Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Implementing Off-Host Processing Solutions
318 VERITAS Volume Manager Administrator’s Guide
Note A volume snapshot represents the data that exists in a volume at a given point in
time. As such, VxVM does not have any knowledge of data that is cached by the
overlying file system, or by applications such as databases that have files open in
the file system. If the fsgen volume usage type is set on a volume that contains a
VERITAS File System (VxFS), intent logging of the file system metadata ensures the
internal consistency of the file system that is backed up. For other file system types,
depending on the intent logging capabilities of the file system, there may
potentially be inconsistencies between in-memory data and the data in the snapshot
image.
For databases, a suitable mechanism must additionally be used to ensure the
integrity of tablespace data when the volume snapshot is taken. The facility to
temporarily suspend file system I/O is provided by most modern database
software. For ordinary files in a file system, which may be open to a wide variety of
different applications, there may be no way to ensure the complete integrity of the
file data other than by shutting down the applications and temporarily unmounting
the file system. In many cases, it may only be important to ensure the integrity of
file data that is not in active use at the time that you take the snapshot.
Implementing Online Backup
This section describes a procedure for implementing off-host online backup for a volume
in a private disk group. The intention is to present an outline of how to set up a regular
backup cycle by combining the Persistent FastResync and disk group split and join
features of VxVM. It is beyond the scope of this guide to describe how to configure a
database to use this procedure, or how to perform the backup itself.
To back up a volume in a private disk group, use the following procedure.
1. Use the following command on the primary host to see if the volume is associated
with a version 20 data change object (DCO) and DCO volume that allow instant
snapshots and Persistent FastResync to be used with the volume:
# vxprint -g volumedg -F%instant volume
This command returns on if the volume can be used for instant snapshot operations;
otherwise, it returns off.