mergerfs/man/mergerfs.1

.\"t
.\" Automatically generated by Pandoc 1.16.0.2
.\"
.TH "mergerfs" "1" "2017\-05\-26" "mergerfs user manual" ""
.hy
.SH NAME
.PP
mergerfs \- a featureful union filesystem
.SH SYNOPSIS
.PP
mergerfs \-o<options> <srcmounts> <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
.IP \[bu] 2
Support for extended attributes (xattrs)
.IP \[bu] 2
Support for file attributes (chattr)
.IP \[bu] 2
Runtime configurable (via xattrs)
.IP \[bu] 2
Safe to run as root
.IP \[bu] 2
Opportunistic credential caching
.IP \[bu] 2
Works with heterogeneous filesystem types
.IP \[bu] 2
Handling of writes to full drives (transparently move file to drive with
capacity)
.IP \[bu] 2
Handles pool of readonly and read/write drives
.IP \[bu] 2
Turn read\-only files into symlinks to increase read performance
.SH OPTIONS
.SS mount options
.IP \[bu] 2
\f[B]defaults\f[]: a shortcut for FUSE\[aq]s \f[B]atomic_o_trunc\f[],
\f[B]auto_cache\f[], \f[B]big_writes\f[], \f[B]default_permissions\f[],
\f[B]splice_move\f[], \f[B]splice_read\f[], and \f[B]splice_write\f[].
These options seem to provide the best performance.
.IP \[bu] 2
\f[B]direct_io\f[]: causes FUSE to bypass caching which can increase
write speeds at the detriment of reads.
Note that not enabling \f[C]direct_io\f[] will cause double caching of
files and therefore less memory for caching generally.
However, \f[C]mmap\f[] does not work when \f[C]direct_io\f[] is enabled.
.IP \[bu] 2
\f[B]minfreespace\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\f[]: when enabled (set to \f[B]true\f[]) if a
\f[B]write\f[] fails with \f[B]ENOSPC\f[] or \f[B]EDQUOT\f[] a scan of
all drives will be done looking for the drive with most free space which
is at least the size of the file plus the amount which failed to write.
An attempt to move the file to that drive will occur (keeping all
metadata possible) and if successful the original is unlinked and the
write retried.
(default: false)
.IP \[bu] 2
\f[B]use_ino\f[]: causes mergerfs to supply file/directory inodes rather
than libfuse.
While not a default it is generally recommended it be enabled so that
hard linked files share the same inode value.
.IP \[bu] 2
\f[B]dropcacheonclose\f[]: when a file is requested to be closed call
\f[C]posix_fadvise\f[] on it first to instruct the kernel that we no
longer need the data and it can drop its cache.
Recommended when \f[B]direct_io\f[] is not enabled to limit double
caching.
(default: false)
.IP \[bu] 2
\f[B]symlinkify\f[]: when enabled (set to \f[B]true\f[]) and a file is
not writable and its mtime or ctime is older than
\f[B]symlinkify_timeout\f[] files will be reported as symlinks to the
original files.
Please read more below before using.
(default: false)
.IP \[bu] 2
\f[B]symlinkify_timeout\f[]: time to wait, in seconds, to activate the
\f[B]symlinkify\f[] behavior.
(default: 3600)
.IP \[bu] 2
\f[B]nullrw\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]fsname\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.
Example: \f[B]category.create=mfs\f[]
.PP
\f[B]NOTE:\f[] Options are evaluated in the order listed so if the
options are \f[B]func.rmdir=rand,category.action=ff\f[] the
\f[B]action\f[] category setting will override the \f[B]rmdir\f[]
setting.
.SS srcmounts
.PP
The srcmounts (source mounts) argument is a colon (\[aq]:\[aq])
delimited list of paths to be included in the pool.
It does not matter if the paths are on the same or different drives nor
does it matter the filesystem.
Used and available space will not be duplicated for paths on the same
device and any features which aren\[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 source mounts 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 expand it.\f[]
.IP
.nf
\f[C]
$\ mergerfs\ \-o\ defaults,allow_other,use_ino\ /mnt/disk\\*:/mnt/cdrom\ /media/drives
\f[]
.fi
.PP
The above line will use all mount points in /mnt prefixed with
\f[B]disk\f[] and the \f[B]cdrom\f[].
.PP
To have the pool mounted at boot or otherwise accessable from related
tools use \f[B]/etc/fstab\f[].
.IP
.nf
\f[C]
#\ <file\ system>\ \ \ \ \ \ \ \ <mount\ point>\ \ <type>\ \ \ \ \ \ \ \ \ <options>\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ <dump>\ \ <pass>
/mnt/disk*:/mnt/cdrom\ \ /media/drives\ \ fuse.mergerfs\ \ defaults,allow_other,use_ino\ \ 0\ \ \ \ \ \ \ 0
\f[]
.fi
.PP
\f[B]NOTE:\f[] the globbing is done at mount or xattr update time (see
below).
If a new directory is added matching the glob after the fact it will not
be automatically included.
.PP
\f[B]NOTE:\f[] for mounting via \f[B]fstab\f[] to work you must have
\f[B]mount.fuse\f[] installed.
For Ubuntu/Debian it is included in the \f[B]fuse\f[] package.
.SS symlinkify
.PP
Due to the levels of indirection introduced by mergerfs and the
underlying technology FUSE there can be varying levels of performance
degredation.
This feature will turn non\-directories which are not writable into
symlinks to the original file found by the \f[C]readlink\f[] policy
after the mtime and ctime are older than the timeout.
