VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Chapter 8, Administering Volumes
Enabling Persistent FastResync on Existing Volumes with Associated Snapshots
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4. Use the following command on the original volume and on each of its snapshot
volumes (if any) to add a DCO and DCO volume.
# vxassist [-g diskgroup] addlog volume logtype=dco \
dcolen=loglen ndcomirror=number [storage_attribute ...]
Set the value of ndcomirror equal to the number of data and snapshot plexes in the
volume.
The ndcomirror attribute specifies the number ofDCO plexes that arecreated in the
DCO volume. It is recommended that you configure as many DCO plexes as there are
data plexesin the volume. For example,specify ndcomirror=3 when addinga DCO
to a 3-way mirrored volume. If a volume has any snapshot plexes, a separate DCO
plex must also be reserved for each of these plexes. These 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 ndcomirror=5 for a volume with 3 data
plexes and2 snapshotplexes. For a snapshot volume, setthe valueof ndcomirror to
the number of plexes in the volume.
The value of the dcolen attribute specifies the size of a DCO plex, and must be a
integer multipleof 33 blocks from 33 to2112 blocks. The default value is 132blocks. A
larger value requires more disk space, but the finer granularity provided by the
FastResync maps provides faster resynchronization.
If a snapshot volume is to be moved to a separate disk group (using the disk group
move, split and join feature), you must ensure that the plexes its DCO volume are not
set up on the same physical disk as the plexes of any volume that is to remain in the
original disk group. To ensure this, specifyappropriate storage attributesto definethe
disks that can and/or cannot be used. For example, the following command would
allow the DCO plex for the volume SNAP-vol to be set up on disk disk03, but not
on disk01 or disk02:
# vxassist -g egdg addlog SNAP-vol logtype=dco \
dcolen=264 ndcomirror=1 !disk01 !disk02 disk03
Note If the DCO plexes of the snapshot volume are configured on disks that also contain
the plexes of other volumes, this prevents the snapshot volume from being moved
to a different disk group. See “Considerations for Placing DCO Plexes” on page 126
for more information.
5. Perform this step for each snapshot volume and for each snapshot plex in the original
volume. It is optional for the data plexes of the original volume. If the
dco_plex_rid attribute is not set, or if it is set incorrectly on a plex in a snapshot
volume, then Persistent FastResync is configured incorrectly, and a full
resynchronization is required on snapback. You can omit this step if you chose to
reattach all the snapshot volumes in step 2.