unionfs-fuse - A userspace unionfs implementation
unionfs-fuse [-o option1 -o option2 ... -o optionN ]
unionfs-fuse overlays several directory into one single mount point.
It first tries to access the file on the top branch and if the file
does not exist there, it continues on lower level branches. If the
user tries to modify a file on a lower level read-only branch the file
is copied to to a higher level read-write branch if the copy-on-write
(cow) mode was enabled.
Below is a summary of unionfs-fuse options
-o cow Enable copy-on-write
Show statistics in the file ’stats’ under the mountpoint.
By default blocks of all branches are counted in statfs() calls
(e.g. by ’df’). On setting this option read-only branches will
be omitted for the summary of blocks. This may sound weird but
it actually fixes "wrong" percentage of free space.
Since version 0.23 without any effect, just left over for
compatibility. Might be removed in future versions.
Path to chroot into. By using this option unionfs-fuse may be
used for live CDs or live USB sticks, etc. So it can serve "/"
as filesystem. If you do not specify this option and try to use
it for "/" it will deadlock on calling ’pivot_root’. If you do
set this option, you also need to specify the branches relativly
to the given chroot directory. See examples/S01a-unionfs-fuse-
live-cd.sh for an example.
Maximum number of open files. Most system have a default of 1024
open files per process. For example if unionfs-fuse servs "/"
applications like KDE or GNOME might have much more open files,
which will make the unionfs-fuse process to exceed this limit.
Suggested for "/" is >16000 or even >32000 files. If this limit
exceeds unionfs-fuse will not be able to open further files.
Options to libfuse
There are several further options available, which don’t directly apply
to unionfs, but to libfuse. Please run "unionfs-fuse --help" to see
these. We already set the "-o default-permissions" options on our own.
unionfs-fuse -o cow,max_files=32768 \
-o allow_other,use_ino,suid,dev,nonempty \
Like other filesystems unionfs-fuse also needs to store meta data.
Well, presently only information about deleted files and directories
need to be stored, but in future releases more information might be
required, e.g. inode-numbers for persistent inode information. Meta
data information are saved and looked for in the .unionfs/ directories
of each branch-root. So in the example above, these are
/u/host/etc/.unionfs, /u/group/etc/.unionfs and /u/common/etc/.unionfs.
Within these directories a complete directory structure may be found.
Example: If the admin decides to delete the file /etc/test/testfile,
which only exists in /u/unionfs/etc/test/testfile, unionfs-fuse can’t
delete this file, since it is on a read-only branch. So instead the
whiteout file /u/host/etc/.unionfs/test/testfile_HIDDEN~ will be
created. So on accessing the union filesystem, test/testfile will not
be visible. Please also note that whiteout files/directories will only
hide the files in lower level branches. So for example whiteouts in the
group directory (/u/group/etc/.unionfs of the example above) will only
hide file of the common branch (/u/common/etc), but not these of the
group and host branches. Especially for diskless-booted environments
it is rather useful for the admin to create whiteout files him/her-
self. For example one should blacklist network re-initializations,
/etc/mtab, /etc/nologin of the server and several cron-scripts. This
can be easily achieved by creating whiteout files for these scripts in
the group meta directory.
1) Another issue is that presently there is no support for read-only branches
when copy-on-write is disabled, thus, -ocow is NOT specified! Support for
that might be added in later releases.
unionfs-fuse Original implemention by Radek Podgorny
Radek Podgorny <firstname.lastname@example.org>, Bernd Schubert
Many thanks to the author of the FUSE filesystem Miklos Szeredi.