% mergerfs(1) mergerfs user manual
% Antonio SJ Musumeci trapexit@spawn.link
% June 9, 2014

NAME

mergerfs - another FUSE union filesystem

SYNOPSIS

mergerfs -ocreate=epmfs,search=ff,action=ff <srcpoints> <mountpoint>

DESCRIPTION

mergerfs is similar to mhddfs, unionfs, and aufs. Like mhddfs in that it too uses FUSE. Like aufs in that it provides multiple policies for how to handle behavior.

Why create mergerfs when those exist? mhddfs isn't really maintained or flexible. There are also issues with running as root. aufs is more flexible but contains some hard to debug inconsistencies in behavior. Neither support file attributes (chattr).

OPTIONS

###options###

Option	Default
search	ff
action	ff
create	epmfs

###srcpoints###

The source points argument is a colon (':') delimited list of paths. To make it simplier to include multiple source points without having to modify your fstab we also support globbing.

$ mergerfs /mnt/disk*:/mnt/cdrom /media/drives

The above line will use all points in /mnt prefixed with disk and the directory cdrom.

In /etc/fstab it'd look like the following:

# <file system>        <mount point>  <type>         <options>    <dump>  <pass>
/mnt/disk*:/mnt/cdrom  /media/drives  fuse.mergerfs  allow_other  0       0

NOTE: the globbing is done at mount time. If a new directory is added matching the glob after the fact it will not be included.

POLICIES

Filesystem calls are broken up into 4 functional categories: search, action, create, and none. These categories can be assigned a policy which dictates how mergerfs behaves while when action on the filesystem. Any policy can be assigned to a category though some aren't terribly practical. For instance: rand (Random) may be useful for create but could lead to very odd behavior if used for search or action. Since the input for any policy is the source mounts and fusepath and the output a vector of targets the choice was made to simplify the implementation and allow a policies usage in any category. NOTE: In any policy which can return more than one location (currently only all) the first value will be used in search and create policies since they can only ever act on 1 filepath.

Functional classifications

Class	FUSE calls
search	access, getattr, getxattr, listxattr, open, readlink
action	chmod, link, removexattr, rmdir, setxattr, truncate, unlink, utimens
create	create, mkdir, mknod
none	fallocate, fgetattr, fsync, ftruncate, ioctl, read, readdir, rename, statfs, symlink, write, release

Policy descriptions

Policy	Description
ff (first found)	Given the order the paths were provided at mount time act on the first one found (regardless if stat would return EACCES).
ffwp (first found w/ permissions)	Given the order the paths were provided at mount time act on the first one found which you have access (stat does not error with EACCES).
newest (newest file)	If multiple files exist return the one with the most recent mtime.
all (all files found)	Attempt to apply the call to each file found. If any sub call succeeds the entire operation succeeds and other errors ignored. If all fail then the last error is reported.
mfs (most free space)	Assuming the path is found to exist (ENOENT would not be returned) use the drive with the most free space available.
epmfs (existing path, most free space)	If the path exists in multiple locations use the one with the most free space. Otherwise fall back to mfs.
rand (random)	Pick a destination at random. Again the dirname of the full path must exist somewhere.

statvfs

It 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's space.

NOTE: create is really a search for existence and then create. The 'search' policy applies to the first part. If the dirname of the full path is not found to exist ENOENT is returned.

BUILDING

• Need to install FUSE development libraries (libfuse-dev).
• Optionally need libattr1 (libattr1-dev).

[trapexit:~/dev/mergerfs] $ make help
usage: make
make XATTR_AVAILABLE=0 - to build program without xattrs functionality (auto discovered otherwise)

Runtime Settings

/.mergerfs pseudo file

<mountpoint>/.mergerfs

There is a pseudo file available at the mountpoint which allows for the runtime modification of certain mergerfs options. The file will not show up in readdirs but can be stat'ed and manipulated via {list,get,set}xattrs calls.

Even if xattrs are disabled the {list,get,set}xattrs calls will still work.

The keys are:

• user.mergerfs.srcmounts
• user.mergerfs.action
• user.mergerfs.create
• user.mergerfs.search

[trapexit:/tmp/mount] $ xattr -l .mergerfs
user.mergerfs.srcmounts: /tmp/a:/tmp/b
user.mergerfs.action: ff
user.mergerfs.create: epmfs
user.mergerfs.search: ff

[trapexit:/tmp/mount] $ xattr -p user.mergerfs.action .mergerfs
ff

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

For user.mergerfs.srcmounts there are several instructions available for manipulating the list. The value provided is just as the value used at mount time. A colon (':') delimited list of full path globs.

Instruction	Description
+[list]	append
+<[list]	prepend
+>[list]	append
-[list]	remove all values provided
-<	remove first in list
->	remove last in list
=[list]	set
[list]	set

mergerfs file xattrs

While they won't show up when using listxattr mergerfs offers a number of special xattrs to query information about the files served. To access the values you will need to issue a getxattr for one of the following:

• user.mergerfs.basepath : gives you the base mount point for the file given the current search policy
• user.mergerfs.fullpath : gives you the full path of the original file given the search policy

[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