Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

363Configuring off-host processing
Implementing off-host processing solutions
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 off-host 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. 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
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.
2 Use the following command on the primary host to check whether FastResync is
enabled on the volume:
# vxprint -g volumedg -F%fastresync volume
This command returns on if FastResync is enabled; otherwise, it returns off.
If FastResync is disabled, enable it using the following command on the primary
host:
# vxvol -g volumedg set fastresync=on volume
3 On the primary host, create a new volume in a separate disk group for use as the
snapshot volume as described in “Creating a volume for use as a full-sized instant or