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

324 Administering volume snapshots
Creating instant snapshots
disk group. Also note that break-off snapshots are used for the redo logs as such volumes
are write intensive.
Creating instant snapshots of volume sets
Volume set names can be used in place of volume names with the following vxsnap
operations on instant snapshots:
addmir, dis, make, prepare, reattach, refresh,
restore, rmmir, split, syncpause, syncresume, syncstart, syncstop,
syncwait, and unprepare.
The procedure for creating an instant snapshot of a volume set is the same as that for a
standalone volume. However, there are certain restrictions if a full-sized instant snapshot
is to be created from a prepared volume set. A full-sized instant snapshot of a volume set
must itself be a volume set with the same number of volumes, and the same volume sizes
and index numbers as the parent. For example, if a volume set contains three volumes with
sizes 1GB, 2GB and 3GB, and indexes 0, 1 and 2 respectively, then the snapshot volume
set must have three volumes with the same sizes matched to the same set of index
numbers. The corresponding volumes in the parent and snapshot volume sets are also
subject to the same restrictions as apply between standalone volumes and their snapshots.
You can use the
vxvset list command to verify that the volume sets have identical
characteristics as shown in this example:
# vxvset -g mydg list vset1
VOLUME INDEX LENGTH KSTATE CONTEXT
vol_0 0 204800 ENABLED -
vol_1 1 409600 ENABLED -
vol_2 2 614400 ENABLED -
# vxvset -g mydg list snapvset1
VOLUME INDEX LENGTH KSTATE CONTEXT
svol_0 0 204800 ENABLED -
svol_1 1 409600 ENABLED -
svol_2 2 614400 ENABLED -
A full-sized instant snapshot of a volume set can be created using a prepared volume set in
which each volume is the same size as the corresponding volume in the parent volume set.
Alternatively, you can use the nmirrors attribute to specify the number of plexes that
are to be broken off provided that sufficient plexes exist for each volume in the volume
set.
The following example shows how to prepare a source volume set, vset1, and an
identical volume set, snapvset1, which is then used to create the snapshot:
# vxsnap -g mydg prepare vset1
# vxsnap -g mydg prepare snapvset1
# vxsnap -g mydg make source=vset1/snapvol=snapvset1
To create a full-sized third-mirror break-off snapshot, you must ensure that each volume
in the source volume set contains sufficient plexes. The following example shows how to