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!