.PP
\f[B]WARNING:\f[] The current implementation has a known issue in which
if the file is open and being used when the file is converted to a
symlink then the application which has that file open will receive an
error when using it.
This is unlikely to occur in practice but is something to keep in mind.
.PP
\f[B]WARNING:\f[] Some backup solutions, such as CrashPlan, do not
backup the target of a symlink.
If using this feature it will be necessary to point any backup software
to the original drives or configure the software to follow symlinks if
such an option is available.
Alternatively create two mounts.
One for backup and one for general consumption.
.SS nullrw
.PP
Due to how FUSE works there is an overhead to all requests made to a
FUSE filesystem.
Meaning that even a simple passthrough will have some slowdown.
However, generally the overhead is minimal in comparison to the cost of
the underlying I/O.
By disabling the underlying I/O we can test the theoretical performance
boundries.
.PP
By enabling \f[C]nullrw\f[] mergerfs will work as it always does
\f[B]except\f[] that all reads and writes will be no\-ops.
A write will succeed (the size of the write will be returned as if it
were successful) but mergerfs does nothing with the data it was given.
Similarly a read will return the size requested but won\[aq]t touch the
buffer.
.PP
Example:
.IP
.nf
\f[C]
$\ dd\ if=/dev/zero\ of=/path/to/mergerfs/mount/benchmark\ ibs=1M\ obs=512\ count=1024
1024+0\ records\ in
2097152+0\ records\ out
1073741824\ bytes\ (1.1\ GB,\ 1.0\ GiB)\ copied,\ 15.4067\ s,\ 69.7\ MB/s

$\ dd\ if=/dev/zero\ of=/path/to/mergerfs/mount/benchmark\ ibs=1M\ obs=1M\ count=1024
1024+0\ records\ in
1024+0\ records\ out
1073741824\ bytes\ (1.1\ GB,\ 1.0\ GiB)\ copied,\ 0.219585\ s,\ 4.9\ GB/s

$\ dd\ if=/path/to/mergerfs/mount/benchmark\ of=/dev/null\ bs=512\ count=102400
102400+0\ records\ in
102400+0\ records\ out
52428800\ bytes\ (52\ MB,\ 50\ MiB)\ copied,\ 0.757991\ s,\ 69.2\ MB/s

$\ dd\ if=/path/to/mergerfs/mount/benchmark\ of=/dev/null\ bs=1M\ count=1024
1024+0\ records\ in
1024+0\ records\ out
1073741824\ bytes\ (1.1\ GB,\ 1.0\ GiB)\ copied,\ 0.18405\ s,\ 5.8\ GB/s
\f[]
.fi
.PP
It\[aq]s important to test with different \f[C]obs\f[] (output block
size) values since the relative overhead is greater with smaller values.
As you can see above the size of a read or write can massively impact
theoretical performance.
If an application performs much worse through mergerfs it could very
well be that it doesn\[aq]t optimally size its read and write requests.
.SH FUNCTIONS / POLICIES / CATEGORIES
.PP
The POSIX filesystem API has a number of functions.
\f[B]creat\f[], \f[B]stat\f[], \f[B]chown\f[], etc.
In mergerfs these functions are grouped into 3 categories:
\f[B]action\f[], \f[B]create\f[], and \f[B]search\f[].
Functions and categories can be assigned a policy which dictates how
\f[B]mergerfs\f[] behaves.
Any policy can be assigned to a function or category though some may not
be very useful in practice.
For instance: \f[B]rand\f[] (random) may be useful for file creation
(create) but could lead to very odd behavior if used for \f[C]chmod\f[]
(though only if there were more than one copy of the file).
.PP
Policies, when called to create, will ignore drives which are readonly.
This allows for readonly and read/write drives to be mixed together.
Note that the drive must be explicitly mounted with the \f[B]ro\f[]
mount option for this to work.
.SS Function / Category classifications
.PP
.TS
tab(@);
lw(7.9n) lw(62.1n).
T{
Category
T}@T{
FUSE Functions
T}
_
T{
action
T}@T{
chmod, chown, link, removexattr, rename, rmdir, setxattr, truncate,
unlink, utimens
T}
T{
create
T}@T{
create, mkdir, mknod, symlink
T}
T{
search
T}@T{
access, getattr, getxattr, ioctl, listxattr, open, readlink
T}
T{
N/A
T}@T{
fallocate, fgetattr, fsync, ftruncate, ioctl, read, readdir, release,
statfs, write
T}
.TE
.PP
Due to FUSE limitations \f[B]ioctl\f[] behaves differently if its acting
on a directory.
It\[aq]ll use the \f[B]getattr\f[] policy to find and open the directory
before issuing the \f[B]ioctl\f[].
In other cases where something may be searched (to confirm a directory
exists across all source mounts) \f[B]getattr\f[] will also be used.
.SS Path Preservation
.PP
Policies, as described below, are of two core types.
\f[C]path\ preserving\f[] and \f[C]non\-path\ preserving\f[].
.PP
All policies which start with \f[C]ep\f[] (\f[B]epff\f[],
\f[B]eplfs\f[], \f[B]eplus\f[], \f[B]epmfs\f[], \f[B]eprand\f[]) are
\f[C]path\ preserving\[aq].\f[]ep\f[C]stands\ for\ \[aq]existing\ path\f[].
.PP
As the descriptions explain a path preserving policy will only consider
drives where the relative path being accessed already exists.
.PP
When using non\-path preserving policies where something is created
paths will be copied to target drives as necessary.
