direct_io\f[]
enabled.
Enabling \f[C]dropcacheonclose\f[] is recommended when
\f[C]cache.files=partial|full|auto\-full\f[] or
\f[C]direct_io=false\f[].
.IP \[bu] 2
Since POSIX functions give only a singular error or success its
difficult to determine the proper behavior when applying the function 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 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.
.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 \[aq]mv /mnt/pool/foo /mnt/disk1/foo\[aq] removes \[aq]foo\[aq]
.PP
This is not a bug.
.PP
Run in verbose mode to better understand what\[aq]s happening:
.IP
.nf
\f[C]
$\ mv\ \-v\ /mnt/pool/foo\ /mnt/disk1/foo
copied\ \[aq]/mnt/pool/foo\[aq]\ \->\ \[aq]/mnt/disk1/foo\[aq]
removed\ \[aq]/mnt/pool/foo\[aq]
$\ ls\ /mnt/pool/foo
ls:\ cannot\ access\ \[aq]/mnt/pool/foo\[aq]:\ No\ such\ file\ or\ directory
\f[]
.fi
.PP
\f[C]mv\f[], when working across devices, is copying the source to
target and then removing the source.
Since the source \f[B]is\f[] the target in this case, depending on the
unlink policy, it will remove the just copied file and other files
across the branches.
.PP
If you want to move files to one drive just copy them there and use
mergerfs.dedup to clean up the old paths or manually remove them from
the branches directly.
.SS cached memory appears greater than it should be
.PP
Use \f[C]cache.files=off\f[] or \f[C]direct_io=true\f[].
See the section on page caching.
.SS NFS clients returning ESTALE / Stale file handle
.PP
Be sure to use \f[C]noforget\f[] and \f[C]use_ino\f[] arguments.
.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 set \f[C]cache.files=partial|full|auto\-full\f[] or 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 fallback to traditional methods.
FUSE does not currently support mmap while using \f[C]direct_io\f[].
There may 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.
Be sure to set \f[C]dropcacheonclose=true\f[] if not using
\f[C]direct_io\f[].
.SS rtorrent fails with files >= 4GiB
.PP
This is a kernel bug with mmap and FUSE on 32bit platforms.
A fix should become available for all LTS releases.
.PP
https://marc.info/?l=linux\-fsdevel&m=155550785230874&w=2
.SS Crashing on OpenVZ
.PP
There appears to be a bug in the OpenVZ kernel with regard to how it
handles ioctl calls.
It is making invalid requests which leads to a crash.
As of 2019\-12\-10 there is a bug report filed with OpenVZ but it is not
yet fixed.
.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 sqlite3 which
apparently needs mmap.
mmap doesn\[aq]t work with \f[C]direct_io\f[].
To fix this place the data elsewhere or disable \f[C]direct_io\f[] (with
\f[C]dropcacheonclose=true\f[]).
Sqlite3 does not need mmap but the developer needs to fall back to
standard IO if mmap fails.
.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/was 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 its 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 situation 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 up to 32 supplemental groups.
Linux >= 2.6.3 allows up to 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.
.PP
While not a bug some users have found when using containers that
supplemental groups defined inside the container don\[aq]t work properly
with regard to permissions.
This is expected as mergerfs lives outside the container and therefore
is querying the host\[aq]s group database.
There might be a hack to work around this (make mergerfs read the
/etc/group file in the container) but it is not yet implemented and
would be limited to Linux and the /etc/group DB.
Preferably users would mount in the host group file into the containers
or use a standard shared user & groups technology like NIS or LDAP.
.SS mergerfs or libfuse crashing
.PP
First...
always upgrade to the latest version unless told otherwise.
.PP
If using mergerfs below 2.22.0:
.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.
.PP
If using mergerfs at or above 2.22.0:
.PP
First upgrade if possible, check the known bugs section, and contact
trapexit.
.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 pressure 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:[<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
\f[]
.fi
.PP
There is a bug in the kernel.
A work around appears to be turning off \f[C]splice\f[].
Don\[aq]t add the \f[C]splice_*\f[] arguments or add
\f[C]no_splice_write,no_splice_move,no_splice_read\f[].
This, however, is not guaranteed to work.
.SS rm: fts_read failed: No such file or directory
.PP
NOTE: This is only relevant to mergerfs versions at or below v2.25.x and
should not occur in more recent versions.
See the notes on \f[C]unlink\f[].
.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 immediately.
That has a side effect however.
Files which are unlinked and then they are still used (in certain forms)
will result in an error (ENOENT).
