Manual Reference Pages - UNIONFS (8)
unionfs-fuse - A userspace unionfs implementation
unionfs-fuse [-o option1 -o option2 ... -o optionN ]
unionfs 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 options
Path to chroot into. By using this option unionfs
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-live-cd.sh
for an example.
In our unionfs root path we have a .unionfs directory that includes
metadata, such as hidden (deleted) files. This options make this
directory invisible from readdir(), so for example "ls -la /union_root/"
will not show it. However, this directory is still there and "cd .unionfs"
or "ls -l .unionfs" still work. Also, libfuse will create .fuse_hidden*
files, if a file is open, but will be deleted. Those fuse meta files also
will be invisble. This option is especially usufull for
Enable debugging for unionfs and libfuse. Useful for developers if the code
if the code does not behave as expected. Debug information will be written
to stderr and a debug file (./unionfs_debug.log by default).
Write unionfs debug information into that file.
Maximum number of open files. Most system have a default of 1024 open
files per process. For example if unionfs serves "/" applications like
KDE or GNOME might have much more open files, which will make the unionfs
process to exceed this limit. Suggested for "/" is >16000 or even >32000 files.
If this limit exceeds unionfs will not be able to open further files.
Since version 0.23 without any effect, just left over for compatibility.
Might be removed in future versions.
Usually we automatically add the libfuse option "-odefault_permissions"
so that libfuse takes over permission checks. However, if running not
as root (so as uid =! 0 and gid != 0), permissions of the underlying
filesystem are already sufficient. In order to prevent from severe
security issues, this option is not allowed if running as root.
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.
Options to libfuse
There are several further options available, which dont directly apply to
unionfs, but to libfuse. Please run "unionfs --help" to see these.
We already set the "-o default-permissions" options on our own.
unionfs -o cow,max_files=32768 \
-o allow_other,use_ino,suid,dev,nonempty \
Like other filesystems unionfs 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 cant 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 <email@example.com>
Radek Podgorny <radekpodgorny.cz>, Bernd Schubert <bernd-schubertgmx.de>
Many thanks to the author of the FUSE filesystem Miklos Szeredi.
|unionfs-fuse 1.0 ||UNIONFS (8) ||2015 |
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