.SS Policy descriptions
.PP
.TS
tab(@);
lw(16.6n) lw(53.4n).
T{
Policy
T}@T{
Description
T}
_
T{
all
T}@T{
Search category: acts like \f[B]ff\f[].
Action category: apply to all found.
Create category: for \f[B]mkdir\f[], \f[B]mknod\f[], and
\f[B]symlink\f[] it will apply to all found.
\f[B]create\f[] works like \f[B]ff\f[].
It will exclude readonly drives and those with free space less than
\f[B]minfreespace\f[].
T}
T{
epall (existing path, all)
T}@T{
Search category: acts like \f[B]epff\f[].
Action category: apply to all found.
Create category: for \f[B]mkdir\f[], \f[B]mknod\f[], and
\f[B]symlink\f[] it will apply to all existing paths found.
\f[B]create\f[] works like \f[B]epff\f[].
Excludes readonly drives and those with free space less than
\f[B]minfreespace\f[].
T}
T{
epff (existing path, first found)
T}@T{
Given the order of the drives, as defined at mount time or configured at
runtime, act on the first one found where the relative path already
exists.
For \f[B]create\f[] category functions it will exclude readonly drives
and those with free space less than \f[B]minfreespace\f[] (unless there
is no other option).
Falls back to \f[B]ff\f[].
T}
T{
eplfs (existing path, least free space)
T}@T{
Of all the drives on which the relative path exists choose the drive
with the least free space.
For \f[B]create\f[] category functions it will exclude readonly drives
and those with free space less than \f[B]minfreespace\f[].
Falls back to \f[B]lfs\f[].
T}
T{
eplus (existing path, least used space)
T}@T{
Of all the drives on which the relative path exists choose the drive
with the least used space.
For \f[B]create\f[] category functions it will exclude readonly drives
and those with free space less than \f[B]minfreespace\f[].
Falls back to \f[B]lus\f[].
T}
T{
epmfs (existing path, most free space)
T}@T{
Of all the drives on which the relative path exists choose the drive
with the most free space.
For \f[B]create\f[] category functions it will exclude readonly drives
and those with free space less than \f[B]minfreespace\f[].
Falls back to \f[B]mfs\f[].
T}
T{
eprand (existing path, random)
T}@T{
Calls \f[B]epall\f[] and then randomizes.
Otherwise behaves the same as \f[B]epall\f[].
T}
T{
erofs
T}@T{
Exclusively return \f[B]\-1\f[] with \f[B]errno\f[] set to
\f[B]EROFS\f[] (Read\-only filesystem).
By setting \f[B]create\f[] functions to this you can in effect turn the
filesystem mostly readonly.
T}
T{
ff (first found)
T}@T{
Given the order of the drives, as defined at mount time or configured at
runtime, act on the first one found.
For \f[B]create\f[] category functions it will exclude readonly drives
and those with free space less than \f[B]minfreespace\f[] (unless there
is no other option).
T}
T{
lfs (least free space)
T}@T{
Pick the drive with the least available free space.
For \f[B]create\f[] category functions it will exclude readonly drives
and those with free space less than \f[B]minfreespace\f[].
Falls back to \f[B]mfs\f[].
T}
T{
lus (least used space)
T}@T{
Pick the drive with the least used space.
For \f[B]create\f[] category functions it will exclude readonly drives
and those with free space less than \f[B]minfreespace\f[].
Falls back to \f[B]mfs\f[].
T}
T{
mfs (most free space)
T}@T{
Pick the drive with the most available free space.
For \f[B]create\f[] category functions it will exclude readonly drives.
Falls back to \f[B]ff\f[].
T}
T{
newest
T}@T{
Pick the file / directory with the largest mtime.
For \f[B]create\f[] category functions it will exclude readonly drives
and those with free space less than \f[B]minfreespace\f[] (unless there
is no other option).
T}
T{
rand (random)
T}@T{
Calls \f[B]all\f[] and then randomizes.
T}
.TE
.SS Defaults
.PP
.TS
tab(@);
l l.
T{
Category
T}@T{
Policy
T}
_
T{
action
T}@T{
all
T}
T{
create
T}@T{
epmfs
T}
T{
search
T}@T{
ff
T}
.TE
.SS rename & link
.PP
\f[B]NOTE:\f[] If you\[aq]re receiving errors from software when files
are moved / renamed then you should consider changing the create policy
to one which is \f[B]not\f[] path preserving or contacting the author of
the offending software and requesting that \f[C]EXDEV\f[] be properly
handled.
.PP
rename (http://man7.org/linux/man-pages/man2/rename.2.html) is a tricky
function in a merged system.
Under normal situations rename only works within a single filesystem or
device.
If a rename can\[aq]t be done atomically due to the source and
destination paths existing on different mount points it will return
\f[B]\-1\f[] with \f[B]errno = EXDEV\f[] (cross device).
.PP
Originally mergerfs would return EXDEV whenever a rename was requested
which was cross directory in any way.
This made the code simple and was technically complient with POSIX
requirements.
However, many applications fail to handle EXDEV at all and treat it as a
normal error or otherwise handle it poorly.
Such apps include: gvfsd\-fuse v1.20.3 and prior, Finder / CIFS/SMB
client in Apple OSX 10.9+, NZBGet, Samba\[aq]s recycling bin feature.
.PP
As a result a compromise was made in order to get most software to work
while still obeying mergerfs\[aq] policies.
Below is the rather complicated logic.
