Backup and Read-only Volumes
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Backup and Read-only Volumes
There are three types of volumes: read-write,
read-only, and backup. A read-write volume is a regular
volume that can be read and written-just as the name implies. A
read-write volume may have associated with it zero, one, or many
read-only volumes. Read-only volumes cannot be modified by normal
users. They have special properties, the most important of which is
that many copies of a read-only volume can exist at once. If an AFS\
mountpoint is read-only and a read-only volume exists with the right
name, AFS just picks one read-only volume to read from. If that
volume disappears or somehow becomes unreachable, AFS will start using
another one without the user ever knowing the difference. Backup
volumes are also special. There can be only one backup volume for a
read-write volume. Read-only volumes cannot have backup volumes. In
other words, a backup volume can be associated only with a read-write
volume. A backup volume is a read-only copy of a read-write volume
that actually shares the same disk space as the read-write
volume. These volumes are often known as
clones. When a volume is backed up, that volume
initially takes a very small amount of space on disk. As the
read-write volume and the backup volume get further out of
synchronization, data is actually copied. The next time the volume is
backed up, the old copied data is destroyed. That means that, once a
volume has been backed up once, subsequent backups of the volume may
actually reduce the total amount of disk spaced used!