Example implementation of off-host processing shows an example implementation of off-host processing.
Example implementation of off-host processing
Click the thumbnail above to view full-sized image.
By accessing snapshot volumes from a lightly loaded host (shown here as the OHP host), CPU- and I/O-intensive operations for online backup and decision support do not degrade the performance of the primary host that is performing the main production activity (such as running a database).If you also place the snapshot volumes on disks that are attached to different host controllers than the disks in the primary volumes, it is possible to avoid contending with the primary host for I/O resources.
The following sections describe how you can apply off-host processing to implement regular online backup of a volume in a private disk group, and to set up a replica of a production database for decision support. Two applications are outlined
See "Implementing off-host online backup" on page 375.
See "Implementing decision support" on page 378.
These applications use the Persistent FastResync feature of VxVM in conjunction with linked break-off snapshots.
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.