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357Administering volume snapshots
Adding a version 0 DCO and DCO volume
3 Use the following command to add a DCO and DCO volume to the existing
volume:
# vxassist [-g diskgroup] addlog volume logtype=dco \
[ndcomirror=number] [dcolen=size] [storage_attributes]
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 357 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