.IP \[bu] 2
If using a \f[B]create\f[] policy which tries to preserve directory
paths (epff,eplfs,eplus,epmfs)
.IP \[bu] 2
Using the \f[B]rename\f[] policy get the list of files to rename
.IP \[bu] 2
For each file attempt rename:
.RS 2
.IP \[bu] 2
If failure with ENOENT run \f[B]create\f[] policy
.IP \[bu] 2
If create policy returns the same drive as currently evaluating then
clone the path
.IP \[bu] 2
Re\-attempt rename
.RE
.IP \[bu] 2
If \f[B]any\f[] of the renames succeed the higher level rename is
considered a success
.IP \[bu] 2
If \f[B]no\f[] renames succeed the first error encountered will be
returned
.IP \[bu] 2
On success:
.RS 2
.IP \[bu] 2
Remove the target from all drives with no source file
.IP \[bu] 2
Remove the source from all drives which failed to rename
.RE
.IP \[bu] 2
If using a \f[B]create\f[] policy which does \f[B]not\f[] try to
preserve directory paths
.IP \[bu] 2
Using the \f[B]rename\f[] policy get the list of files to rename
.IP \[bu] 2
Using the \f[B]getattr\f[] policy get the target path
.IP \[bu] 2
For each file attempt rename:
.RS 2
.IP \[bu] 2
If the source drive != target drive:
.IP \[bu] 2
Clone target path from target drive to source drive
.IP \[bu] 2
Rename
.RE
.IP \[bu] 2
If \f[B]any\f[] of the renames succeed the higher level rename is
considered a success
.IP \[bu] 2
If \f[B]no\f[] renames succeed the first error encountered will be
returned
.IP \[bu] 2
On success:
.RS 2
.IP \[bu] 2
Remove the target from all drives with no source file
.IP \[bu] 2
Remove the source from all drives which failed to rename
.RE
.PP
The the removals are subject to normal entitlement checks.
.PP
The above behavior will help minimize the likelihood of EXDEV being
returned but it will still be possible.
.PP
\f[B]link\f[] uses the same basic strategy.
.SS readdir
.PP
readdir (http://linux.die.net/man/3/readdir) is different from all other
filesystem functions.
While it could have it\[aq]s own set of policies to tweak its behavior
at this time it provides a simple union of files and directories found.
Remember that any action or information queried about these files and
directories come from the respective function.
For instance: an \f[B]ls\f[] is a \f[B]readdir\f[] and for each
file/directory returned \f[B]getattr\f[] is called.
Meaning the policy of \f[B]getattr\f[] is responsible for choosing the
file/directory which is the source of the metadata you see in an
\f[B]ls\f[].
.SS statvfs
.PP
statvfs (http://linux.die.net/man/2/statvfs) normalizes the source
drives based on the fragment size and sums the number of adjusted blocks
and inodes.
This means you will see the combined space of all sources.
Total, used, and free.
The sources however are dedupped based on the drive so multiple sources
on the same drive will not result in double counting it\[aq]s space.
.SH BUILDING
.PP
\f[B]NOTE:\f[] Prebuilt packages can be found at:
https://github.com/trapexit/mergerfs/releases
.PP
First get the code from github (http://github.com/trapexit/mergerfs).
.IP
.nf
\f[C]
$\ git\ clone\ https://github.com/trapexit/mergerfs.git
$\ #\ or
$\ wget\ https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs\-<ver>.tar.gz
\f[]
.fi
.SS Debian / Ubuntu
.IP
.nf
\f[C]
$\ sudo\ apt\-get\ \-y\ update
$\ sudo\ apt\-get\ \-y\ install\ git\ make
$\ cd\ mergerfs
$\ make\ install\-build\-pkgs
$\ make\ deb
$\ sudo\ dpkg\ \-i\ ../mergerfs_version_arch.deb
\f[]
.fi
.SS Fedora
.IP
.nf
\f[C]
$\ su\ \-
#\ dnf\ \-y\ update
#\ dnf\ \-y\ install\ git\ make
#\ cd\ mergerfs
#\ make\ install\-build\-pkgs
#\ make\ rpm
#\ rpm\ \-i\ rpmbuild/RPMS/<arch>/mergerfs\-<verion>.<arch>.rpm
\f[]
.fi
.SS Generically
.PP
Have git, g++, make, python, libattr1, automake, libtool installed.
.IP
.nf
\f[C]
$\ cd\ mergerfs
$\ make
$\ sudo\ make\ install
\f[]
.fi
.SH RUNTIME
.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
Even if xattrs are disabled for mergerfs the
{list,get,set}xattrs (http://linux.die.net/man/2/listxattr) calls
against this pseudo file will still work.
.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 (fstab).
.SS Keys
.PP
Use \f[C]xattr\ \-l\ /mount/point/.mergerfs\f[] to see all supported
keys.
Some are informational and therefore readonly.
.SS user.mergerfs.srcmounts
.PP
Used to query or modify the list of source mounts.
When modifying there are several shortcuts to easy manipulation of the
list.
.PP
.TS
tab(@);
l l.
T{
Value
T}@T{
Description
T}
_
T{
[list]
T}@T{
set
T}
T{
+<[list]
T}@T{
prepend
T}
T{
+>[list]
T}@T{
append
T}
T{
\-[list]
T}@T{
remove all values provided
T}
T{
\-<
T}@T{
remove first in list
T}
T{
\->
T}@T{
remove last in list
T}
.TE
.SS minfreespace
.PP
Input: interger with an optional multiplier suffix.
