VSP Overview

NetBackup uses VSP to back up open and active files on Windows NT, Windows 2000, Windows XP, and Windows Server 2003 (32 and 64-bit) clients. To make backing up open and active files possible, VSP first captures a snapshot of each volume that needs to be backed up. After creating a snapshot of the volume, a virtual drive representing a static copy of the volume in a point-in-time is created along with a corresponding VSP cache file. NetBackup backs up files using the virtual drive instead of the actual drive. For each snapshot that is created for a volume, a VSP cache file is created to maintain the integrity of the snapshot. The original data corresponding to the changes that occur during the backup is stored in the cache file that was created along with the volume snapshot.

VSP is similar to OTM (used in previous releases) in that VSP creates volume snapshots using a caching mechanism. However, it is important to keep in mind the following considerations when using VSP:

• VSP uses a cache file for each volume that requires a snapshot, while OTM uses only one cache for all snapshots.
• Using VSP, a snapshot of a volume cannot be created if the volume already contains a VSP cache file.
• Using VSP, a cache file cannot be placed on a volume that has had a snapshot taken of it, or is in the process of having a snapshot taken. Only when the snapshot for the volume has been destroyed can it be used as a location for a VSP cache file.
• All VSP cache files are placed at the root level of a volume and are removed when its VSP snapshot has been destroyed.
• VSP cannot be used to perform hot database backups. See Using VSP with Databases.

This property appears for Windows clients only.

Stepping through the Backup Process with VSP

The following steps describe the sequence of events during a backup using VSP:

1. Before the backup begins, NetBackup uses VSP to create snapshots for the backup job. NetBackup waits for a quiet period to occur when no writes are being performed on the drives that contain data to be backed up. This wait is required to ensure that the file system is in a consistent state. The length of the quiet period is defined by the Busy File Timeout property. If a quiet period of sufficient length does not occur within the time specified by Busy File Timeout, the backup proceeds without VSP.
2. If a quiet period of sufficient length is detected, NetBackup performs the actions necessary to record the VSP snapshot.
3. The backup begins and NetBackup starts reading data from the client. If an application requests a read or write during the backup, VSP reads or writes the disk or its cache as necessary to maintain the snapshot and provide accurate data to the application.
4. Once the backup completes, NetBackup attempts to destroy the VSP volume snapshots created for the backup job while deleting the VSP cache files for the volume snapshots.