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
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.
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.