About snapshot rotation
NetBackup Snapshot Client implements a rotation scheme for managing snapshot resources for some snapshot methods.
In the following bulleted example, the system is configured to have three snapshots available for instant restore ("undo") at any time:
The state of the client's backup selection list is captured in the first Instant Recovery snapshot, and the snapshot is retained. In other words, it is not deleted, even if the backup image is also copied to a storage unit.
When the backup policy runs again, recent changes in the client's backup selection list are captured in a second snapshot. This snapshot is created on a second device that was allocated for this purpose. The first snapshot is retained, in case a restore is needed from that data.
When the backup policy runs a third time, a third snapshot is created on the third allocated device. At this point three snapshots representing three different states of the client data are available for restore.
When the policy runs a fourth time, no additional snapshot device is available: one of the existing devices must be reused. The system "rotates" the devices, overwriting the first snapshot with the fourth (most recent) one. Although the earliest snapshot is now gone, the three most recent snapshots are available for restore.
In this figure, the next Instant Recovery backup overwrites the snapshot that was made at 12:00 noon.
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