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MOUNT_OVERLAY(8)	  BSD System Manager's Manual	      MOUNT_OVERLAY(8)

NAME
     mount_overlay — mount an overlay filesystem; demonstrate the use of an
     overlay file system layer

SYNOPSIS
     mount_overlay [-o options] /overlay mount-point

DESCRIPTION
     The mount_overlay command creates an overlay layer, interposing the over‐
     lay filesystem between the over-mounted file store and future pathname
     lookups.

     A different device number for the virtual copy is returned by stat(2),
     but in other respects it is indistinguishable from the original.

     The mount_overlay filesystem differs from the null filesystem in that the
     mount_overlay filesystem does not replicate the sub-tree, it places
     itself between the sub-tree and all future access.

     The overlay layer has two purposes.  First, it serves as a demonstration
     of layering by providing a layer which does nothing other than insert
     itself over the over-mounted file system.	Second, the overlay layer can
     serve as a prototype layer.  Since it provides all necessary layer frame‐
     work, new file system layers which need to block access to the overlayed
     file system can be created very easily by starting with an overlay layer.

     The internal operation of the overlay layer is identical to that of the
     null layer.  See its documentation for details.

SEE ALSO
     mount(8), mount_null(8)

     UCLA Technical Report CSD-910056, Stackable Layers: an Architecture for
     File System Development.

HISTORY
     The mount_overlay utility first appeared in NetBSD 1.5.

BSD			       January 18, 2000				   BSD

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