Administering volume snapshots 321

Creating instant snapshots

Preparing to create instant and break-off snapshotsTo prepare a volume for the creation of instant and break-off snapshots

1Use the following commands to see if the volume is associated with a version 20 data change object (DCO) and DCO volume that allow instant snapshots and Persistent FastResync to be used with the volume, and to check that FastResync is enabled on the volume:

#vxprint -g volumedg -F%instant volume#vxprint -g volumedg -F%fastresync volume

If both commands return a value of on, the volume can be used for instant snapshot operations, and you should skip to step 3. Otherwise continue with step 2.

2To 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 252, “Preparing a volume for DRL and instant snapshots” on page 275 and “Removing support for DRL and instant snapshots from a volume” on page 279 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.

3If 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 322 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