VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
Performing Online Backup
Chapter 394
Performing Online Backup
Volume Manager provides snapshot backups of volume devices. This is
done through the vxassist command and other commands. There are
various procedures for doing backups, depending upon the requirements
for integrity of the volume contents. These procedures have the same
starting requirement: a plex that is large enough to store the complete
contents of the volume. The plex can be larger than necessary, but if a
plex that is too small is used, an incomplete copy results.
The recommended approach to volume backup is by using the vxassist
command which is easy to use.The vxassist snapstart, snapwait, and
snapshot tasks provide a way to do online backup of volumes with
minimal disruption to users.
The vxassist snapshot procedure consists of two steps:
Step 1. Running vxassist snapstart to create a snapshot mirror
Step 2. Running vxassist snapshot to create a snapshot volume
You can use the vxassist command to create a snapshot of a RAID-5
volume by using the recommended approach to volume backup described
in this section.
The vxassist snapstart step creates a write-only backup plex which
gets attached to and synchronized with the volume. When synchronized
with the volume, the backup plex is ready to be used as a snapshot
mirror. The end of the update procedure is indicated by the new snapshot
mirror changing its state to SNAPDONE. This change can be tracked by the
vxassist snapwait task, which waits until at least one of the mirrors
changes its state to SNAPDONE. If the attach process fails, the snapshot
mirror is removed and its space is released.
Once the snapshot mirror is synchronized, it continues being updated
until it is detached. You can then select a convenient time at which to
create a snapshot volume as an image of the existing volume. You can
also ask users to refrain from using the system during the brief time
required to perform the snapshot (typically less than a minute). The
amount of time involved in creating the snapshot mirror is long in
contrast to the brief amount of time that it takes to create the snapshot
volume.