Merge pull request #165 from trapexit/manpage

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 .\"t
 .TH "mergerfs" "1" "2015\-10\-11" "mergerfs user manual" ""
 .SH NAME
 .PP
 mergerfs \- another FUSE union filesystem
 .SH SYNOPSIS
 .PP
 mergerfs \-o<options> <srcpoints> <mountpoint>
 .SH DESCRIPTION
 .PP
 \f[B]mergerfs\f[] is similar to \f[B]mhddfs\f[], \f[B]unionfs\f[], and
 \f[B]aufs\f[].
 Like \f[B]mhddfs\f[] in that it too uses \f[B]FUSE\f[].
 Like \f[B]aufs\f[] in that it provides multiple policies for how to
 handle behavior.
 .PP
 Why \f[B]mergerfs\f[] when those exist?
 \f[B]mhddfs\f[] has not been updated in some time nor very flexible.
 There are also security issues when with running as root.
 \f[B]aufs\f[] is more flexible than \f[B]mhddfs\f[] but kernel based and
 difficult to debug when problems arise.
 Neither support file attributes
 (chattr (http://linux.die.net/man/1/chattr)).
 .SH FEATURES
 .IP \[bu] 2
 Runs in userspace (FUSE)
 .IP \[bu] 2
 Configurable behaviors
 .IP \[bu] 2
 Supports extended attributes (xattrs)
 .IP \[bu] 2
 Supports file attributes (chattr)
 .IP \[bu] 2
 Dynamically 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
 .SH OPTIONS
 .SS 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 an addition caching step which
 can increase write speeds at the detriment of read speed.
 .IP \[bu] 2
 \f[B]minfreespace\f[]: the minimum space value used for the
 \f[B]lfs\f[], \f[B]fwfs\f[], and \f[B]epmfs\f[] 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[] 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]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 srcpoints
 .PP
 The source points argument is a colon (\[aq]:\[aq]) delimited list of
 paths.
 To make it simpler to include multiple source points without having to
 modify your fstab (http://linux.die.net/man/5/fstab) we also support
 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 probably expand it.\f[]
 .IP
 .nf
 \f[C]
 $\ mergerfs\ /mnt/disk\\*:/mnt/cdrom\ /media/drives
 \f[]
 .fi
 .PP
 The above line will use all points in /mnt prefixed with \f[I]disk\f[]
 and the directory \f[I]cdrom\f[].
 .PP
 In /etc/fstab it\[aq]d look like the following:
 .IP
 .nf
 \f[C]
 #\ <file\ system>\ \ \ \ \ \ \ \ <mount\ point>\ \ <type>\ \ \ \ \ \ \ \ \ <options>\ \ \ \ \ \ \ \ \ \ \ \ \ <dump>\ \ <pass>
 /mnt/disk*:/mnt/cdrom\ \ /media/drives\ \ fuse.mergerfs\ \ defaults,allow_other\ \ 0\ \ \ \ \ \ \ 0
 \f[]
 .fi
 .PP
 \f[B]NOTE:\f[] the globbing is done at mount or xattr update time.
 If a new directory is added matching the glob after the fact it will not
 be included.
 .SH POLICIES
 .PP
 Filesystem calls are broken up into 3 categories: \f[B]action\f[],
 \f[B]create\f[], \f[B]search\f[].
 There are also some calls which have no policy attached due to state
 being kept between calls.
 These categories can be assigned a policy which dictates how
 \f[B]mergerfs\f[] behaves.
 Any policy can be assigned to a category though some aren\[aq]t terribly
 practical.
 For instance: \f[B]rand\f[] (Random) may be useful for \f[B]create\f[]
 but could lead to very odd behavior if used for \f[B]search\f[].
 .SS Functional classifications
 .PP
 .TS
 tab(@);
 l l.
 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
 \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) then \f[B]getattr\f[] will be used.
 .SS Policy descriptions
 .PP
 .TS
 tab(@);
 l l.
 T{
 Policy
 T}@T{
 Description
 T}
 _
 T{
 ff (first found)
 T}@T{
 Given the order of the drives act on the first one found (regardless if
 stat would return EACCES).
