310Administering volume snapshotsEmulation of third-mirror break-off snapshots

As for instant snapshots, space-optimized snapshots use a copy-on-write mechanism to make them immediately available for use when they are first created, or when their data is refreshed. Unlike instant snapshots, however, you cannot enable synchronization on space-optimized snapshots, reattach them to their original volume, or turn them into independent volumes.

See “Creating and managing space-optimized instant snapshots” on page 324 for details of the procedures for creating and using this type of snapshot.

For information about how to set up a cache for use by space-optimized instant snapshots, see “Creating a shared cache object” on page 322.

Emulation of third-mirror break-off snapshots

Third-mirror break-off snapshots are suitable for write-intensive volumes (such as for database redo logs) where the copy-on-write mechanism of space- optimized or full-sized instant snapshots might degrade the performance of the volume.

If you use the vxsnap prepare command to enable a volume for use with instant and space-optimized snapshots, you cannot use the vxassist snapshot commands to administer snapshots that you create for the volume. If you require snapshots that behave as third-mirror break-off snapshots (that is, they must be fully synchronized before they can be used), there are three ways to achieve this:

■Use the vxsnap addmir command to create and attach one or more snapshot mirrors to the volume. When the plexes have been synchronized and are in the SNAPDONE state, the vxsnap make command can then be used with the nmirror attribute to create the snapshot volume. This technique is similar to using the vxassist snapstart and vxassist snapshot commands that are described in “Traditional third-mirrorbreak-off snapshots” on

page 305.

■Use the vxsnap make command with the plex attribute to use one or more existing plexes of a volume as snapshot plexes. The volume must have a sufficient number of available plexes that are in the ACTIVE state.

Note: The volume must be a non-layered volume with a mirror or mirror- stripe layout, or a RAID-5 volume that you have converted to a special layered volume (see “Using a DCO and DCO volume with a RAID-5 volume” on page 277) and then mirrored.

The plexes in a volume with a stripe-mirrorlayout are mirrored at the sub-volume level, and cannot be broken off.