Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

The ndcomirs attribute specifies the number of DCO plexes that are created in
the DCO volume. You should 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.
To enable DRL logging on the volume, specify drl=on (this is the default). For
sequential DRL, specify drl=sequential. If you do not need DRL, specify drl=off.
You can also specify vxassist-style storage attributes to define the disks that
can or cannot be used for the plexes of the DCO volume.
See “Specifying storage for version 20 DCO plexes” on page 319.
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.
See “Using a DCO and DCO volume with a RAID-5 volume” on page 320.
By default, a version 20 DCO volume contains 32 per-volume maps. If you require
more maps, you can use the vxsnap addmap command to add them.
See the vxsnap(1M) manual page.
Specifying storage for version 20 DCO plexes
If you move the disks that contain volumes and their snapshots into different disk
groups, you must ensure that the disks that contain their DCO plexes can
accompany them. You can use storage attributes to specify which disks to use for
the DCO plexes. (If you do not want to use dirty region logging (DRL) with a volume,
you can specify the same disks as those on which the volume is configured,
assuming that space is available on the disks). For example, to add a DCO object
and mirrored DCO volume with plexes on disk05 and disk06 to the volume, myvol,
use the following command:
# vxsnap -g mydg prepare myvol ndcomirs=2 alloc=disk05,disk06
319Administering volumes
Preparing a volume for DRL and instant snapshots