 T}
 T{
 ffwp (first found w/ permissions)
 T}@T{
 Given the order of the drives act on the first one found which you have
 access (stat does not error with EACCES).
 T}
 T{
 newest (newest file)
 T}@T{
 If multiple files exist return the one with the most recent mtime.
 T}
 T{
 mfs (most free space)
 T}@T{
 Use the drive with the most free space available.
 T}
 T{
 epmfs (existing path, most free space)
 T}@T{
 If the path exists on multiple drives use the one with the most free
 space and is greater than \f[B]minfreespace\f[].
 If no drive has at least \f[B]minfreespace\f[] then fallback to
 \f[B]mfs\f[].
 T}
 T{
 fwfs (first with free space)
 T}@T{
 Pick the first drive which has at least \f[B]minfreespace\f[].
 T}
 T{
 lfs (least free space)
 T}@T{
 Pick the drive with least available space but more than
 \f[B]minfreespace\f[].
 T}
 T{
 rand (random)
 T}@T{
 Pick an existing drive at random.
 T}
 T{
 all
 T}@T{
 Applies action to all found.
 For searches it will behave like first found \f[B]ff\f[].
 T}
 T{
 enosys, einval, enotsup, exdev, erofs
 T}@T{
 Exclusively return \f[C]\-1\f[] with \f[C]errno\f[] set to the
 respective value.
 Useful for debugging other applications\[aq] behavior to errors.
 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
 .PP
 rename (http://man7.org/linux/man-pages/man2/rename.2.html) is a tricky
 function in a merged system.
 Normally if a rename can\[aq]t be done atomically due to the from and to
 paths existing on different mount points it will return \f[C]\-1\f[]
 with \f[C]errno\ =\ EXDEV\f[].
 The atomic rename is most critical for replacing files in place
 atomically (such as securing writing to a temp file and then replacing a
 target).
 The problem is that by merging multiple paths you can have N instances
 of the source and destinations on different drives.
 Meaning that if you just renamed each source locally you could end up
 with the destination files not overwriten / replaced.
 To address this mergerfs works in the following way.
 If the source and destination exist in different directories it will
 immediately return \f[C]EXDEV\f[].
 Generally it\[aq]s not expected for cross directory renames to work so
 it should be fine for most instances (mv,rsync,etc.).
 If they do belong to the same directory it then runs the \f[C]rename\f[]
 policy to get the files to rename.
 It iterates through and renames each file while keeping track of those
 paths which have not been renamed.
 If all the renames succeed it will then \f[C]unlink\f[] or
 \f[C]rmdir\f[] the other paths to clean up any preexisting target files.
 This allows the new file to be found without the file itself ever
 disappearing.
 There may still be some issues with this behavior.
 Particularly on error.
 At the moment however this seems the best policy.
 .SS readdir
 .PP
 readdir (http://linux.die.net/man/3/readdir) is very different from most
 functions in this realm.
 It certainly could have it\[aq]s own set of policies to tweak its
 behavior.
 At this time it provides a simple \f[B]first found\f[] merging of
 directories and file found.
 That is: only the first file or directory found for a directory is
 returned.
 Given how FUSE works though the data representing the returned entry
 comes from \f[B]getattr\f[].
 .PP
 It could be extended to offer the ability to see all files found.
 Perhaps concatenating \f[B]#\f[] and a number to the name.
 But to really be useful you\[aq]d need to be able to access them which
 would complicate file lookup.
 .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 points
 on the same drive will not result in double counting it\[aq]s space.
 .PP
 \f[B]NOTE:\f[] Since we can not (easily) replicate the atomicity of an
 \f[B]mkdir\f[] or \f[B]mknod\f[] without side effects those calls will
 first do a scan to see if the file exists and then attempts a create.
 This means there is a slight race condition.
 Worse case you\[aq]d end up with the directory or file on more than one
 mount.
