Figure: Creating a snapshot of a snapshot creation of a snapshot of an existing snapshot.
Even though the arrangement of the snapshots in this figure appears similar to a snapshot cascade, the relationship between the snapshots is not recursive. When reading from the snapshot S2, data is obtained directly from the original volume, V, if it does not exist in S1 itself.
Such an arrangement may be useful if the snapshot volume, S1, is critical to the operation. For example, S1 could be used as a stable copy of the original volume, V. The additional snapshot volume, S2, can be used to restore the original volume if that volume becomes corrupted. For a database, you might need to replay a redo log on S2 before you could use it to restore V.
Figure: Using a snapshot of a snapshot to restore a database shows the sequence of steps that would be required to restore a database.
If you have configured snapshots in this way, you may wish to make one or more of the snapshots into independent volumes. There are two vxsnap commands that you can use to do this:
vxsnap dis dissociates a snapshot and turns it into an independent volume. The snapshot to be dissociated must have been fully synchronized from its parent. If a snapshot volume has a child snapshot volume, the child must also have been fully synchronized. If the command succeeds, the child snapshot becomes a snapshot of the original volume.
Figure: Dissociating a snapshot volume shows the effect of applying the vxsnap dis command to snapshots with and without dependent snapshots.
vxsnap split dissociates a snapshot and its dependent snapshots from its parent volume. The snapshot that is to be split must have been fully synchronized from its parent volume.
Figure: Splitting snapshots shows the operation of the vxsnap split command.
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