Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Emulation of Third-Mirror Break-Off Snapshots
268 VERITAS Volume Manager Administrator’s Guide
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 Instant Snapshots” on page 275 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 294.
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 is similar to using the vxassist snapstart and
vxassist snapshot commands that are described in “Traditional Third-Mirror
Break-Off Snapshots” on page 263.
◆ 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 237) 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.