HP Veritas Volume Manager 5.0 -UX 11i v3 manual Space-optimized instant snapshots

Administering volume snapshots

Space-optimized instant snapshots

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Space-optimized instant snapshots

Volume snapshots, such as those described in “Traditional third-mirror break- off snapshots” on page 305 and “Full-sized instant snapshots” on page 307, require the creation of a complete copy of the original volume, and use as much storage space as the original volume.

Instead of requiring a complete copy of the original volume’s storage space, space-optimizedinstant snapshots use a storage cache. The size of this cache may be configured when the snapshot is created.

Note: A storage cache may be named and shared among several volumes in the same disk group. If so, the size of the cache that is declared must be the same for each volume’s space-optimized snapshot. You may find it convenient to configure a single storage cache in a disk group that can be shared by all the volumes in that disk group. See “Creating a shared cache object” on page 322 for details.

When the original volume is written to, VxVM preserves the original data contents in the cache before the write is committed. As the storage cache can be configured to require much less storage than the original volume, it is referred to as being space-optimized. If the cache becomes too full, you can configure VxVM to grow the size of the cache automatically using any available free space in the disk group.

The instant space-optimized snapshot model is illustrated in Figure 9-3.

Figure 9-3	Space-optimized instant snapshot creation and usage in a backup
	cycle
Start		vxsnap make		vxsnap refresh
vxsnap prepare
					Backup
	Original		Snapshot
	volume		volume		Cycle

Back up to disk, tape or other media.