Its a way of mounting a filesystem readonly under another one, so
	files in the lower level will 'show thru' into the upper layer
	(man page enclosed)

		David/abs

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Network Analyst, UCS, City University   System Manager, Southern Studios Ltd
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MOUNT_UNION(8)          NetBSD System Manager's Manual          MOUNT_UNION(8)

NAME
     mount_union - mount union filesystems

SYNOPSIS
     mount_union [-br] [-o options] directory uniondir

DESCRIPTION
     The mount_union command attaches directory above uniondir in such a way
     that the contents of both directory trees remain visible.  By default,
     directory becomes the upper layer and uniondir becomes the lower layer.

     The options are as follows:

     -b      Invert the default position, so that directory becomes the lower
             layer and uniondir becomes the upper layer.  However, uniondir
             remains the mount point.

     -o      Options are specified with a -o flag followed by a comma separat-
             ed string of options.  See the mount(8) man page for possible op-
             tions and their meanings.

     -r      Hide the lower layer completely in the same way as mounting with
             mount_lofs(8) or mount_null(8).

     To enforce filesystem security, the user mounting the filesystem must be
     superuser or else have write permission on the mounted-on directory.

     Filenames are looked up in the upper layer and then in the lower layer.
     If a directory is found in the lower layer, and there is no entry in the
     upper layer, then a shadow directory will be created in the upper layer.
     It will be owned by the user who originally did the union mount, with
     mode ``rwxrwxrwx'' (0777) modified by the umask in effect at that time.

     If a file exists in the upper layer then there is no way to access a file
     with the same name in the lower layer.  If necessary, a combination of
     loopback and union mounts can be made which will still allow the lower
     files to be accessed by a different pathname.

     Except in the case of a directory, access to an object is granted via the
     normal filesystem access checks.  For directories, the current user must
     have access to both the upper and lower directories (should they both ex-
     ist).

     Requests to create or modify objects in uniondir are passed to the upper
     layer with the exception of a few special cases.  An attempt to open for
     writing a file which exists in the lower layer causes a copy of the
     entire file to be made to the upper layer, and then for the upper layer
     copy to be opened.  Similarly, an attempt to truncate a lower layer file
     to zero length causes an empty file to be created in the upper layer.
     Any other operation which would ultimately require modification to the
     lower layer fails with EROFS.

     The union filesystem manipulates the namespace, rather than individual
     filesystems.  The union operation applies recursively down the directory
     tree now rooted at uniondir. Thus any filesystems which are mounted under
     uniondir will take part in the union operation.  This differs from the
     union option to mount(8) which only applies the union operation to the
     mount point itself, and then only for lookups.

EXAMPLES
     The commands

           mount -t cd9660 -o ro /dev/cd0a /usr/src
           mount -t union /var/obj /usr/src

     mount the CD-ROM drive /dev/cd0a on /usr/src and then attaches /var/obj
     on top.  For most purposes the effect of this is to make the source tree
     appear writable even though it is stored on a CD-ROM.

     The command

           mount -t union -o -b /sys $HOME/sys

     attaches the system source tree below the sys directory in the user's
     home directory.  This allows individual users to make private changes to
     the source, and build new kernels, without those changes becoming visible
     to other users.  Note that the files in the lower layer remain accessible
     via /sys.

SEE ALSO
     intro(2),  mount(2),  unmount(2),  fstab(5),  mount(8),  mount_lofs(8),
     mount_null(8)

BUGS
     Without whiteout support from the filesystem backing the upper layer,
     there is no way that delete and rename operations on lower layer objects
     can be done.  EROFS is returned for this kind of operations along with
     any others which would make modifications to the lower layer, such as
     chmod(1).

     Running find(1) over a union tree has the side-effect of creating a tree
     of shadow directories in the upper layer.

HISTORY
     The mount_union command first appeared in 4.4BSD.

BSD Experimental              September 30, 1995                             2