\f[B]K\f[], \f[B]M\f[], or \f[B]G\f[].
.PP
Output: value in bytes
.SS moveonenospc
.PP
Input: \f[B]true\f[] and \f[B]false\f[]
.PP
Ouput: \f[B]true\f[] or \f[B]false\f[]
.SS categories / funcs
.PP
Input: short policy string as described elsewhere in this document
.PP
Output: the policy string except for categories where its funcs have
multiple types.
In that case it will be a comma separated list
.SS Example
.IP
.nf
\f[C]
[trapexit:/tmp/mount]\ $\ xattr\ \-l\ .mergerfs
user.mergerfs.srcmounts:\ /tmp/a:/tmp/b
user.mergerfs.minfreespace:\ 4294967295
user.mergerfs.moveonenospc:\ false
\&...

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

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

[trapexit:/tmp/mount]\ $\ xattr\ \-w\ user.mergerfs.srcmounts\ +/tmp/c\ .mergerfs
[trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.srcmounts\ .mergerfs
/tmp/a:/tmp/b:/tmp/c

[trapexit:/tmp/mount]\ $\ xattr\ \-w\ user.mergerfs.srcmounts\ =/tmp/c\ .mergerfs
[trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.srcmounts\ .mergerfs
/tmp/c

[trapexit:/tmp/mount]\ $\ xattr\ \-w\ user.mergerfs.srcmounts\ \[aq]+</tmp/a:/tmp/b\[aq]\ .mergerfs
[trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.srcmounts\ .mergerfs
/tmp/a:/tmp/b:/tmp/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:/tmp/mount]\ $\ ls
A\ B\ C
[trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.fullpath\ A
/mnt/a/full/path/to/A
[trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.basepath\ A
/mnt/a
[trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.relpath\ A
/full/path/to/A
[trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.allpaths\ A\ |\ tr\ \[aq]\\0\[aq]\ \[aq]\\n\[aq]
/mnt/a/full/path/to/A
/mnt/b/full/path/to/A
\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.balance: Rebalance files across drives by moving them from the
most filled to the least filled
.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
.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 TIPS / NOTES
.IP \[bu] 2
The recommended options are
\f[B]defaults,allow_other,direct_io,use_ino\f[].
(\f[B]use_ino\f[] will only work when used with mergerfs 2.18.0 and
above.)
.IP \[bu] 2
Run mergerfs as \f[C]root\f[] unless you\[aq]re merging paths which are
owned by the same user otherwise strange permission issues may arise.
.IP \[bu] 2
https://github.com/trapexit/backup\-and\-recovery\-howtos : A set of
guides / howtos on creating a data storage system, backing it up,
maintaining it, and recovering from failure.
.IP \[bu] 2
If you don\[aq]t see some directories and files you expect in a merged
point or policies seem to skip drives be sure the user has permission to
all the underlying directories.
Use \f[C]mergerfs.fsck\f[] to audit the drive for out of sync
permissions.
.IP \[bu] 2
Do \f[I]not\f[] use \f[C]direct_io\f[] if you expect applications (such
as rtorrent) to mmap (http://linux.die.net/man/2/mmap) files.
It is not currently supported in FUSE w/ \f[C]direct_io\f[] enabled.
.IP \[bu] 2
Since POSIX gives you only error or success on calls its difficult to
determine the proper behavior when applying the behavior to multiple
targets.
\f[B]mergerfs\f[] will return an error only if all attempts of an action
fail.
Any success will lead to a success returned.
This means however that some odd situations may arise.
.IP \[bu] 2
Kodi (http://kodi.tv), Plex (http://plex.tv),
Subsonic (http://subsonic.org), etc.
can use directory mtime (http://linux.die.net/man/2/stat) to more
efficiently determine whether to scan for new content rather than simply
performing a full scan.
If using the default \f[B]getattr\f[] policy of \f[B]ff\f[] its possible
\f[B]Kodi\f[] 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 attempt to merge together multiple sources of data which
could be out of sync due to the different policies.
.IP \[bu] 2
For consistency its generally best to set \f[B]category\f[] wide
policies rather than individual \f[B]func\f[]\[aq]s.
This will help limit the confusion of tools such as
rsync (http://linux.die.net/man/1/rsync).
However, the flexibility is there if needed.
.SH KNOWN ISSUES / BUGS
.SS directory mtime is not being updated
.PP
Remember that the default policy for \f[C]getattr\f[] is \f[C]ff\f[].
The information for the first directory found will be returned.
If it wasn\[aq]t the directory which had been updated then it will
appear outdated.
.PP
The reason this is the default is because any other policy would be far
more expensive and for many applications it is unnecessary.
To always return the directory with the most recent mtime or a faked
value based on all found would require a scan of all drives.
That alone is far more expensive than \f[C]ff\f[] but would also
possibly spin up sleeping drives.
.PP
If you always want the directory information from the one with the most
recent mtime then use the \f[C]newest\f[] policy for \f[C]getattr\f[].
.SS cached memory appears greater than it should be
.PP
Use the \f[C]direct_io\f[] option as described above.
Due to what mergerfs is doing there ends up being two caches of a file
under normal usage.
One from the underlying filesystem and one from mergerfs.
Enabling \f[C]direct_io\f[] removes the mergerfs cache.
This saves on memory but means the kernel needs to communicate with
mergerfs more often and can therefore result in slower speeds.
.PP
Since enabling \f[C]direct_io\f[] disables \f[C]mmap\f[] this is not an
ideal situation however write speeds should be increased.
