Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
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+Unionfs is a stackable unification file system, which can appear to merge
+the contents of several directories (branches), while keeping their physical
+content separate. Unionfs is useful for unified source tree management,
+merged contents of split CD-ROM, merged separate software package
+directories, data grids, and more. Unionfs allows any mix of read-only and
+read-write branches, as well as insertion and deletion of branches anywhere
+in the fan-out. To maintain Unix semantics, Unionfs handles elimination of
+duplicates, partial-error conditions, and more.
+
+# mount -t unionfs -o branch-option[,union-options[,...]] none MOUNTPOINT
+
+The available branch-option for the mount command is:
+
+ dirs=branch[=ro|=rw][:...]
+
+specifies a separated list of which directories compose the union.
+Directories that come earlier in the list have a higher precedence than
+those which come later. Additionally, read-only or read-write permissions of
+the branch can be specified by appending =ro or =rw (default) to each
+directory.
+
+Syntax:
+
+ dirs=/branch1[=ro|=rw]:/branch2[=ro|=rw]:...:/branchN[=ro|=rw]
+
+Example:
+
+ dirs=/writable_branch=rw:/read-only_branch=ro
+
+
+DYNAMIC BRANCH MANAGEMENT AND REMOUNTS
+======================================
+
+You can remount a union and change its overall mode, or reconfigure the
+branches, as follows.
+
+To downgrade a union from read-write to read-only:
+
+# mount -t unionfs -o remount,ro none MOUNTPOINT
+
+To upgrade a union from read-only to read-write:
+
+# mount -t unionfs -o remount,rw none MOUNTPOINT
+
+To delete a branch /foo, regardless where it is in the current union:
+
+# mount -t unionfs -o remount,del=/foo none MOUNTPOINT
+
+To insert (add) a branch /foo before /bar:
+
+# mount -t unionfs -o remount,add=/bar:/foo none MOUNTPOINT
+
+To insert (add) a branch /foo (with the "rw" mode flag) before /bar:
+
+# mount -t unionfs -o remount,add=/bar:/foo=rw none MOUNTPOINT
+
+To insert (add) a branch /foo (in "rw" mode) at the very beginning (i.e., a
+new highest-priority branch), you can use the above syntax, or use a short
+hand version as follows:
+
+# mount -t unionfs -o remount,add=/foo none MOUNTPOINT
+
+To append a branch to the very end (new lowest-priority branch):
+
+# mount -t unionfs -o remount,add=:/foo none MOUNTPOINT
+
+To append a branch to the very end (new lowest-priority branch), in
+read-only mode:
+
+# mount -t unionfs -o remount,add=:/foo=ro none MOUNTPOINT
+
+Finally, to change the mode of one existing branch, say /foo, from read-only
+to read-write, and change /bar from read-write to read-only:
+
+# mount -t unionfs -o remount,mode=/foo=rw,mode=/bar=ro none MOUNTPOINT
+
+Note: in Unionfs 2.x, you cannot set the leftmost branch to readonly because
+then Unionfs won't have any writable place for copyups to take place.
+Moreover, the VFS can get confused when it tries to modify something in a
+file system mounted read-write, but isn't permitted to write to it.
+Instead, you should set the whole union as readonly, as described above.
+If, however, you must set the leftmost branch as readonly, perhaps so you
+can get a snapshot of it at a point in time, then you should insert a new
+writable top-level branch, and mark the one you want as readonly. This can
+be accomplished as follows, assuming that /foo is your current leftmost
+branch:
+
+# mount -t tmpfs -o size=NNN /new
+# mount -t unionfs -o remount,add=/new,mode=/foo=ro none MOUNTPOINT
+<do what you want safely in /foo>
+# mount -t unionfs -o remount,del=/new,mode=/foo=rw none MOUNTPOINT
+<check if there's anything in /new you want to preserve>
+# umount /new
+
+CACHE CONSISTENCY
+=================
+
+If you modify any file on any of the lower branches directly, while there is
+a Unionfs 2.1 mounted above any of those branches, you should tell Unionfs
+to purge its caches and re-get the objects. To do that, you have to
+increment the generation number of the superblock using the following
+command:
+
+# mount -t unionfs -o remount,incgen none MOUNTPOINT
+
+Note that the older way of incrementing the generation number using an
+ioctl, is no longer supported in Unionfs 2.0 and newer. Ioctls in general
+are not encouraged. Plus, an ioctl is per-file concept, whereas the
+generation number is a per-file-system concept. Worse, such an ioctl
+requires an open file, which then has to be invalidated by the very nature
+of the generation number increase (read: the old generation increase ioctl
+was pretty racy).
+
+
+For more information, see <http://unionfs.filesystems.org/>.
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1.5.2.2
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