Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

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 performance.
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 that 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.
See “Traditional third-mirror break-off snapshots” on page 201.
■ 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.
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
and then mirrored.
The plexes in a volume with a stripe-mirror layout are mirrored at the
sub-volume level, and cannot be used for snapshots.
■ 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 “Adding snapshot mirrors to a volume” on page 231.
See “Creating and managing third-mirror break-off snapshots” on page 223.
Linked break-off snapshot volumes
A variant of third-mirror break-off snapshots are linked break-off snapshots,
which use the vxsnap addmir command to link a specially prepared volume with
the data volume. The volume that is used for the snapshot is prepared in the same
way as for full-sized instant snapshots. However, unlike full-sized instant
snapshots, this volume can be set up in a different disk group from the data volume.
This makes linked break-off snapshots especially suitable for recurring off-host
processing applications as it avoids the disk group split/join administrative step.
205Administering volume snapshots
Emulation of third-mirror break-off snapshots