MOUNT_UNION(8) NetBSD System Manager's Manual MOUNT_UNION(8)
mount_union -- mount union filesystems
mount_union [-b] [-o options] directory uniondir
The mount_union command attaches directory above uniondir in such a way that the contents of both directory trees remain visible. By default, directory becomes the upper layer and uniondir becomes the lower layer. Both directory and uniondir are converted to absolute paths before use. The options are as follows: -b Invert the default position, so that directory becomes the lower layer and uniondir becomes the upper layer. However, uniondir remains the mount point. -o Options are specified with a -o flag followed by a comma sepa- rated string of options. See the mount(8) man page for possible options and their meanings. Filenames are looked up in the upper layer and then in the lower layer. If a directory is found in the lower layer, and there is no entry in the upper layer, then a shadow directory will be created in the upper layer. It will be owned by the user who originally did the union mount, with mode "rwxrwxrwx" (0777) modified by the umask in effect at that time. If a file exists in the upper layer then there is no way to access a file with the same name in the lower layer. If necessary, a combination of loopback and union mounts can be made which will still allow the lower files to be accessed by a different pathname. Except in the case of a directory, access to an object is granted via the normal filesystem access checks. For directories, the current user must have access to both the upper and lower directories (should they both exist). Requests to create or modify objects in uniondir are passed to the upper layer with the exception of a few special cases. An attempt to open for writing a file which exists in the lower layer causes a copy of the entire file to be made to the upper layer, and then for the upper layer copy to be opened. Similarly, an attempt to truncate a lower layer file to zero length causes an empty file to be created in the upper layer. Any other operation which would ultimately require modification to the lower layer fails with EROFS. The union filesystem manipulates the namespace, rather than individual filesystems. The union operation applies recursively down the directory tree now rooted at uniondir. Thus any filesystems which are mounted under uniondir will take part in the union operation. This differs from the union option to mount(8) which only applies the union operation to the mount point itself, and then only for lookups.
The commands mount -t cd9660 -o ro /dev/cd0a /usr/src mount -t union /var/obj /usr/src mount the CD-ROM drive /dev/cd0a on /usr/src and then attaches /var/obj on top. For most purposes the effect of this is to make the source tree appear writable even though it is stored on a CD-ROM. The command mount -t union -o -b /sys $HOME/sys attaches the system source tree below the sys directory in the user's home directory. This allows individual users to make private changes to the source, and build new kernels, without those changes becoming visible to other users. Note that the files in the lower layer remain accessible via /sys.
intro(2), mount(2), unmount(2), fstab(5), fsck_ffs(8), mount(8), mount_null(8), sysctl(8)
The mount_union command first appeared in 4.4BSD.
Without whiteout support from the filesystem backing the upper layer, there is no way that delete and rename operations on lower layer objects can be done. An attempt to mount a union directory under one which does not have whiteout support will return EOPNOTSUPP ("Operation not supported"). Whiteout support can be added to an existing FFS filesystem by using the -c option of fsck_ffs(8). Running find(1) over a union tree has the side-effect of creating a tree of shadow directories in the upper layer. NetBSD 8.1 February 5, 2008 NetBSD 8.1
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