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Veritas NetBackup™ Administrator's Guide, Volume I

Last Published: 2019-07-01

Product(s): NetBackup (8.2)

Accelerator forced rescan option (schedule attribute)

The policy Schedules tab contains an option called Accelerator forced rescan. This option creates a checksum of the content of each file during backup. It uses the checksums for change detection. It provides a safety net by establishing a new baseline for the next Accelerator backup.

Note:

The following information is only applicable to Files and Folder (Unix/Windows) backups and NDMP backups. The Accelerator forced rescan schedule is not necessary for other backup types such as VMware and Hyper-V.

The Accelerator forced rescan option detects the following events:

The file's data changes but the file's metadata does not change.
The file's metadata becomes corrupted such that it does not indicate that the file has changed.
A malicious user or application changes the file's metadata such that it does not indicate that the file has changed.

Note:

If Accelerator forced rescan is enabled, NetBackup uses the Windows change journal to help determine if a file has changed. NetBackup also examines the file system metadata for each file to help detect which files have changed.

For the most efficient use of Accelerator, Accelerator policies must include at least two full-backup schedules: one full schedule with the Accelerator forced rescan option disabled, and another full schedule with Accelerator forced rescan enabled. See Table: Required full-backup schedules for each Accelerator policy.

Table: Required full-backup schedules for each Accelerator policy

Full backup schedules	Notes on schedule frequency
First schedule: Accelerator forced rescan disabled	Configure this schedule to run most of your Accelerator full backups.
Second schedule: Accelerator forced rescan enabled	Configure this schedule to run less often than the first full-backup schedule. For example: If the first full-backup schedule runs weekly, run the second schedule (with the Accelerator forced rescan option enabled) every few months. However, the best frequency for this schedule depends upon your environment. Note: If the policy has no schedule that enables the Accelerator forced rescan option, all full backups automatically enable that option and backup performance is reduced.

Full backup schedules

Notes on schedule frequency

First schedule:

Accelerator forced rescan disabled

Configure this schedule to run most of your Accelerator full backups.

Second schedule:

Accelerator forced rescan enabled

Configure this schedule to run less often than the first full-backup schedule.

For example: If the first full-backup schedule runs weekly, run the second schedule (with the Accelerator forced rescan option enabled) every few months. However, the best frequency for this schedule depends upon your environment.

Note:

If the policy has no schedule that enables the Accelerator forced rescan option, all full backups automatically enable that option and backup performance is reduced.

Note the following about the Accelerator forced rescan option:

The Accelerator forced rescan option is grayed out if the Use Accelerator option on the Attributes tab is not selected.
Because of the checksum processing on the client, this option reduces backup speed as compared to the Use Accelerator option on its own. The speed reduction depends on the client's configuration and its current processing load. If the client is busy with many jobs when Accelerator backup begins, checksum processing can reduce backup speed.
If the Windows volume is not NTFS or ReFS, and the volume has no full backup schedule that is configured with the Accelerator forced rescan option, note: NetBackup uses Accelerator forced rescan on every full backup. The Accelerator forced rescan option is not enabled in the schedule but is in fact used, as indicated in the NetBackup log messages. This option may reduce the speed of the backup as compared to the previous backups that did not use Accelerator forced rescan.
See Table: Required full-backup schedules for each Accelerator policy for recommended full-backup schedules.
For an MS-Windows or Standard policy (to back up files and folders), all the data on the client is read. However, NetBackup sends only the changed data over the network to be included in the backup image. Sending only the changed data is similar to a regular Accelerator full backup. Thus, for an Accelerator forced rescan backup, the optimization percentage is similar to a regular Accelerator full backup. Note that the duration of the backup falls somewhere between a non-Accelerator full backup and a regular Accelerator full backup.