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37 .\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95
38 .\" $FreeBSD: src/sbin/mount_null/mount_null.8,v 1.11.2.6 2001/12/20 16:40:00 ru Exp $
39 .\" $DragonFly: src/sbin/mount_null/mount_null.8,v 1.2 2003/06/17 04:27:33 dillon Exp $
46 .Nd "mount a loopback filesystem sub-tree; demonstrate the use of a null file system layer"
56 null layer, duplicating a sub-tree of the file system
57 name space under another part of the global file system namespace.
58 This allows existing files and directories to be accessed
59 using a different pathname.
61 The primary differences between a virtual copy of the filesystem
62 and a symbolic link are that the
64 functions work correctly in the virtual copy, and that other filesystems
65 may be mounted on the virtual copy without affecting the original.
66 A different device number for the virtual copy is returned by
68 but in other respects it is indistinguishable from the original.
72 filesystem differs from a traditional
73 loopback file system in two respects: it is implemented using
74 a stackable layers techniques, and it's
77 all lower-layer vnodes, not just over directory vnodes.
79 The options are as follows:
80 .Bl -tag -width indent
82 Options are specified with a
84 flag followed by a comma separated string of options.
87 man page for possible options and their meanings.
90 The null layer has two purposes.
91 First, it serves as a demonstration of layering by providing a layer
93 (It actually does everything the loopback file system does,
94 which is slightly more than nothing.)
95 Second, the null layer can serve as a prototype layer.
96 Since it provides all necessary layer framework,
97 new file system layers can be created very easily by starting
100 The remainder of this man page examines the null layer as a basis
101 for constructing new layers.
104 .Sh INSTANTIATING NEW NULL LAYERS
105 New null layers are created with
108 takes two arguments, the pathname
109 of the lower vfs (target-pn) and the pathname where the null
110 layer will appear in the namespace (mount-point-pn). After
111 the null layer is put into place, the contents
112 of target-pn subtree will be aliased under mount-point-pn.
115 .Sh OPERATION OF A NULL LAYER
116 The null layer is the minimum file system layer,
117 simply bypassing all possible operations to the lower layer
118 for processing there. The majority of its activity centers
119 on the bypass routine, through which nearly all vnode operations
122 The bypass routine accepts arbitrary vnode operations for
123 handling by the lower layer. It begins by examining vnode
124 operation arguments and replacing any null-nodes by their
125 lower-layer equivalents. It then invokes the operation
126 on the lower layer. Finally, it replaces the null-nodes
127 in the arguments and, if a vnode is returned by the operation,
128 stacks a null-node on top of the returned vnode.
130 Although bypass handles most operations,
138 must change the fsid being returned.
142 are not bypassed so that
143 they can handle freeing null-layer specific data.
145 is not bypassed to avoid excessive debugging
149 .Sh INSTANTIATING VNODE STACKS
150 Mounting associates the null layer with a lower layer,
151 in effect stacking two VFSes. Vnode stacks are instead
152 created on demand as files are accessed.
154 The initial mount creates a single vnode stack for the
155 root of the new null layer. All other vnode stacks
156 are created as a result of vnode operations on
157 this or other null vnode stacks.
159 New vnode stacks come into existence as a result of
160 an operation which returns a vnode.
161 The bypass routine stacks a null-node above the new
162 vnode before returning it to the caller.
164 For example, imagine mounting a null layer with
165 .Bd -literal -offset indent
166 mount_null /usr/include /dev/layer/null
168 Changing directory to
171 the root null-node (which was created when the null layer was mounted).
174 A vop_lookup would be
175 done on the root null-node. This operation would bypass through
176 to the lower layer which would return a vnode representing
179 Null_bypass then builds a null-node
182 and returns this to the caller.
183 Later operations on the null-node
186 process when constructing other vnode stacks.
189 .Sh CREATING OTHER FILE SYSTEM LAYERS
190 One of the easiest ways to construct new file system layers is to make
191 a copy of the null layer, rename all files and variables, and
192 then begin modifying the copy.
194 can be used to easily rename
197 The umap layer is an example of a layer descended from the
201 .Sh INVOKING OPERATIONS ON LOWER LAYERS
202 There are two techniques to invoke operations on a lower layer
203 when the operation cannot be completely bypassed. Each method
204 is appropriate in different situations. In both cases,
205 it is the responsibility of the aliasing layer to make
206 the operation arguments "correct" for the lower layer
207 by mapping a vnode argument to the lower layer.
209 The first approach is to call the aliasing layer's bypass routine.
210 This method is most suitable when you wish to invoke the operation
211 currently being handled on the lower layer.
212 It has the advantage that
213 the bypass routine already must do argument mapping.
214 An example of this is
218 A second approach is to directly invoke vnode operations on
219 the lower layer with the
220 .Em VOP_OPERATIONNAME
222 The advantage of this method is that it is easy to invoke
223 arbitrary operations on the lower layer. The disadvantage
224 is that vnode arguments must be manually mapped.
230 UCLA Technical Report CSD-910056,
231 .Em "Stackable Layers: an Architecture for File System Development" .
233 THIS FILESYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
234 AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR
235 OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET.
237 This code also needs an owner in order to be less dangerous - serious
238 hackers can apply by sending mail to
239 .Aq hackers@FreeBSD.org
241 their intent to take it over.
245 utility first appeared in