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324 Administering volume snapshots
Creating instant snapshots
4 Use the vxassist command to create a volume, snapvol, of the required size
and redundancy, together with a version 20 DCO volume with the correct
region size:
# vxassist [-g diskgroup] make snapvol $LEN \
[layout=mirror nmirror=number] logtype=dco drl=off \
dcoversion=20 [ndcomirror=number] regionsz=$RSZ \
init=active [storage_attributes]
Specify the same number of DCO mirrors (ndcomirror) as the number of
mirrors in the volume (nmirror). The init=active attribute is used to
make the volume available immediately. You can use storage attributes to
specify which disks should be used for the volume.
As an alternative to creating the snapshot volume and its DCO volume in a
single step, you can first create the volume, and then prepare it for instant
snapshot operations as shown here:
# vxassist [-g diskgroup] make snapvol $LEN \
[layout=mirror nmirror=number] init=active \
[storage_attributes]
# vxsnap [-g diskgroup] prepare snapvol [ndcomirs=number] \
regionsize=$RSZ [storage_attributes]
Creating and managing space-optimized instant snapshots
Note: Space-optimized instant snapshots are not suitable for write-intensive
volumes (such as for database redo logs) because the copy-on-write mechanism
may degrade the performance of the volume.
If you intend to split the volume and snapshot into separate disk groups (for
example, to perform off-host processing), you must use a fully synchronized
full-sized instant, third-mirror break-off or linked break-off snapshot (which do
not require a cache object). You cannot use a space-optimized instant snapshot
for this purpose.
Creation of space-optimized snapshots that use a shared cache fails if the region
size specified for the volume is smaller than the region size set on the cache.
If the region size of a space-optimized snapshot differs from the region size of
the cache, this can degrade the system’s performance compared to the case
where the region sizes are the same.