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.\"     @(#)mount_null.8	8.6 (Berkeley) 5/1/95
.\"
.\"
.Dd January 18, 2000
.Dt MOUNT_OVERLAY 8
.Os
.Sh NAME
.Nm mount_overlay
.Nd mount an overlay filesystem;
demonstrate the use of an overlay file system layer
.Sh SYNOPSIS
.Nm
.Op Fl o Ar options
.Ar /overlay
.Ar mount-point
.Sh DESCRIPTION
The
.Nm
command creates an
overlay layer, interposing the overlay filesystem between the over-mounted
file store and future pathname lookups.
.Pp
A different device number for the virtual copy is returned by
.Xr stat 2 ,
but in other respects it is indistinguishable from the original.
.Pp
The
.Nm
filesystem differs from the null filesystem in that the
.Nm
filesystem does not replicate the sub-tree, it places itself between
the sub-tree and all future access.
.Pp
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 framework,
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.
.Pp
The internal operation of the overlay layer is identical to that of the
null layer.
See its documentation for details.
.Sh SEE ALSO
.Xr mount 8 ,
.Xr mount_null 8
.Pp
UCLA Technical Report CSD-910056,
.Em "Stackable Layers: an Architecture for File System Development" .
.Sh HISTORY
The
.Nm
utility first appeared in
.Nx 1.5 .