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Union Filesystem (`type:=union')
================================

   The "union" filesystem type allows the contents of several
directories to be merged and made visible in a single directory.  This
can be used to overcome one of the major limitations of the Unix mount
mechanism which only allows complete directories to be mounted.

   For example, supposing `/tmp' and `/var/tmp' were to be merged into
a new directory called `/mtmp', with files in `/var/tmp' taking
precedence.  The following command could be used to achieve this effect:

     amd ... /mtmp union:/tmp:/var/tmp -type:=union

   Currently, the unioned directories must *not* be automounted.  That
would cause a deadlock.  This seriously limits the current usefulness of
this filesystem type and the problem will be addressed in a future
release of Amd.

   Files created in the union directory are actually created in the last
named directory.  This is done by creating a wildcard entry which points
to the correct directory.  The wildcard entry is visible if the union
directory is listed, so allowing you to see which directory has
priority.

   The files visible in the union directory are computed at the time
Amd is started, and are not kept uptodate with respect to the
underlying directories.  Similarly, if a link is removed, for example
with the `rm' command, it will be lost forever.