NetBSD/sbin/mount_overlay/mount_overlay.8

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.\" $NetBSD: mount_overlay.8,v 1.6 2002/10/01 13:40:42 wiz Exp $
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.\"
.\" Copyright (c) 1992, 1993, 1994
.\" The Regents of the University of California. All rights reserved.
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.\" This code is derived from software donated to Berkeley by
.\" John Heidemann of the UCLA Ficus project.
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.\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95
.\"
.\"
.Dd January 18, 2000
.Dt MOUNT_OVERLAY 8
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.Os
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.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.
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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
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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.
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.Sh SEE ALSO
.Xr mount 8 ,
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.Xr mount_null 8
.sp
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 .