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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 1.6 2007/07/14 21:48:16 swildner 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
99 variable must be set to 1 to permit file system mounting by ordinary users.
101 Filenames are looked up in the upper layer and then in the
103 If a directory is found in the lower layer, and there is no entry
104 in the upper layer, then a
106 directory will be created in the upper layer.
107 It will be owned by the user who originally did the union mount,
110 (0777) modified by the umask in effect at that time.
112 If a file exists in the upper layer then there is no way to access
113 a file with the same name in the lower layer.
114 If necessary, a combination of loopback and union mounts can be made
115 which will still allow the lower files to be accessed by a different
118 Except in the case of a directory,
119 access to an object is granted via the normal filesystem access checks.
120 For directories, the current user must have access to both the upper
121 and lower directories (should they both exist).
123 Requests to create or modify objects in
125 are passed to the upper layer with the exception of a few special cases.
126 An attempt to open for writing a file which exists in the lower layer
129 file to be made to the upper layer, and then for the upper layer copy
131 Similarly, an attempt to truncate a lower layer file to zero length
132 causes an empty file to be created in the upper layer.
133 Any other operation which would ultimately require modification to
134 the lower layer fails with
137 The union filesystem manipulates the namespace, rather than
138 individual filesystems.
139 The union operation applies recursively down the directory tree
142 Thus any filesystems which are mounted under
144 will take part in the union operation.
145 This differs from the
149 which only applies the union operation to the mount point itself,
150 and then only for lookups.
153 .Bd -literal -offset indent
154 mount -t cd9660 -o ro /dev/cd0a /usr/src
155 mount -t union /var/obj /usr/src
158 mount the CD-ROM drive
165 For most purposes the effect of this is to make the
166 source tree appear writable
167 even though it is stored on a CD-ROM.
170 .Bd -literal -offset indent
171 mount -t union -o -b /sys $HOME/sys
174 attaches the system source tree below the
176 directory in the user's home directory.
177 This allows individual users to make private changes
178 to the source, and build new kernels, without those
179 changes becoming visible to other users.
180 Note that the files in the lower layer remain
193 command first appeared in
196 .Fx Ns -(fill this in) .
198 THIS FILESYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
199 AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR
200 OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET.
202 This code also needs an owner in order to be less dangerous - serious
203 hackers can apply by sending mail to
204 .Aq kernel@crater.dragonflybsd.org
206 their intent to take it over.
208 Without whiteout support from the filesystem backing the upper layer,
209 there is no way that delete and rename operations on lower layer
212 is returned for this kind of operations along with any others
213 which would make modifications to the lower layer, such as
218 over a union tree has the side-effect of creating
219 a tree of shadow directories in the upper layer.