1.\" $NetBSD: mount_overlay.8,v 1.9 2004/06/10 14:13:36 uebayasi Exp $ 2.\" 3.\" Copyright (c) 1992, 1993, 1994 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" This code is derived from software donated to Berkeley by 7.\" John Heidemann of the UCLA Ficus project. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. Neither the name of the University nor the names of its contributors 18.\" may be used to endorse or promote products derived from this software 19.\" without specific prior written permission. 20.\" 21.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 22.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 23.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 24.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 25.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 26.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 27.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 28.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 29.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 30.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31.\" SUCH DAMAGE. 32.\" 33.\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95 34.\" 35.\" 36.Dd January 18, 2000 37.Dt MOUNT_OVERLAY 8 38.Os 39.Sh NAME 40.Nm mount_overlay 41.Nd mount an overlay filesystem; 42demonstrate the use of an overlay file system layer 43.Sh SYNOPSIS 44.Nm 45.Op Fl o Ar options 46.Ar /overlay 47.Ar mount-point 48.Sh DESCRIPTION 49The 50.Nm 51command creates an 52overlay layer, interposing the overlay filesystem between the over-mounted 53file store and future pathname lookups. 54.Pp 55A different device number for the virtual copy is returned by 56.Xr stat 2 , 57but in other respects it is indistinguishable from the original. 58.Pp 59The 60.Nm 61filesystem differs from the null filesystem in that the 62.Nm 63filesystem does not replicate the sub-tree, it places itself between 64the sub-tree and all future access. 65.Pp 66The overlay layer has two purposes. 67First, it serves as a demonstration of layering by providing a layer 68which does nothing other than insert itself over the over-mounted 69file system. 70Second, the overlay layer can serve as a prototype layer. 71Since it provides all necessary layer framework, 72new file system layers which need to block access to the overlayed 73file system can be created very easily by starting 74with an overlay layer. 75.Pp 76The internal operation of the overlay layer is identical to that of the 77null layer. 78See its documentation for details. 79.Sh SEE ALSO 80.Xr mount 8 , 81.Xr mount_null 8 82.Pp 83UCLA Technical Report CSD-910056, 84.Em "Stackable Layers: an Architecture for File System Development" . 85.Sh HISTORY 86The 87.Nm 88utility first appeared in 89.Nx 1.5 . 90