.PP
If \f[C]direct_io\f[] is disabled it is probably a good idea to enable
\f[C]dropcacheonclose\f[] to minimize double caching.
.SS NFS clients don\[aq]t work
.PP
Some NFS clients appear to fail when a mergerfs mount is exported.
Kodi in particular seems to have issues.
.PP
Try enabling the \f[C]use_ino\f[] option.
Some have reported that it fixes the issue.
.SS rtorrent fails with ENODEV (No such device)
.PP
Be sure to turn off \f[C]direct_io\f[].
rtorrent and some other applications use
mmap (http://linux.die.net/man/2/mmap) to read and write to files and
offer no failback to traditional methods.
FUSE does not currently support mmap while using \f[C]direct_io\f[].
There will be a performance penalty on writes with \f[C]direct_io\f[]
off as well as the problem of double caching but it\[aq]s the only way
to get such applications to work.
If the performance loss is too high for other apps you can mount
mergerfs twice.
Once with \f[C]direct_io\f[] enabled and one without it.
.SS mmap performance is really bad
.PP
There is a bug (https://lkml.org/lkml/2016/3/16/260) in caching which
affects overall performance of mmap through FUSE in Linux 4.x kernels.
It is fixed in 4.4.10 and 4.5.4 (https://lkml.org/lkml/2016/5/11/59).
.SS When a program tries to move or rename a file it fails
.PP
Please read the section above regarding rename & link (#rename--link).
.PP
The problem is that many applications do not properly handle
\f[C]EXDEV\f[] errors which \f[C]rename\f[] and \f[C]link\f[] may return
even though they are perfectly valid situations which do not indicate
actual drive or OS errors.
The error will only be returned by mergerfs if using a path preserving
policy as described in the policy section above.
If you do not care about path preservation simply change the mergerfs
policy to the non\-path preserving version.
For example: \f[C]\-o\ category.create=mfs\f[]
.PP
Ideally the offending software would be fixed and it is recommended that
if you run into this problem you contact the software\[aq]s author and
request proper handling of \f[C]EXDEV\f[] errors.
.SS Samba: Moving files / directories fails
.PP
Workaround: Copy the file/directory and then remove the original rather
than move.
.PP
This isn\[aq]t an issue with Samba but some SMB clients.
GVFS\-fuse v1.20.3 and prior (found in Ubuntu 14.04 among others) failed
to handle certain error codes correctly.
Particularly \f[B]STATUS_NOT_SAME_DEVICE\f[] which comes from the
\f[B]EXDEV\f[] which is returned by \f[B]rename\f[] when the call is
crossing mount points.
When a program gets an \f[B]EXDEV\f[] it needs to explicitly take an
alternate action to accomplish it\[aq]s goal.
In the case of \f[B]mv\f[] or similar it tries \f[B]rename\f[] and on
\f[B]EXDEV\f[] falls back to a manual copying of data between the two
locations and unlinking the source.
In these older versions of GVFS\-fuse if it received \f[B]EXDEV\f[] it
would translate that into \f[B]EIO\f[].
This would cause \f[B]mv\f[] or most any application attempting to move
files around on that SMB share to fail with a IO error.
.PP
GVFS\-fuse v1.22.0 (https://bugzilla.gnome.org/show_bug.cgi?id=734568)
and above fixed this issue but a large number of systems use the older
release.
On Ubuntu the version can be checked by issuing
\f[C]apt\-cache\ showpkg\ gvfs\-fuse\f[].
Most distros released in 2015 seem to have the updated release and will
work fine but older systems may not.
Upgrading gvfs\-fuse or the distro in general will address the problem.
.PP
In Apple\[aq]s MacOSX 10.9 they replaced Samba (client and server) with
their own product.
It appears their new client does not handle \f[B]EXDEV\f[] either and
responds similar to older release of gvfs on Linux.
.SS Trashing files occasionally fails
.PP
This is the same issue as with Samba.
\f[C]rename\f[] returns \f[C]EXDEV\f[] (in our case that will really
only happen with path preserving policies like \f[C]epmfs\f[]) and the
software doesn\[aq]t handle the situtation well.
This is unfortunately a common failure of software which moves files
around.
The standard indicates that an implementation \f[C]MAY\f[] choose to
support non\-user home directory trashing of files (which is a
\f[C]MUST\f[]).
The implementation \f[C]MAY\f[] also support "top directory trashes"
which many probably do.
.PP
To create a \f[C]$topdir/.Trash\f[] directory as defined in the standard
use the mergerfs\-tools (https://github.com/trapexit/mergerfs-tools)
tool \f[C]mergerfs.mktrash\f[].
.SS Supplemental user groups
.PP
Due to the overhead of
getgroups/setgroups (http://linux.die.net/man/2/setgroups) mergerfs
utilizes a cache.
This cache is opportunistic and per thread.
Each thread will query the supplemental groups for a user when that
particular thread needs to change credentials and will keep that data
for the lifetime of the thread.
This means that if a user is added to a group it may not be picked up
without the restart of mergerfs.
However, since the high level FUSE API\[aq]s (at least the standard
version) thread pool dynamically grows and shrinks it\[aq]s possible
that over time a thread will be killed and later a new thread with no
cache will start and query the new data.
.PP
The gid cache uses fixed storage to simplify the design and be
compatible with older systems which may not have C++11 compilers.
There is enough storage for 256 users\[aq] supplemental groups.
Each user is allowed upto 32 supplemental groups.
Linux >= 2.6.3 allows upto 65535 groups per user but most other *nixs
allow far less.
