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

Use the following command to add a version 20 DCO and DCO volume to an

existing volume:

# vxsnap [-g diskgroup] prepare volume [ndcomirs=number] \

[regionsize=size] [alloc=storage_attribute[,...]]

The ndcomirs attribute specifies the number of DCO plexes that are created

in the DCO volume. It is recommended that you configure as many DCO plexes

as there are data and snapshot plexes in the volume. The DCO plexes are used

to set up a DCO volume for any snapshot volume that you subsequently create

from the snapshot plexes. For example, specify ndcomirs=5 for a volume with

3 data plexes and 2 snapshot plexes.

The value of the regionsize attribute specifies the size of the tracked regions

in the volume. A write to a region is tracked by setting a bit in the change

map. The default value is 64k (64KB). A smaller value requires more disk

space for the change maps, but the finer granularity provides faster

resynchronization.

You can also specify vxassist-style storage attributes to define the disks

that can and/or cannot be used for the plexes of the DCO volume.You can

specify the disks, controller and enclosure to be included or excluded in the

alloc attributes. An example of the alloc usage to allocate storage from

disks excluding disk01 and disk02:

alloc=!disk01,!disk02

Note: The vxsnap prepare command automatically enables persistent

FastResync on the volume. Persistent FastResync is also set automatically

on any snapshots that are generated from a volume on which this feature is

enabled.

If the volume is a RAID-5 volume, it is converted to a layered volume that can

be used with instant snapshots and persistent FastResync.

By default, a new-style DCO volume contains 32 per-volume maps. If you

require more maps than this, you can use the vxsnap addmap command to

add more maps. See the vxsnap(1M) manual page for details of this command.

145Setting up volumes for instant snapshots

Preparing a volume for instant snapshot operations