 .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/archive/master.zip
 \f[]
 .fi
 .SS Debian / Ubuntu
 .IP
 .nf
 \f[C]
 $\ sudo\ apt\-get\ install\ g++\ pkg\-config\ git\ git\-buildpackage\ pandoc\ debhelper\ libfuse\-dev\ libattr1\-dev
 $\ cd\ mergerfs
 $\ make\ deb
 $\ sudo\ dpkg\ \-i\ ../mergerfs_version_arch.deb
 \f[]
 .fi
 .SS Fedora
 .IP
 .nf
 \f[C]
 $\ su\ \-
 #\ dnf\ install\ rpm\-build\ fuse\-devel\ libattr\-devel\ pandoc\ gcc\-c++\ git\ make\ which
 #\ cd\ mergerfs
 #\ make\ rpm
 #\ rpm\ \-i\ rpmbuild/RPMS/<arch>/mergerfs\-<verion>.<arch>.rpm
 \f[]
 .fi
 .SS Generically
 .PP
 Have pkg\-config, pandoc, libfuse, libattr1 installed.
 .IP
 .nf
 \f[C]
 $\ cd\ mergerfs
 $\ make
 $\ make\ man
 $\ 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 the
 {list,get,set}xattrs (http://linux.die.net/man/2/listxattr) calls will
 still work.
 .SS Keys
 .PP
 Use \f[C]xattr\ \-l\ /mount/point/.mergerfs\f[] to see all supported
 keys.
 .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
 user.mergerfs.policies:\ all,einval,enosys,enotsup,epmfs,erofs,exdev,ff,ffwp,fwfs,lfs,mfs,newest,rand
 user.mergerfs.version:\ x.y.z
 user.mergerfs.category.action:\ all
 user.mergerfs.category.create:\ epmfs
 user.mergerfs.category.search:\ ff
 user.mergerfs.func.access:\ ff
 user.mergerfs.func.chmod:\ all
 user.mergerfs.func.chown:\ all
 user.mergerfs.func.create:\ epmfs
 user.mergerfs.func.getattr:\ ff
 user.mergerfs.func.getxattr:\ ff
 user.mergerfs.func.link:\ all
 user.mergerfs.func.listxattr:\ ff
 user.mergerfs.func.mkdir:\ epmfs
 user.mergerfs.func.mknod:\ epmfs
 user.mergerfs.func.open:\ ff
 user.mergerfs.func.readlink:\ ff
 user.mergerfs.func.removexattr:\ all
 user.mergerfs.func.rename:\ all
 user.mergerfs.func.rmdir:\ all
 user.mergerfs.func.setxattr:\ all
 user.mergerfs.func.symlink:\ epmfs
 user.mergerfs.func.truncate:\ all
 user.mergerfs.func.unlink:\ all
 user.mergerfs.func.utimens:\ all
 [trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.category.search\ .mergerfs
 ff
 [trapexit:/tmp/mount]\ $\ xattr\ \-w\ user.mergerfs.category.search\ ffwp\ .mergerfs
 [trapexit:/tmp/mount]\ $\ xattr\ \-p\ user.mergerfs.category.search\ .mergerfs
 ffwp
 [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 user.mergerfs.srcmounts
 .PP
 For \f[B]user.mergerfs.srcmounts\f[] there are several instructions
 available for manipulating the list.
 The value provided is just as the value used at mount time.
 A colon (\[aq]:\[aq]) delimited list of full path globs.
 .PP
 .TS
 tab(@);
 l l.
 T{
 Instruction
 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 suffix.
 \f[B]K\f[], \f[B]M\f[], or \f[B]G\f[].
 Output: value in bytes
 .SS moveonenospc
 .PP
 Input: \f[B]true\f[] and \f[B]false\f[] Ouput: \f[B]true\f[] or
 \f[B]false\f[]
 .SS categories / funcs
 .PP
 Input: short policy string as described elsewhere in this document
 Output: the policy string except for categories where its funcs have
 multiple types.
 In that case it will be a comma separated list.
 .SS mergerfs file 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 search 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 search 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
 /usr/sbin/fsck.mergerfs: Provides permissions and ownership auditing and
 the ability to fix them.
 .SH TIPS / NOTES
 .IP \[bu] 2
 If you don\[aq]t see some directories / files you expect in a merged
 point be sure the user has permission to all the underlying directories.
 If \f[C]/drive0/a\f[] has is owned by \f[C]root:root\f[] with ACLs set
 to \f[C]0700\f[] and \f[C]/drive1/a\f[] is \f[C]root:root\f[] and
 \f[C]0755\f[] you\[aq]ll see only \f[C]/drive1/a\f[].