NFS allowing only 16.
The system does handle overflow gracefully.
If the user has more than 32 supplemental groups only the first 32 will
be used.
If more than 256 users are using the system when an uncached user is
found it will evict an existing user\[aq]s cache at random.
So long as there aren\[aq]t more than 256 active users this should be
fine.
If either value is too low for your needs you will have to modify
\f[C]gidcache.hpp\f[] to increase the values.
Note that doing so will increase the memory needed by each thread.
.SS mergerfs or libfuse crashing
.PP
\f[B]NOTE:\f[] as of mergerfs 2.22.0 it includes the most recent version
of libfuse so any crash should be reported.
For older releases continue reading...
.PP
If suddenly the mergerfs mount point disappears and
\f[C]Transport\ endpoint\ is\ not\ connected\f[] is returned when
attempting to perform actions within the mount directory \f[B]and\f[]
the version of libfuse (use \f[C]mergerfs\ \-v\f[] to find the version)
is older than \f[C]2.9.4\f[] its likely due to a bug in libfuse.
Affected versions of libfuse can be found in Debian Wheezy, Ubuntu
Precise and others.
.PP
In order to fix this please install newer versions of libfuse.
If using a Debian based distro (Debian,Ubuntu,Mint) you can likely just
install newer versions of
libfuse (https://packages.debian.org/unstable/libfuse2) and
fuse (https://packages.debian.org/unstable/fuse) from the repo of a
newer release.
.SS mergerfs appears to be crashing or exiting
.PP
There seems to be an issue with Linux version \f[C]4.9.0\f[] and above
in which an invalid message appears to be transmitted to libfuse (used
by mergerfs) causing it to exit.
No messages will be printed in any logs as its not a proper crash.
Debugging of the issue is still ongoing and can be followed via the
fuse\-devel
thread (https://sourceforge.net/p/fuse/mailman/message/35662577).
.SS mergerfs under heavy load and memory preasure leads to kernel panic
.PP
https://lkml.org/lkml/2016/9/14/527
.IP
.nf
\f[C]
[25192.515454]\ kernel\ BUG\ at\ /build/linux\-a2WvEb/linux\-4.4.0/mm/workingset.c:346!
[25192.517521]\ invalid\ opcode:\ 0000\ [#1]\ SMP
[25192.519602]\ Modules\ linked\ in:\ netconsole\ ip6t_REJECT\ nf_reject_ipv6\ ipt_REJECT\ nf_reject_ipv4\ configfs\ binfmt_misc\ veth\ bridge\ stp\ llc\ nf_conntrack_ipv6\ nf_defrag_ipv6\ xt_conntrack\ ip6table_filter\ ip6_tables\ xt_multiport\ iptable_filter\ ipt_MASQUERADE\ nf_nat_masquerade_ipv4\ xt_comment\ xt_nat\ iptable_nat\ nf_conntrack_ipv4\ nf_defrag_ipv4\ nf_nat_ipv4\ nf_nat\ nf_conntrack\ xt_CHECKSUM\ xt_tcpudp\ iptable_mangle\ ip_tables\ x_tables\ intel_rapl\ x86_pkg_temp_thermal\ intel_powerclamp\ eeepc_wmi\ asus_wmi\ coretemp\ sparse_keymap\ kvm_intel\ ppdev\ kvm\ irqbypass\ mei_me\ 8250_fintek\ input_leds\ serio_raw\ parport_pc\ tpm_infineon\ mei\ shpchp\ mac_hid\ parport\ lpc_ich\ autofs4\ drbg\ ansi_cprng\ dm_crypt\ algif_skcipher\ af_alg\ btrfs\ raid456\ async_raid6_recov\ async_memcpy\ async_pq\ async_xor\ async_tx\ xor\ raid6_pq\ libcrc32c\ raid0\ multipath\ linear\ raid10\ raid1\ i915\ crct10dif_pclmul\ crc32_pclmul\ aesni_intel\ i2c_algo_bit\ aes_x86_64\ drm_kms_helper\ lrw\ gf128mul\ glue_helper\ ablk_helper\ syscopyarea\ cryptd\ sysfillrect\ sysimgblt\ fb_sys_fops\ drm\ ahci\ r8169\ libahci\ mii\ wmi\ fjes\ video\ [last\ unloaded:\ netconsole]
[25192.540910]\ CPU:\ 2\ PID:\ 63\ Comm:\ kswapd0\ Not\ tainted\ 4.4.0\-36\-generic\ #55\-Ubuntu
[25192.543411]\ Hardware\ name:\ System\ manufacturer\ System\ Product\ Name/P8H67\-M\ PRO,\ BIOS\ 3904\ 04/27/2013
[25192.545840]\ task:\ ffff88040cae6040\ ti:\ ffff880407488000\ task.ti:\ ffff880407488000
[25192.548277]\ RIP:\ 0010:[<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[].
Add \f[C]no_splice_write,no_splice_move,no_splice_read\f[] to
mergerfs\[aq] options.
Should be placed after \f[C]defaults\f[] if it is used since it will
turn them on.
This however is not guaranteed to work.
.SH FAQ
.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
If you wish to get similar behavior to mhddfs from mergerfs then set
\f[C]category.create=ff\f[].
.SS Why use mergerfs over aufs?
.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 LVM/ZFS/BTRFS/RAID0 drive concatenation /
striping?
.PP
With simple JBOD / drive concatenation / stripping / RAID0 a single
drive failure will result in full pool failure.
mergerfs performs a similar behavior without the possibility of
catastrophic failure and difficulties in recovery.
