mergerfs

another FUSE union filesystem

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).

Policies

Filesystem calls are broken up into 4 categories of policies: search, action, create, and none.

Below shows each policy class, the FUSE calls they impact, and the policy names.

Policy classifications

Class	FUSE calls	Policies
search	access, getattr, getxattr, listxattr, open, readlink	First Found (ff), First Found w/ Permission (ffwp), Newest (newest)
action	chmod, link, removexattr, rmdir, setxattr, truncate, unlink, utimens	First Found (ff), First Found w/ Permission (ffwp), Newest (newest), All Found (all)
create	create, mkdir, mknod	Existing Path (ep), Most Free Space (mfs), Existing Path Most Free Space (epmfs), Random (rand)
none	fallocate, fsync, ftruncate, ioctl, read, readdir, rename, statfs, symlink, write, release

Descriptions

Class/Policy	Description
search/ff	Given the order the paths were provided at mount time act on the first one found (regardless if stat would return EACCES).
search/ffwp	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).
search/newest	If multiple files exist return the one with the most recent mtime.
action/ff	(same as search/ff)
action/ffwp	(same as search/ffwp)
action/newest	(same as search/newest)
action/all	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.
create/ep	Choose the first path which is found.
create/mfs	Assuming the path is found to exist (ENOENT would not be returned) use the drive with the most free space available.
create/epmfs	If the path exists in multiple locations use the one with the most free space. Otherwise fall back to mfs.
create/rand	Pick a destination at random. Again the dirname of the full path must exist somewhere.

statvfs

Since we aren't trying to stripe data across drives the free space of the mountpoint is just that of the source mount with the most free space at the moment.

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.

Usage

$ mergerfs -o create=epmfs,search=ff,action=ff <mountpoint> <dir0>:<dir1>:<dir2>

Option	Values	Default
search	ff, ffwp, newest	ff
action	ff, ffwp, newest, all	ff
create	ep, mfs, epmfs, rand	epmfs

Building

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

[trapexit:~/dev/mergerfs] $ make help
usage: make
make WITHOUT_XATTR=1 - to build program without xattrs functionality

Runtime Settings

/.mergerfs pseudo file

<mountpoint>/.mergerfs

There is a pseudo file available at the mountpoint which currently allows for the runtime modification of policies. The file will not show up in readdirs but can be stat'ed, read, and writen. Most other calls will fail with EPERM, EINVAL, or whatever may be appropriate for that call. Anything not understood while writing will result in EINVAL otherwise the number of bytes written will be returned.

Reading the file will result in a newline delimited list of current settings as followed:

[trapexit:/tmp/mount] $ cat .mergerfs
action=ff
create=epmfs
search=ff

Writing to the file is buffered and waits till a newline to process. Meaning echo works well.

[trapexit:/tmp/mount] $ echo "search=newest" >> .mergerfs
[trapexit:/tmp/mount] $ cat .mergerfs
action=ff
create=epmfs
search=newest

NOTE: offset is not supported and ignored in both read and write. There is also a safety check which limits buffered + incoming length to a max of 1024 bytes.

xattrs

If xattrs has been enabled you can also use {list,get,set}xattrs on the pseudo file .mergerfs to modify the policies. The keys are mergerfs.action, mergerfs.create, and mergerfs.search.

[trapexit:/tmp/mount] $ attr -l .mergerfs
Attribute "mergerfs.action" has a 3 byte value for .mergerfs
Attribute "mergerfs.create" has a 6 byte value for .mergerfs
Attribute "mergerfs.search" has a 3 byte value for .mergerfs

[trapexit:/tmp/mount] $ attr -g mergerfs.action .mergerfs
Attribute "mergerfs.action" had a 3 byte value for .mergerfs:
ff

[trapexit:/tmp/mount] 1 $ attr -s mergerfs.action -V ffwp .mergerfs
Attribute "mergerfs.action" set to a 3 byte value for .mergerfs:
ffwp