 Use \f[C]fsck.mergerfs\f[] to audit the drive for out of sync
 permissions.
 .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.
 Generally if something succeeds when reading it returns the data it can.
 If something fails when making an action we continue on and return the
 last error.
 .IP \[bu] 2
 The recommended options are \f[B]defaults,allow_other\f[].
 The \f[B]allow_other\f[] is to allow users who are not the one which
 executed mergerfs access to the mountpoint.
 \f[B]defaults\f[] is described above and should offer the best
 performance.
 It\[aq]s possible that if you\[aq]re running on an older platform the
 \f[B]splice\f[] features aren\[aq]t available and could error.
 In that case simply use the other options manually.
 .IP \[bu] 2
 If write performance is valued more than read it may be useful to enable
 \f[B]direct_io\f[].
 .IP \[bu] 2
 Remember that 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
 An example: Kodi (http://kodi.tv) and Plex (http://plex.tv) can
 apparently use directory mtime (http://linux.die.net/man/2/stat) to more
 efficiently determine whether or not to scan for new content rather than
 simply performing a full scan.
 If using the current 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
 Due to previously mentioned issues 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).
 .SH Known Issues / Bugs
 .SS Samba
 .IP \[bu] 2
 Moving files or directories between directories on a SMB share fail with
 IO errors.
 .RS 2
 .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 mountpoints.
 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.
 .RE
 .SS Supplemental groups
 .IP \[bu] 2
 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 mount or 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.
 .RS 2
 .PP
 The gid cache uses fixed storage to simplify the design and be
 compatible with older systems which may not have C++11 compilers (as the
 original design required).
 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.
 .RE
 .SH FAQ
 .PP
 \f[I]It\[aq]s mentioned that there are some security issues with mhddfs.
 What are they? How does mergerfs address them?\f[]
 .PP
 mhddfs (https://github.com/trapexit/mhddfs) tries to handle being run 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 behaviors the proper
 behavior is to use seteuid (http://linux.die.net/man/2/seteuid) and
 setegid (http://linux.die.net/man/2/setegid), become the user making the
 original call and perform the action as them.
 This is how mergerfs (https://github.com/trapexit/mergerfs) handles
 things.
 .PP
 If you are familiar with POSIX standards you\[aq]ll know that this
 behavior poses a problem.
 \f[B]seteuid\f[] and \f[B]setegid\f[] affect the whole process and
 \f[B]libfuse\f[] is multithreaded by default.
 We\[aq]d need to lock access to \f[B]seteuid\f[] and \f[B]setegid\f[]
 with a mutex so that the several threads aren\[aq]t stepping on one
 another and files end up with weird permissions and ownership.
 This however wouldn\[aq]t scale well.
 With lots of calls the contention on that mutex would be extremely high.
 Thankfully on Linux and OSX we have a better solution.
 .PP
 OSX has a non\-portable pthread
 extension (https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/pthread_setugid_np.2.html)
 for per\-thread user and group impersonation.
 .PP
 Linux does not support
 pthread_setugid_np (https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/pthread_setugid_np.2.html)
 but user and group IDs are a per\-thread attribute though documentation
 on that fact or how to manipulate them is not well distributed.
 From the \f[B]4.00\f[] release of the Linux man\-pages project for
 setuid (http://man7.org/linux/man-pages/man2/setuid.2.html)
 .RS
 .PP
 At the kernel level, user IDs and group IDs are a per\-thread attribute.
 However, POSIX requires that all threads in a process share the same
 credentials.
 The NPTL threading implementation handles the POSIX requirements by
 providing wrapper functions for the various system calls that change
 process UIDs and GIDs.
 These wrapper functions (including the one for setuid()) employ a
 signal\-based technique to ensure that when one thread changes
 credentials, all of the other threads in the process also change their
 credentials.
 For details, see nptl(7).
 .RE
 .PP
 Turns out the setreuid syscalls apply only to the thread.
 GLIBC hides this away using RT 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).
 .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
 allow 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.
 As new platforms are supported if they offer per thread credentials
 those APIs will be adopted.
 .SH AUTHORS
 Antonio SJ Musumeci <trapexit@spawn.link>.