.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 as
its 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 What policies should I use?
.PP
Unless you\[aq]re doing something more niche the average user is
probably best off using \f[C]mfs\f[] for \f[C]category.create\f[].
It will spread files out across your branches based on available space.
You may want to use \f[C]lus\f[] if you prefer a slightly different
distribution of data if you have a mix of smaller and larger drives.
Generally though \f[C]mfs\f[], \f[C]lus\f[], or even \f[C]rand\f[] are
good for the general use case.
If you are starting with an imbalanced pool you can use the tool
\f[B]mergerfs.balance\f[] to redistribute files across the pool.
.PP
If you really wish to try to colocate files based on directory you can
set \f[C]func.create\f[] to \f[C]epmfs\f[] or similar and
\f[C]func.mkdir\f[] to \f[C]rand\f[] or \f[C]eprand\f[] depending on if
you just want to colocate generally or on specific branches.
Either way the \f[I]need\f[] to colocate is rare.
For instance: if you wish to remove the drive regularly and want the
data to predictably be on that drive or if you don\[aq]t use backup at
all and don\[aq]t wish to replace that data piecemeal.
In which case using path preservation can help but will require some
manual attention.
Colocating after the fact can be accomplished using the
\f[B]mergerfs.consolidate\f[] tool.
.PP
Ultimately there is no correct answer.
It is a preference or based on some particular need.
mergerfs is very easy to test and experiment with.
I suggest creating a test setup and experimenting to get a sense of what
you want.
.PP
The reason \f[C]mfs\f[] is not the default \f[C]category.create\f[]
policy is historical.
When/if a 3.X gets released it will be changed to minimize confusion
people often have with path preserving policies.
.SS What settings should I use?
.PP
Depends on what features you want.
Generally speaking there are no "wrong" settings.
All settings are performance or feature related.
The best bet is to read over the available options and choose what fits
your situation.
If something isn\[aq]t clear from the documentation please reach out and
the documentation will be improved.
.PP
That said, for the average person, the following should be fine:
.PP
\f[C]\-o\ use_ino,cache.files=off,dropcacheonclose=true,allow_other,category.create=mfs\f[]
.SS Why are all my files ending up on 1 drive?!
.PP
Did you start with empty drives?
Did you explicitly configure a \f[C]category.create\f[] policy?
Are you using a path preserving policy?
.PP
The default create policy is \f[C]epmfs\f[].
That is a path preserving algorithm.
With such a policy for \f[C]mkdir\f[] and \f[C]create\f[] with a set of
empty drives it will select only 1 drive when the first directory is
created.
Anything, files or directories, created in that first directory will be
placed on the same branch because it is preserving paths.
.PP
This catches a lot of new users off guard but changing the default would
break the setup for many existing users.
If you do not care about path preservation and wish your files to be
spread across all your drives change to \f[C]mfs\f[] or similar policy
as described above.
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
transferring your data.
Setting \f[C]func.mkdir=epall\f[] can simplify managing path
preservation for \f[C]create\f[].
Or use \f[C]func.mkdir=rand\f[] if you\[aq]re intersted in just grouping
together directory content by drive.
.SS Do hard links work?
.PP
Yes.
You need to use \f[C]use_ino\f[] to support proper reporting of inodes.
.PP
What mergerfs does not do is fake hard links across branches.
Read the section "rename & link" for how it works.
.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 hard link breaking (copy to temp file then rename over original)
which can be useful when wanting to save space by hardlinking duplicate
files but wish to treat each name as if it were a unique and separate
file.
.SS Why can\[aq]t I see my files / directories?
.PP
It\[aq]s almost always a permissions issue.
Unlike mhddfs and unionfs\-fuse, which runs as root and attempts to
access content as such, mergerfs always changes its 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 Is my OS\[aq]s libfuse needed for mergerfs to work?
.PP
No.
Normally \f[C]mount.fuse\f[] is needed to get mergerfs (or any FUSE
filesystem to mount using the \f[C]mount\f[] command but in vendoring
the libfuse library the \f[C]mount.fuse\f[] app has been renamed to
\f[C]mount.mergerfs\f[] meaning the filesystem type in \f[C]fstab\f[]
can simply be \f[C]mergerfs\f[].
.SS Why was libfuse embedded into mergerfs?
.IP "1." 3
A significant number of users use mergerfs on distros with old versions
of libfuse which have serious bugs.
Requiring updated versions of libfuse on those distros isn\[aq]t
practical (no package offered, user inexperience, etc.).
