236 lines
7.9 KiB
Groff
236 lines
7.9 KiB
Groff
.\"
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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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.\"
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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. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. 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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.\" $Id: mount_null.8,v 1.8 1997/03/11 12:33:34 peter Exp $
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.\"
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.Dd May 1, 1995
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.Dt MOUNT_NULL 8
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.Os BSD 4.4
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.Sh NAME
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.Nm mount_null
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.Nd mount a loopback filesystem sub-tree;
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demonstrate the use of a null file system layer
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.Sh SYNOPSIS
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.Nm mount_null
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.Op Fl o Ar options
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.Ar target
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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 a
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null layer, duplicating a sub-tree of the file system
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name space under another part of the global file system namespace.
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This allows existing files and directories to be accessed
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using a different pathname.
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.Pp
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The primary differences between a virtual copy of the filesystem
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and a symbolic link are that
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.Xr getcwd 3
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functions correctly in the virtual copy, and that other filesystems
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may be mounted on the virtual copy without affecting the original.
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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 a traditional
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loopback file system in two respects: it is implemented using
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a stackable layers techniques, and it's
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.Do
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null-node
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.Dc s
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stack above
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all lower-layer vnodes, not just over directory vnodes.
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.Pp
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The options are as follows:
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.Bl -tag -width indent
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.It Fl o
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Options are specified with a
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.Fl o
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flag followed by a comma separated string of options.
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See the
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.Xr mount 8
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man page for possible options and their meanings.
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.El
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.Pp
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The null 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.
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(It actually does everything the loopback file system does,
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which is slightly more than nothing.)
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Second, the null 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 can be created very easily be starting
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with a null layer.
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.Pp
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The remainder of this man page examines the null layer as a basis
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for constructing new layers.
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.\"
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.\"
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.Sh INSTANTIATING NEW NULL LAYERS
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New null layers are created with
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.Xr mount_null 8 .
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.Xr Mount_null 8
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takes two arguments, the pathname
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of the lower vfs (target-pn) and the pathname where the null
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layer will appear in the namespace (mount-point-pn). After
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the null layer is put into place, the contents
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of target-pn subtree will be aliased under mount-point-pn.
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.\"
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.\"
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.Sh OPERATION OF A NULL LAYER
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The null layer is the minimum file system layer,
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simply bypassing all possible operations to the lower layer
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for processing there. The majority of its activity centers
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on the bypass routine, though which nearly all vnode operations
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pass.
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.Pp
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The bypass routine accepts arbitrary vnode operations for
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handling by the lower layer. It begins by examining vnode
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operation arguments and replacing any null-nodes by their
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lower-layer equivalents. It then invokes the operation
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on the lower layer. Finally, it replaces the null-nodes
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in the arguments and, if a vnode is returned by the operation,
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stacks a null-node on top of the returned vnode.
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.Pp
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Although bypass handles most operations,
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.Em vop_getattr ,
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.Em vop_inactive ,
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.Em vop_reclaim ,
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and
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.Em vop_print
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are not bypassed.
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.Em Vop_getattr
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must change the fsid being returned.
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.Em Vop_inactive
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and vop_reclaim are not bypassed so that
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they can handle freeing null-layer specific data.
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.Em Vop_print
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is not bypassed to avoid excessive debugging
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information.
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.\"
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.\"
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.Sh INSTANTIATING VNODE STACKS
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Mounting associates the null layer with a lower layer,
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in effect stacking two VFSes. Vnode stacks are instead
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created on demand as files are accessed.
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.Pp
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The initial mount creates a single vnode stack for the
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root of the new null layer. All other vnode stacks
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are created as a result of vnode operations on
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this or other null vnode stacks.
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.Pp
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New vnode stacks come into existence as a result of
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an operation which returns a vnode.
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The bypass routine stacks a null-node above the new
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vnode before returning it to the caller.
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.Pp
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For example, imagine mounting a null layer with
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.Bd -literal -offset indent
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mount_null /usr/include /dev/layer/null
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.Ed
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Changing directory to
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.Pa /dev/layer/null
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will assign
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the root null-node (which was created when the null layer was mounted).
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Now consider opening
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.Pa sys .
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A vop_lookup would be
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done on the root null-node. This operation would bypass through
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to the lower layer which would return a vnode representing
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the UFS
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.Pa sys .
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Null_bypass then builds a null-node
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aliasing the UFS
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.Pa sys
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and returns this to the caller.
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Later operations on the null-node
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.Pa sys
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will repeat this
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process when constructing other vnode stacks.
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.\"
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.\"
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.Sh CREATING OTHER FILE SYSTEM LAYERS
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One of the easiest ways to construct new file system layers is to make
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a copy of the null layer, rename all files and variables, and
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then begin modifying the copy.
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.Xr Sed 1
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can be used to easily rename
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all variables.
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.Pp
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The umap layer is an example of a layer descended from the
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null layer.
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.\"
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.\"
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.Sh INVOKING OPERATIONS ON LOWER LAYERS
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There are two techniques to invoke operations on a lower layer
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when the operation cannot be completely bypassed. Each method
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is appropriate in different situations. In both cases,
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it is the responsibility of the aliasing layer to make
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the operation arguments "correct" for the lower layer
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by mapping an vnode arguments to the lower layer.
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.Pp
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The first approach is to call the aliasing layer's bypass routine.
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This method is most suitable when you wish to invoke the operation
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currently being handled on the lower layer. It has the advantage
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the bypass routine already must do argument mapping.
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An example of this is
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.Em null_getattrs
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in the null layer.
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.Pp
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A second approach is to directly invoked vnode operations on
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the lower layer with the
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.Em VOP_OPERATIONNAME
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interface.
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The advantage of this method is that it is easy to invoke
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arbitrary operations on the lower layer. The disadvantage
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is that vnodes arguments must be manually mapped.
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.\"
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.\"
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.Sh SEE ALSO
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.Xr mount 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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.Bx 4.4 .
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