VERITAS Volume Manager 4.1 Administrator's Guide

Configuring Off-Host Processing
Implementing Off-Host Processing Solutions
Chapter 11388
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 177. 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 236 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 a backup, you should usually only
require the default of one.
Step 4. If the volume to be backed up contains database tables in a file system,
suspend updates to the volume. The database may have a hot backup
mode that allows you to do this by temporarily suspending writes to its
tables.
Step 5. On the primary host, make a snapshot volume, snapvol, using the
following command:
# vxassist -g volumedg snapshot [nmirrors= N ] volume snapvol
If required, use the nmirrors attribute to specify the number of mirrors
in the snapshot volume.