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

267Administering volumes
Preparing a volume for DRL and instant snapshots
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 setting). If
sequential DRL is required, specify
drl=sequential. If DRL is not required, specify
drl=off.
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. See “Specifying storage for version 20
DCO plexes” on page 267 for details.
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. See “Using a DCO and DCO volume
with a RAID-5 volume” on page 268 for details.
By default, a version 20 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.
Specifying storage for version 20 DCO plexes
If the disks that contain volumes and their snapshots are to be moved 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 disk05 disk06
To view the details of the DCO object and DCO volume that are associated with a volume,
use the
vxprint command. The following is example vxprint -vh output for the
volume named vol1 (the TUTIL0 and PUTIL0 columns are omitted for clarity):
TY NAME ASSOC KSTATE LENGTH PLOFFS STATE ...
v vol1 fsgen ENABLED 1024 - ACTIVE
pl vol1-01 vol1 ENABLED 1024 - ACTIVE