Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

302 Administering volume snapshots
Emulation of third-mirror break-off snapshots
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 315 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 312.
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-mirror break-off snapshots” on page 297.
■ 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 269)
and then mirrored.
The plexes in a volume with a stripe-mirror layout are mirrored at the sub-
volume level, and cannot be broken off.
■ Use the vxsnap make command with the sync=yes and type=full attributes
specified to create the snapshot volume, and then use the
vxsnap syncwait
command to wait for synchronization of the snapshot volume to complete.
See “Creating and managing third-mirror break-off snapshots” on page 319 for details of
the procedures for creating and using this type of snapshot.