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

345Administering volume snapshots
Adding a version 0 DCO and DCO volume
For non-layered volumes, the default number of plexes in the mirrored DCO volume
is equal to the lesser of the number of plexes in the data volume or 2. For layered
volumes, the default number of DCO plexes is always 2. If required, use the
ndcomirror attribute to specify a different number. It is recommended that you
configure as many DCO plexes as there are existing data and snapshot plexes in the
volume. For example, specify
ndcomirror=3 when adding a DCO to a 3-way
mirrored volume.
The default size of each plex is 132 blocks. You can use the dcolen attribute to
specify a different size. If specified, the size of the plex must be an integer multiple
of 33 blocks from 33 up to a maximum of 2112 blocks.
You can 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 0
DCO plexes” on page 345 for details.
Specifying storage for version 0 DCO plexes
Note: The operations in this section relate to version 0 DCO volumes. They are not
supported for the version 20 DCO volume layout that was introduced in VxVM 4.0.
If the disks that contain volumes and their snapshots are to be moved or split into different
disk groups, the disks that contain their respective DCO plexes must be able to accompany
them. By default, VxVM attempts to place the DCO plexes on the same disks as the data
plexes of the parent volume. However, this may be impossible if there is insufficient space
available on those disks. In this case, VxVM uses any available space on other disks in the
disk group. If the DCO plexes are placed on disks which are used to hold the plexes of
other volumes, this may cause problems when you subsequently attempt to move volumes
into other disk groups.
You can use storage attributes to specify explicitly which disks to use for the DCO plexes.
If possible, specify the same disks as those on which the volume is configured. For
example, to add a DCO object and DCO volume with plexes on mydg05 and mydg06,
and a plex size of 264 blocks to the volume, myvol, in the disk group, mydg, use the
following command:
# vxassist -g mydg addlog myvol logtype=dco dcolen=264 \
mydg05 mydg06
To view the details of the DCO object and DCO volume that are associated with a volume,
use the
vxprint command. The following is partial vxprint 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
sd disk01-01 vol1-01 ENABLED 1024 0 -
pl vol1-02 vol1 ENABLED 1024 - ACTIVE