Backup Optimization

Rules for Identifying Identical Files for Backup Optimization

Table 2-2 describes criteria that RMAN uses to determine whether a file is identical to a file that it already backed up.

If RMAN determines that a file is identical and it has already been backed up, then it is a candidate to be skipped. However, RMAN must do further checking to determine whether to skip the file, because both the retention policy and the backup duplexing feature are factors in the algorithm that determines whether RMAN has sufficient backups on the specified device type.

Backup Optimization Algorithm

Table 2-3 describes the algorithm that backup optimization uses when determining whether to skip the backup of an identical file.

For example, assume that at 9 a.m. you back up three copies of all existing archived logs to tape:

BACKUP DEVICE TYPE sbt COPIES 3 ARCHIVELOG ALL;

Later, you enable the following configuration setting in preparation for another backup:

CONFIGURE ARCHIVELOG BACKUP COPIES FOR DEVICE TYPE sbt TO 4;
CONFIGURE BACKUP OPTIMIZATION ON;

Then, you run the following archived log backup at noon:

BACKUP DEVICE TYPE sbt COPIES 2 ARCHIVELOG ALL;

In this case, the BACKUP ... COPIES setting overrides the CONFIGURE ... COPIES setting, so RMAN sets n=2. RMAN skips the backup of a log only if at least two copies of the log exist on the sbt device. Because three copies of each log exist on sbt of all the logs generated before 9 a.m., RMAN skips the backups of these logs. However, RMAN backs up two copies of all logs generated after 9 a.m. because these logs have not yet been backed up to tape.

At this point, three copies of the logs created before 9 a.m. exist on tape, and two copies of the logs created after 9 a.m. exist on tape. Assume that you run the following backup at 3 p.m.:

RUN
{ 
  SET BACKUP COPIES 3;
  BACKUP DEVICE TYPE sbt ARCHIVELOG ALL;
}

In this case, RMAN sets n=3 and so will not back up the logs created before 9 a.m. because three copies already exist on tape. However, only two copies of the logs created after 9 a.m. exist on tape, so RMAN does not optimize backups of these logs. Hence, RMAN backs up three copies of the logs created after 9 a.m.

Requirements for Backup Optimization

Backup optimization is used when the following conditions are true:

• The CONFIGURE BACKUP OPTIMIZATION ON command has been run to enable backup optimization.

• You run BACKUP DATABASE, BACKUP ARCHIVELOG with ALL or LIKE options, or BACKUP BACKUPSET ALL.

• Only one type of channel is allocated, that is, you do not mix channels of type DISK and sbt in the same backup command.

For example, assume that you run these commands:

BACKUP DEVICE TYPE sbt DATABASE PLUS ARCHIVELOG;
BACKUP DEVICE TYPE sbt BACKUPSET ALL;

If none of these files has changed since the last backup, then RMAN does not back up the files again, nor signal an error if it skips all files specified in the command.

Overriding and Disabling Backup Optimization

To override backup optimization and back up all files whether or not they have changed, specify the FORCE option on the BACKUP command. For example:

BACKUP DEVICE TYPE sbt BACKUPSET ALL FORCE;

To disable backup optimization on a persistent basis, use the following command:

RMAN> CONFIGURE BACKUP OPTIMIZATION OFF;

Effect of Retention Policies on Backup Optimization for SBT Backups

Backup optimization is not always applied when backing up to SBT. The exceptions to normal backup optimization behavior for recovery window-based and redundancy-based retention policies are described in the following sections.

CONFIGURE DEFAULT DEVICE TYPE TO sbt;
CONFIGURE BACKUP OPTIMIZATION ON;
CONFIGURE RETENTION POLICY TO REDUNDANCY 2;