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

312 Administering volume snapshots
Creating instant snapshots
2 To prepare a volume for instant snapshots, use the following command:
# vxsnap [-g diskgroup] prepare volume [regionsize=size] \
[ndcomirs=number] [alloc=storage_attributes]
Note: It is only necessary to run the vxsnap prepare command on a volume if it
does not already have a version 20 DCO volume (for example, if you have run the
vxsnap unprepare command on the volume). See “Creating a volume with a
version 20 DCO volume” on page 244, “Preparing a volume for DRL and instant
snapshots” on page 267 and “Removing support for DRL and instant snapshots from
a volume” on page 271 for more information.
For example, to prepare the volume, myvol, in the disk group, mydg, use the
following command:
# vxsnap -g mydg prepare myvol regionsize=128k ndcomirs=2 \
alloc=mydg10,mydg11
This example creates a DCO object and redundant DCO volume with two plexes
located on disks mydg10 and mydg11, and associates them with myvol. The
region size is also increased to 128KB from the default size of 64KB. The region size
must be a power of 2, and be greater than or equal to 16KB. A smaller value requires
more disk space for the change maps, but the finer granularity provides faster
resynchronization.
3 If you need to create several space-optimized instant snapshots for the volumes in a
disk group, you may find it more convenient to create a single shared cache object in
the disk group rather than a separate cache object for each snapshot. Follow the
procedure in “Creating a shared cache object” on page 312 to create the cache object.
For full-sized instant snapshots and linked break-off snapshots, you must prepare a
volume that is to be used as the snapshot volume. This volume must be the same size
as the data volume for which the snapshot is being created, and it must also have the
same region size. See “Creating a volume for use as a full-sized instant or linked
break-off snapshot” on page 314 for details.
Creating a shared cache object
To create a shared cache object
1 Decide on the following characteristics that you want to allocate to the cache volume
that underlies the cache object:
■ The size of the cache volume should be sufficient to record changes to the parent
volumes during the interval between snapshot refreshes. A suggested value is
10% of the total size of the parent volumes for a refresh interval of 24 hours.
■ If redundancy is a desired characteristic of the cache volume, it should be
mirrored. This increases the space that is required for the cache volume in
proportion to the number of mirrors that it has.