The only practical way to provide a stable runtime on those systems was
to "vendor" / embed the library into the project.
.IP "2." 3
mergerfs was written to use the high level API.
There are a number of limitations in the HLAPI that make certain
features difficult or impossible to implement.
While some of these features could be patched into newer versions of
libfuse without breaking the public API some of them would require hacky
code to provide backwards compatibility.
While it may still be worth working with upstream to address these
issues in future versions, since the library needs to be vendored for
stability and compatibility reasons it is preferable / easier to modify
the API.
Longer term the plan is to rewrite mergerfs to use the low level API.
.SS Why did support for system libfuse get removed?
.PP
See above first.
.PP
If/when mergerfs is rewritten to use the low\-level API then it\[aq]ll
be plausible to support system libfuse but till then its simply too much
work to manage the differences across the versions.
.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,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 than 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 function without the possibility of
catastrophic failure and the difficulties in recovery.
Drives may fail, however, all other data will continue to be accessible.
.PP
When combined with something like SnapRaid (http://www.snapraid.it)
and/or an offsite backup solution you can have the flexibility 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 a number of costs and limitations as
described
here (http://louwrentius.com/the-hidden-cost-of-using-zfs-for-your-home-nas.html),
here (https://markmcb.com/2020/01/07/five-years-of-btrfs/), and
here (https://utcc.utoronto.ca/~cks/space/blog/solaris/ZFSWhyNoRealReshaping).
.SS What should mergerfs NOT be used for?
.IP \[bu] 2
databases: Even if the database stored data in separate files (mergerfs
wouldn\[aq]t offer much otherwise) the higher latency of the indirection
will kill performance.
If it is a lightly used SQLITE database then it may be fine but
you\[aq]ll need to test.
.IP \[bu] 2
VM images: For the same reasons as databases.
VM images are accessed very aggressively and mergerfs will introduce too
much latency (if it works at all).
.IP \[bu] 2
As replacement for RAID: mergerfs is just for pooling branches.
If you need that kind of device performance aggregation or high
availability you should stick with RAID.
.SS Can drives be written to directly? Outside of mergerfs while pooled?
.PP
Yes, however its not recommended to use the same file from within the
pool and from without at the same time (particularly writing).
Especially if using caching of any kind (cache.files, cache.entry,
cache.attr, cache.negative_entry, cache.symlinks, cache.readdir, etc.)
as there could be a conflict between cached data and not.
.SS Why do I get an "out of space" / "no space left on device" / ENOSPC
error even though there appears to be lots of space available?
.PP
First make sure you\[aq]ve read the sections above about policies, path
preservation, branch filtering, and the options \f[B]minfreespace\f[],
\f[B]moveonenospc\f[], \f[B]statfs\f[], and \f[B]statfs_ignore\f[].
.PP
mergerfs is simply presenting a union of the content within multiple
branches.
The reported free space is an aggregate of space available within the
pool (behavior modified by \f[B]statfs\f[] and \f[B]statfs_ignore\f[]).
It does not represent a contiguous space.
In the same way that read\-only filesystems, those with quotas, or
reserved space report the full theoretical space available.
.PP
Due to path preservation, branch tagging, read\-only status, and
\f[B]minfreespace\f[] settings it is perfectly valid that
\f[C]ENOSPC\f[] / "out of space" / "no space left on device" be
returned.
It is doing what was asked of it: filtering possible branches due to
those settings.
Only one error can be returned and if one of the reasons for filtering a
branch was \f[B]minfreespace\f[] then it will be returned as such.
\f[B]moveonenospc\f[] is only relevant to writing a file which is too
large for the drive its currently on.
.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.
.PP
If you don\[aq]t care about path preservation then simply change the
\f[C]create\f[] policy to one which isn\[aq]t.
\f[C]mfs\f[] is probably what most are looking for.
The reason its not default is because it was originally set to
\f[C]epmfs\f[] and changing it now would change people\[aq]s setup.
Such a setting change will likely occur in mergerfs 3.
.SS Can mergerfs mounts be exported over NFS?
.PP
Yes.
Due to current usage of libfuse by mergerfs and how NFS interacts with
it it is necessary to add \f[C]noforget\f[] to mergerfs options to keep
from getting "stale file handle" errors.
.PP
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
https://github.com/trapexit/mergerfs/blob/master/src/fs_inode.hpp
.PP
Originally tried to simply OR st_ino and (st_dev << 32) for 64bit
systems.
