1.\" $NetBSD: mount_overlay.8,v 1.5 2001/06/07 13:58:24 wiz 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. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the University of 20.\" California, Berkeley and its contributors. 21.\" 4. Neither the name of the University nor the names of its contributors 22.\" may be used to endorse or promote products derived from this software 23.\" without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 26.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 27.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 28.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 29.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 30.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 31.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 32.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 33.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 34.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 35.\" SUCH DAMAGE. 36.\" 37.\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95 38.\" 39.\" 40.Dd January 18, 2000 41.Dt MOUNT_OVERLAY 8 42.Os 43.Sh NAME 44.Nm mount_overlay 45.Nd mount an overlay filesystem; 46demonstrate the use of an overlay file system layer 47.Sh SYNOPSIS 48.Nm "" 49.Op Fl o Ar options 50.Ar /overlay 51.Ar mount-point 52.Sh DESCRIPTION 53The 54.Nm 55command creates an 56overlay layer, interposing the overlay filesystem between the over-mounted 57file store and future pathname lookups. 58.Pp 59A different device number for the virtual copy is returned by 60.Xr stat 2 , 61but in other respects it is indistinguishable from the original. 62.Pp 63The 64.Nm 65filesystem differs from the null filesystem in that the 66.Nm 67filesystem does not replicate the sub-tree, it places itself between 68the sub-tree and all future access. 69.Pp 70The overlay layer has two purposes. 71First, it serves as a demonstration of layering by providing a layer 72which does nothing other than insert itself over the over-mounted 73file system. 74Second, the overlay layer can serve as a prototype layer. 75Since it provides all necessary layer framework, 76new file system layers which need to block access to the overlayed 77file system can be created very easily by starting 78with an overlay layer. 79.Pp 80The internal operation of the overlay layer is identical to that of the 81null layer. See its documentation for details. 82.Sh SEE ALSO 83.Xr mount 8 , 84.Xr mount_null 8 85.sp 86UCLA Technical Report CSD-910056, 87.Em "Stackable Layers: an Architecture for File System Development" . 88.Sh HISTORY 89The 90.Nm 91utility first appeared in 92.Nx 1.5 . 93