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371Configuring off-host processing
Implementing off-host processing solutions
■ Implementing decision support
These applications use the Persistent FastResync feature of VxVM in
conjunction with linked break-off snapshots.
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.