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35 .\" @(#)mount_union.8 8.6 (Berkeley) 3/27/94
36 .\" $FreeBSD: src/sbin/mount_union/mount_union.8,v 1.6.2.2 2001/12/20 16:46:05 ru Exp $
37 .\" $DragonFly: src/sbin/mount_union/mount_union.8,v 1.2 2003/06/17 04:27:33 dillon Exp $
44 .Nd mount union filesystems
59 in such a way that the contents of both directory trees remain visible.
70 The options are as follows:
71 .Bl -tag -width indent
73 Invert the default position, so that
75 becomes the lower layer and
77 becomes the upper layer.
80 remains the mount point.
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.
89 Hide the lower layer completely in the same way as mounting with
93 To enforce filesystem security, the user mounting the filesystem
94 must be superuser or else have write permission on the mounted-on
97 Filenames are looked up in the upper layer and then in the
99 If a directory is found in the lower layer, and there is no entry
100 in the upper layer, then a
102 directory will be created in the upper layer.
103 It will be owned by the user who originally did the union mount,
106 (0777) modified by the umask in effect at that time.
108 If a file exists in the upper layer then there is no way to access
109 a file with the same name in the lower layer.
110 If necessary, a combination of loopback and union mounts can be made
111 which will still allow the lower files to be accessed by a different
114 Except in the case of a directory,
115 access to an object is granted via the normal filesystem access checks.
116 For directories, the current user must have access to both the upper
117 and lower directories (should they both exist).
119 Requests to create or modify objects in
121 are passed to the upper layer with the exception of a few special cases.
122 An attempt to open for writing a file which exists in the lower layer
125 file to be made to the upper layer, and then for the upper layer copy
127 Similarly, an attempt to truncate a lower layer file to zero length
128 causes an empty file to be created in the upper layer.
129 Any other operation which would ultimately require modification to
130 the lower layer fails with
133 The union filesystem manipulates the namespace, rather than
134 individual filesystems.
135 The union operation applies recursively down the directory tree
138 Thus any filesystems which are mounted under
140 will take part in the union operation.
141 This differs from the
145 which only applies the union operation to the mount point itself,
146 and then only for lookups.
149 .Bd -literal -offset indent
150 mount -t cd9660 -o ro /dev/cd0a /usr/src
151 mount -t union /var/obj /usr/src
154 mount the CD-ROM drive
161 For most purposes the effect of this is to make the
162 source tree appear writable
163 even though it is stored on a CD-ROM.
166 .Bd -literal -offset indent
167 mount -t union -o -b /sys $HOME/sys
170 attaches the system source tree below the
172 directory in the user's home directory.
173 This allows individual users to make private changes
174 to the source, and build new kernels, without those
175 changes becoming visible to other users.
176 Note that the files in the lower layer remain
187 THIS FILESYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
188 AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR
189 OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET.
191 This code also needs an owner in order to be less dangerous - serious
192 hackers can apply by sending mail to
193 .Aq hackers@FreeBSD.org
195 their intent to take it over.
197 Without whiteout support from the filesystem backing the upper layer,
198 there is no way that delete and rename operations on lower layer
201 is returned for this kind of operations along with any others
202 which would make modifications to the lower layer, such as
207 over a union tree has the side-effect of creating
208 a tree of shadow directories in the upper layer.
212 command first appeared in
215 .Fx Ns -(fill this in) .