Provided by: unionfs-fuse_0.24-2.1ubuntu1_i386 bug


       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

       -o stats
              Show statistics in the file 'stats' under the mountpoint.

       -o statfs_omit_ro
              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.

       -o noinitgroups
              Since version 0.23  without  any  effect,  just  left  over  for
              compatibility.  Might be removed in future versions.

       -o chroot=path
              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-
     for an example.

       -o max_files=number
              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 \
                     /u/host/etc=RW:/u/group/etc=RO:/u/common/etc=RO \

Meta data

       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      <>,      Bernd      Schubert


       Many thanks to the author of the FUSE filesystem Miklos Szeredi.