Veritas FlashSnap Point-In-Time Copy Solutions 5.0 AdministratorÆs Guide, HP-UX 11i v3, First Edition, May 2008

23Setting up volumes for instant snapshots
Preparing a volume for instant snapshot operations
Preparing a volume for instant snapshot operations
To prepare a volume for instant snapshot operations, a version 20 Data Change
Object (DCO) and DCO volume must first be associated with that volume.
To add a version 20 DCO object and DCO volume to an existing volume
1 Ensure that the disk group containing the existing volume has been
upgraded to at least version 110. Use the following command to check the
version of a disk group:
# vxprint -l diskgroup | egrep ’version:’
To upgrade a disk group, use the following command:
# vxdg upgrade diskgroup
2 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.
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.