After a number of years someone finally ran into a collision that lead
to some problems.
Traditionally \f[C]dev_t\f[] was 16bit and \f[C]ino_t\f[] was 32bit so
merging into one 64bit value worked but with both types being able to be
up to 64bit that is no longer as simple.
A proper hash seems like the best compromise.
While totally unique inodes are preferred the overhead which would be
needed does not seem to be outweighed 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
.PP
Generally collision, if it occurs, shouldn\[aq]t be a problem.
You can turn off the calculation by not using \f[C]use_ino\f[].
In the future it might be worth creating different strategies for users
to select from.
.SS I notice massive slowdowns of writes when enabling cache.files.
.PP
When file caching is enabled in any form (\f[C]cache.files!=off\f[] or
\f[C]direct_io=false\f[]) it will issue \f[C]getxattr\f[] requests for
\f[C]security.capability\f[] prior to \f[I]every single write\f[].
This will usually result in a performance degregation, especially when
using a network filesystem (such as NFS or CIFS/SMB/Samba.)
Unfortunately at this moment the kernel is not caching the response.
.PP
To work around this situation mergerfs offers a few solutions.
.IP "1." 3
Set \f[C]security_capability=false\f[].
It will short curcuit any call and return \f[C]ENOATTR\f[].
This still means though that mergerfs will receive the request before
every write but at least it doesn\[aq]t get passed through to the
underlying filesystem.
.IP "2." 3
Set \f[C]xattr=noattr\f[].
Same as above but applies to \f[I]all\f[] calls to getxattr.
Not just \f[C]security.capability\f[].
This will not be cached by the kernel either but mergerfs\[aq] runtime
config system will still function.
.IP "3." 3
Set \f[C]xattr=nosys\f[].
Results in mergerfs returning \f[C]ENOSYS\f[] which \f[I]will\f[] be
cached by the kernel.
No future xattr calls will be forwarded to mergerfs.
The downside is that also means the xattr based config and query
functionality won\[aq]t work either.
.IP "4." 3
Disable file caching.
If you aren\[aq]t using applications which use \f[C]mmap\f[] it\[aq]s
probably simplier to just disable it all together.
The kernel won\[aq]t send the requests when caching is disabled.
.SS What are these .fuse_hidden files?
.PP
NOTE: mergerfs >= 2.26.0 will not have these temporary files.
See the notes on \f[C]unlink\f[].
.PP
When not using \f[B]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.
They are safe to remove as they are already unlinked files.
.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 other
situations.
Rather than attempting to simulate POSIX ACL behavior the proper way to
manage this is to use seteuid (http://linux.die.net/man/2/seteuid) and
setegid (http://linux.die.net/man/2/setegid), in effect becoming the
user making the original call, and perform the action as them.
This is what mergerfs does and why mergerfs should always run as root.
.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 SUPPORT
.PP
Filesystems are 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.
.PP
\f[B]Please make sure you are using the latest
release (https://github.com/trapexit/mergerfs/releases) or have tried it
in comparison. Old versions, which are often included in distros like
Debian and Ubuntu, are not ever going to be updated and your bug may
have been addressed already.\f[]
.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
\f[B]All\f[] information about the relevant branches and paths:
permissions, etc.
.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\ <cmd>\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\ <mergerfsPID>\ \-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
.IP \[bu] 2
reddit: https://www.reddit.com/user/trapexit
.IP \[bu] 2
discord: https://discord.gg/MpAr69V
.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
PayPal: https://paypal.me/trapexit
.IP \[bu] 2
GitHub Sponsors: https://github.com/sponsors/trapexit
.IP \[bu] 2
Patreon: https://www.patreon.com/trapexit
.IP \[bu] 2
SubscribeStar: https://www.subscribestar.com/trapexit
.IP \[bu] 2
Ko\-Fi: https://ko\-fi.com/trapexit
.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
ZCoin (XZC): a8L5Vz35KdCQe7Y7urK2pcCGau7JsqZ5Gw
.SH LINKS
.IP \[bu] 2
https://spawn.link
.IP \[bu] 2
https://github.com/trapexit/mergerfs
.IP \[bu] 2
https://github.com/trapexit/mergerfs\-tools
.IP \[bu] 2
https://github.com/trapexit/scorch
.IP \[bu] 2
https://github.com/trapexit/bbf
.IP \[bu] 2
https://github.com/trapexit/backup\-and\-recovery\-howtos
.SH AUTHORS
Antonio SJ Musumeci <trapexit@spawn.link>.