source: src-sh/pbi-manager/mount_nullfs/mount_nullfs.8 @ 7ff5d68

releng/10.0releng/10.0.1releng/10.0.2releng/10.0.3
Last change on this file since 7ff5d68 was 7ff5d68, checked in by Kris Moore <kris@…>, 10 months ago

Rename pbi-manager10 -> pbi-manager for "master" branch

  • Property mode set to 100644
File size: 7.7 KB
Line 
1.\"
2.\" Copyright (c) 1992, 1993, 1994
3.\"     The Regents of the University of California.  All rights reserved.
4.\"
5.\" This code is derived from software donated to Berkeley by
6.\" John Heidemann of the UCLA Ficus project.
7.\"
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.\" 4. 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.\" $FreeBSD: release/9.1.0/sbin/mount_nullfs/mount_nullfs.8 157877 2006-04-19 19:21:42Z brueffer $
35.\"
36.Dd May 1, 1995
37.Dt MOUNT_NULLFS 8
38.Os
39.Sh NAME
40.Nm mount_nullfs
41.Nd "mount a loopback file system sub-tree; demonstrate the use of a null file system layer"
42.Sh SYNOPSIS
43.Nm
44.Op Fl o Ar options
45.Ar target
46.Ar mount-point
47.Sh DESCRIPTION
48The
49.Nm
50utility creates a
51null layer, duplicating a sub-tree of the file system
52name space under another part of the global file system namespace.
53This allows existing files and directories to be accessed
54using a different pathname.
55.Pp
56The primary differences between a virtual copy of the file system
57and a symbolic link are that the
58.Xr getcwd 3
59functions work correctly in the virtual copy, and that other file systems
60may be mounted on the virtual copy without affecting the original.
61A different device number for the virtual copy is returned by
62.Xr stat 2 ,
63but in other respects it is indistinguishable from the original.
64.Pp
65The
66.Nm
67file system differs from a traditional
68loopback file system in two respects: it is implemented using
69a stackable layers techniques, and its
70.Do null-node Dc Ns s
71stack above
72all lower-layer vnodes, not just over directory vnodes.
73.Pp
74The options are as follows:
75.Bl -tag -width indent
76.It Fl o
77Options are specified with a
78.Fl o
79flag followed by a comma separated string of options.
80See the
81.Xr mount 8
82man page for possible options and their meanings.
83.El
84.Pp
85The null layer has two purposes.
86First, it serves as a demonstration of layering by providing a layer
87which does nothing.
88(It actually does everything the loopback file system does,
89which is slightly more than nothing.)
90Second, the null layer can serve as a prototype layer.
91Since it provides all necessary layer framework,
92new file system layers can be created very easily by starting
93with a null layer.
94.Pp
95The remainder of this man page examines the null layer as a basis
96for constructing new layers.
97.\"
98.\"
99.Sh INSTANTIATING NEW NULL LAYERS
100New null layers are created with
101.Nm .
102The
103.Nm
104utility takes two arguments, the pathname
105of the lower vfs (target-pn) and the pathname where the null
106layer will appear in the namespace (mount-point-pn).
107After
108the null layer is put into place, the contents
109of target-pn subtree will be aliased under mount-point-pn.
110.\"
111.\"
112.Sh OPERATION OF A NULL LAYER
113The null layer is the minimum file system layer,
114simply bypassing all possible operations to the lower layer
115for processing there.
116The majority of its activity centers
117on the bypass routine, through which nearly all vnode operations
118pass.
119.Pp
120The bypass routine accepts arbitrary vnode operations for
121handling by the lower layer.
122It begins by examining vnode
123operation arguments and replacing any null-nodes by their
124lower-layer equivalents.
125It then invokes the operation
126on the lower layer.
127Finally, it replaces the null-nodes
128in the arguments and, if a vnode is returned by the operation,
129stacks a null-node on top of the returned vnode.
130.Pp
131Although bypass handles most operations,
132.Em vop_getattr ,
133.Em vop_inactive ,
134.Em vop_reclaim ,
135and
136.Em vop_print
137are not bypassed.
138.Em Vop_getattr
139must change the fsid being returned.
140.Em Vop_inactive
141and
142.Em vop_reclaim
143are not bypassed so that
144they can handle freeing null-layer specific data.
145.Em Vop_print
146is not bypassed to avoid excessive debugging
147information.
148.\"
149.\"
150.Sh INSTANTIATING VNODE STACKS
151Mounting associates the null layer with a lower layer,
152in effect stacking two VFSes.
153Vnode stacks are instead
154created on demand as files are accessed.
155.Pp
156The initial mount creates a single vnode stack for the
157root of the new null layer.
158All other vnode stacks
159are created as a result of vnode operations on
160this or other null vnode stacks.
161.Pp
162New vnode stacks come into existence as a result of
163an operation which returns a vnode.
164The bypass routine stacks a null-node above the new
165vnode before returning it to the caller.
166.Pp
167For example, imagine mounting a null layer with
168.Bd -literal -offset indent
169mount_nullfs /usr/include /dev/layer/null
170.Ed
171.Pp
172Changing directory to
173.Pa /dev/layer/null
174will assign
175the root null-node (which was created when the null layer was mounted).
176Now consider opening
177.Pa sys .
178A vop_lookup would be
179done on the root null-node.
180This operation would bypass through
181to the lower layer which would return a vnode representing
182the UFS
183.Pa sys .
184Null_bypass then builds a null-node
185aliasing the UFS
186.Pa sys
187and returns this to the caller.
188Later operations on the null-node
189.Pa sys
190will repeat this
191process when constructing other vnode stacks.
192.\"
193.\"
194.Sh CREATING OTHER FILE SYSTEM LAYERS
195One of the easiest ways to construct new file system layers is to make
196a copy of the null layer, rename all files and variables, and
197then begin modifying the copy.
198The
199.Xr sed 1
200utility can be used to easily rename
201all variables.
202.Pp
203The umap layer is an example of a layer descended from the
204null layer.
205.\"
206.\"
207.Sh INVOKING OPERATIONS ON LOWER LAYERS
208There are two techniques to invoke operations on a lower layer
209when the operation cannot be completely bypassed.
210Each method
211is appropriate in different situations.
212In both cases,
213it is the responsibility of the aliasing layer to make
214the operation arguments "correct" for the lower layer
215by mapping a vnode argument to the lower layer.
216.Pp
217The first approach is to call the aliasing layer's bypass routine.
218This method is most suitable when you wish to invoke the operation
219currently being handled on the lower layer.
220It has the advantage that
221the bypass routine already must do argument mapping.
222An example of this is
223.Em null_getattrs
224in the null layer.
225.Pp
226A second approach is to directly invoke vnode operations on
227the lower layer with the
228.Em VOP_OPERATIONNAME
229interface.
230The advantage of this method is that it is easy to invoke
231arbitrary operations on the lower layer.
232The disadvantage
233is that vnode arguments must be manually mapped.
234.\"
235.\"
236.Sh SEE ALSO
237.Xr mount 8
238.Pp
239UCLA Technical Report CSD-910056,
240.Em "Stackable Layers: an Architecture for File System Development" .
241.Sh HISTORY
242The
243.Nm
244utility first appeared in
245.Bx 4.4 .
Note: See TracBrowser for help on using the repository browser.