Drives may fail however all other data will continue to be accessable.
.PP
When combined with something like SnapRaid (http://www.snapraid.it)
and/or an offsite backup solution you can have the flexibilty of JBOD
without the single point of failure.
.SS Why use mergerfs over ZFS?
.PP
MergerFS is not intended to be a replacement for ZFS.
MergerFS is intended to provide flexible pooling of arbitrary drives
(local or remote), of arbitrary sizes, and arbitrary filesystems.
For \f[C]write\ once,\ read\ many\f[] usecases such as bulk media
storage.
Where data integrity and backup is managed in other ways.
In that situation ZFS can introduce major maintance and cost burdens as
described
here (http://louwrentius.com/the-hidden-cost-of-using-zfs-for-your-home-nas.html).
.SS Can drives be written to directly? Outside of mergerfs while pooled?
.PP
Yes.
It will be represented immediately in the pool as the policies
perscribe.
.SS Why do I get an "out of space" error even though the system says
there\[aq]s lots of space left?
.PP
First make sure you\[aq]ve read the sections above about policies, path
preserving, and the \f[B]moveonenospc\f[] option.
.PP
Remember that mergerfs is simply presenting a logical merging of the
contents of the pooled drives.
The reported free space is the aggregate space available \f[B]not\f[]
the contiguous space available.
MergerFS does not split files across drives.
If the writing of a file fills a drive and \f[B]moveonenospc\f[] is
disabled it will return an ENOSPC error.
.PP
If \f[B]moveonenospc\f[] is enabled but there exists no drives with
enough space for the file and the data to be written (or the drive
happened to fill up as the file was being moved) it will error
indicating there isn\[aq]t enough space.
.PP
It is also possible that the filesystem selected has run out of inodes.
Use \f[C]df\ \-i\f[] to list the total and available inodes per
filesystem.
In the future it might be worth considering the number of inodes
available when making placement decisions in order to minimize this
situation.
.SS Can mergerfs mounts be exported over NFS?
.PP
Yes.
Some clients (Kodi) have issues in which the contents of the NFS mount
will not be presented but users have found that enabling the
\f[C]use_ino\f[] option often fixes that problem.
.SS Can mergerfs mounts be exported over Samba / SMB?
.PP
Yes.
.SS How are inodes calculated?
.PP
mergerfs\-inode = (original\-inode | (device\-id << 32))
.PP
While \f[C]ino_t\f[] is 64 bits only a few filesystems use more than 32.
Similarly, while \f[C]dev_t\f[] is also 64 bits it was traditionally 16
bits.
Bitwise or\[aq]ing them together should work most of the time.
While totally unique inodes are preferred the overhead which would be
needed does not seem to outweighted by the benefits.
.SS It\[aq]s mentioned that there are some security issues with mhddfs.
What are they? How does mergerfs address them?
.PP
mhddfs (https://github.com/trapexit/mhddfs) manages running as
\f[B]root\f[] by calling
getuid() (https://github.com/trapexit/mhddfs/blob/cae96e6251dd91e2bdc24800b4a18a74044f6672/src/main.c#L319)
and if it returns \f[B]0\f[] then it will
chown (http://linux.die.net/man/1/chown) the file.
Not only is that a race condition but it doesn\[aq]t handle many other
situations.
Rather than attempting to simulate POSIX ACL behavior the proper way to
manage this is to use seteuid (http://linux.die.net/man/2/seteuid) and
setegid (http://linux.die.net/man/2/setegid), in effect becoming the
user making the original call, and perform the action as them.
This is what mergerfs does.
.PP
In Linux setreuid syscalls apply only to the thread.
GLIBC hides this away by using realtime signals to inform all threads to
change credentials.
Taking after \f[B]Samba\f[], mergerfs uses
\f[B]syscall(SYS_setreuid,...)\f[] to set the callers credentials for
that thread only.
Jumping back to \f[B]root\f[] as necessary should escalated privileges
be needed (for instance: to clone paths between drives).
.PP
For non\-Linux systems mergerfs uses a read\-write lock and changes
credentials only when necessary.
If multiple threads are to be user X then only the first one will need
to change the processes credentials.
So long as the other threads need to be user X they will take a readlock
allowing multiple threads to share the credentials.
Once a request comes in to run as user Y that thread will attempt a
write lock and change to Y\[aq]s credentials when it can.
If the ability to give writers priority is supported then that flag will
be used so threads trying to change credentials don\[aq]t starve.
This isn\[aq]t the best solution but should work reasonably well
assuming there are few users.
.SH SUPPORT
.SS Issues with the software
.IP \[bu] 2
github.com: https://github.com/trapexit/mergerfs/issues
.IP \[bu] 2
email: trapexit\@spawn.link
.IP \[bu] 2
twitter: https://twitter.com/_trapexit
.SS Support development
.IP \[bu] 2
Gratipay: https://gratipay.com/~trapexit
.IP \[bu] 2
BitCoin: 12CdMhEPQVmjz3SSynkAEuD5q9JmhTDCZA
.SH LINKS
.IP \[bu] 2
http://github.com/trapexit/mergerfs
.IP \[bu] 2
http://github.com/trapexit/mergerfs\-tools
.IP \[bu] 2
http://github.com/trapexit/scorch
.IP \[bu] 2
http://github.com/trapexit/backup\-and\-recovery\-howtos
.SH AUTHORS
Antonio SJ Musumeci <trapexit@spawn.link>.