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31 .\" @(#)mount_union.8 8.6 (Berkeley) 3/27/94
32 .\" $FreeBSD: src/sbin/mount_union/mount_union.8,v 1.6.2.2 2001/12/20 16:46:05 ru Exp $
39 .Nd mount union filesystems
54 in such a way that the contents of both directory trees remain visible.
65 The options are as follows:
66 .Bl -tag -width indent
68 Invert the default position, so that
70 becomes the lower layer and
72 becomes the upper layer.
75 remains the mount point.
77 Options are specified with a
79 flag followed by a comma separated string of options.
82 man page for possible options and their meanings.
84 Hide the lower layer completely in the same way as mounting with
88 To enforce filesystem security, the user mounting the filesystem
89 must be superuser or else have write permission on the mounted-on
94 variable must be set to 1 to permit file system mounting by ordinary users.
96 Filenames are looked up in the upper layer and then in the
98 If a directory is found in the lower layer, and there is no entry
99 in the upper layer, then a
101 directory will be created in the upper layer.
102 It will be owned by the user who originally did the union mount,
105 (0777) modified by the umask in effect at that time.
107 If a file exists in the upper layer then there is no way to access
108 a file with the same name in the lower layer.
109 If necessary, a combination of loopback and union mounts can be made
110 which will still allow the lower files to be accessed by a different
113 Except in the case of a directory,
114 access to an object is granted via the normal filesystem access checks.
115 For directories, the current user must have access to both the upper
116 and lower directories (should they both exist).
118 Requests to create or modify objects in
120 are passed to the upper layer with the exception of a few special cases.
121 An attempt to open for writing a file which exists in the lower layer
124 file to be made to the upper layer, and then for the upper layer copy
126 Similarly, an attempt to truncate a lower layer file to zero length
127 causes an empty file to be created in the upper layer.
128 Any other operation which would ultimately require modification to
129 the lower layer fails with
132 The union filesystem manipulates the namespace, rather than
133 individual filesystems.
134 The union operation applies recursively down the directory tree
137 Thus any filesystems which are mounted under
139 will take part in the union operation.
140 This differs from the
144 which only applies the union operation to the mount point itself,
145 and then only for lookups.
148 .Bd -literal -offset indent
149 mount -t cd9660 -o ro /dev/cd0a /usr/src
150 mount -t union /var/obj /usr/src
153 mount the CD-ROM drive
160 For most purposes the effect of this is to make the
161 source tree appear writable
162 even though it is stored on a CD-ROM.
165 .Bd -literal -offset indent
166 mount -t union -o -b /sys $HOME/sys
169 attaches the system source tree below the
171 directory in the user's home directory.
172 This allows individual users to make private changes
173 to the source, and build new kernels, without those
174 changes becoming visible to other users.
175 Note that the files in the lower layer remain
188 command first appeared in
191 .Fx Ns -(fill this in) .
193 THIS FILESYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
194 AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR
195 OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET.
197 This code also needs an owner in order to be less dangerous - serious
198 hackers can apply by sending mail to
199 .Aq Mt kernel@crater.dragonflybsd.org
201 their intent to take it over.
203 Without whiteout support from the filesystem backing the upper layer,
204 there is no way that delete and rename operations on lower layer
207 is returned for this kind of operations along with any others
208 which would make modifications to the lower layer, such as
213 over a union tree has the side-effect of creating
214 a tree of shadow directories in the upper layer.