VERITAS Volume Manager 3.5 Administrator's Guide (September 2004)

Configuring Off-Host Processing
Implementing Off-Host Processing Solutions
Chapter 11 381
NOTE By default, VxVM attempts to avoid placing a snapshot mirrors on a disk
that already holds any plexes of a data volume. However, this may be
impossible if insufficient space is available in the disk group. In this case,
VxVM uses any available space on other disks in the disk group. If the
snapshot 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 a snapshot volume into another disk group as described in
“Considerations for Placing DCO Plexes” on page 157. To override the
default storage allocation policy, you can use storage attributes to specify
explicitly which disks to use for the snapshot plexes. See “Creating a
Volume on Specific Disks” on page 222 for more information.
If you start vxassist snapstart in the background using the -b option,
you can use the vxassist snapwait command to wait for the creation of
the mirror to complete as shown here:
# vxassist -g volumedg snapwait volume
If vxassist snapstart is not run in the background, it does not exit
until the mirror has been synchronized with the volume. The mirror is
then ready to be used as a plex of a snapshot volume. While attached to
the original volume, its contents continue to be updated until you take
the snapshot.
Use the nmirror attribute to create as many snapshot mirrors as you
need for the snapshot volume. For applications where data redundancy is
required for the volume that contains the replica database, specify a
number greater than one.
Step 4. Prepare the OHP host to receive the snapshot volume that contains the
copy of the database tables. This may involve setting up private volumes
to contain any redo logs, and configuring any files that are used to
initialize the database.
Step 5. On the primary host, suspend updates to the volume that contains the
database tables. The database may have a hot backup mode that allows
you to do this by temporarily suspending writes to its tables.
Step 6. On the primary host, make a snapshot volume, snapvol, using the
following command:
# vxassist -g volumedg snapshot [nmirrors=N] volume snapvol