90 lines
3.3 KiB
Groff
90 lines
3.3 KiB
Groff
.\" $NetBSD: mount_overlay.8,v 1.8 2003/08/07 10:04:30 agc Exp $
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.\"
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.\" Copyright (c) 1992, 1993, 1994
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software donated to Berkeley by
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.\" John Heidemann of the UCLA Ficus project.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95
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.\"
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.\"
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.Dd January 18, 2000
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.Dt MOUNT_OVERLAY 8
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.Os
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.Sh NAME
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.Nm mount_overlay
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.Nd mount an overlay filesystem;
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demonstrate the use of an overlay file system layer
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.Sh SYNOPSIS
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.Nm
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.Op Fl o Ar options
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.Ar /overlay
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.Ar mount-point
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.Sh DESCRIPTION
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The
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.Nm
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command creates an
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overlay layer, interposing the overlay filesystem between the over-mounted
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file store and future pathname lookups.
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.Pp
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A different device number for the virtual copy is returned by
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.Xr stat 2 ,
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but in other respects it is indistinguishable from the original.
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.Pp
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The
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.Nm
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filesystem differs from the null filesystem in that the
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.Nm
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filesystem does not replicate the sub-tree, it places itself between
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the sub-tree and all future access.
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.Pp
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The overlay layer has two purposes.
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First, it serves as a demonstration of layering by providing a layer
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which does nothing other than insert itself over the over-mounted
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file system.
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Second, the overlay layer can serve as a prototype layer.
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Since it provides all necessary layer framework,
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new file system layers which need to block access to the overlayed
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file system can be created very easily by starting
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with an overlay layer.
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.Pp
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The internal operation of the overlay layer is identical to that of the
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null layer.
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See its documentation for details.
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.Sh SEE ALSO
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.Xr mount 8 ,
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.Xr mount_null 8
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.sp
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UCLA Technical Report CSD-910056,
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.Em "Stackable Layers: an Architecture for File System Development" .
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.Sh HISTORY
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The
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.Nm
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utility first appeared in
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.Nx 1.5 .
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