Overview of Backup Optimization

Overview of Backup Optimization

If you enable backup optimization, then the BACKUP command skips backing up files when the identical file has already been backed up to the specified device type. Table 5-4 describes criteria that RMAN uses to determine whether a file is identical to a file that it backed up.

Table 5-4 Criteria to Determine an Identical File

Type of File Criteria to Determine an Identical File

Data file	The data file must have the same DBID, checkpoint SCN, creation SCN, and RESETLOGS SCN and time as a data file in a backup. The data file must be offline-normal, read-only, or closed normally.
Archived log	Same DBID, thread, sequence number, and RESETLOGS SCN and time
Backup set	Same DBID, backup set record ID, and stamp

If RMAN determines that a file is identical and it has been backed up, then it is a candidate to be skipped. RMAN must do further checking to determine whether to skip the file, however, 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.

RMAN uses backup optimization 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, BACKUP RECOVERY AREA, BACKUP RECOVERY FILES, or BACKUP DATAFILECOPY.

  Note:

  When TO DESTINATION is used with BACKUP RECOVERY AREA or BACKUP RECOVERY FILES, RMAN only skips backups of files that have identical backups in the TO DESTINATION location that you provide.

• Only one type of channel is allocated, do not mix disk and SBT channels in the same backup command.

Note:

In backup undo optimization, RMAN excludes undo changes (that are not needed for recovery of a backup) for transactions that have been committed. You can enable and disable backup optimization, but backup undo optimization is built-in behavior.

For example, assume that you have configured backup optimization. These commands back up to tape the database, all archived logs, and all backup sets:

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

If none of the backed-up files has changed since the last backup, then RMAN does not back up the files again. RMAN also does not signal an error if it skips all files specified in the command because the files have already been backed up.

You can override optimization at any time by specifying the FORCE option on the BACKUP command. For example, you can run:

BACKUP DATABASE FORCE;
BACKUP ARCHIVELOG ALL FORCE;

See Also:

The CONFIGURE entry in Oracle Database Backup and Recovery Reference for a complete description of the backup optimization rules

Effect of Retention Policies on Backup Optimization for SBT Backups

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

Note:

Use caution when enabling backup optimization if you use a media manager with its own internal expiration policy. Run the CROSSCHECK command periodically to synchronize the RMAN repository with the media manager. Otherwise, RMAN may skip backups due to optimization without recognizing that the media manager has discarded backups stored on tape.

Backup Optimization for SBT Backups with Recovery Window Retention Policy

Suppose that backup optimization is enabled, and a recovery window backup retention policy is in effect. In this case, when performing SBT backups RMAN always backs up data files whose most recent backup is older than the recovery window. For example, assume the following scenario:

• Today is February 21.

• The recovery window is 7 days.

• The most recent backup of tablespace tools to tape is January 3.

• Tablespace tools is read-only.

On February 21, when you issue a command to back up tablespace tools to tape, RMAN backs it up even though it did not change after the January 3 backup (because it is read-only). RMAN makes the backup because no backup of the tablespace exists within the 7-day recovery window.

This behavior enables the media manager to expire old tapes. Otherwise, the media manager would be forced to keep the January 3 backup of tablespace tools indefinitely. By making a more recent backup of tablespace tools on February 21, RMAN enables the media manager to expire the tape containing the January 3 backup.

Backup Optimization for SBT Backups With Redundancy Retention Policy

Assume that you configure a retention policy for redundancy. In this case, RMAN only skips backups of offline or read-only data files to SBT when there are r + 1 backups of the files, where r is set in CONFIGURE RETENTION POLICY TO REDUNDANCY r.

For example, assume that you enable backup optimization and set the following retention policy:

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

With these settings, RMAN only skips backups when three identical files are already backed up. Also assume that you have never backed up the users tablespace, which is read/write, and that you perform the actions described in Table 5-5 over the course of the week.

Table 5-5 Effect of Redundancy Setting on Backup Optimization

Day Action Result Redundant Backup

Monday	Take users offline normal.
Tuesday	BACKUP DATABASE	The users tablespace is backed up.
Wednesday	BACKUP DATABASE	The users tablespace is backed up.
Thursday	BACKUP DATABASE	The users tablespace is backed up.	Tuesday backup
Friday	BACKUP DATABASE	The users tablespace is not backed up.	Tuesday backup
Saturday	BACKUP DATABASE	The users tablespace is not backed up.	Tuesday backup
Sunday	DELETE OBSOLETE	The Tuesday backup is deleted.
Monday	BACKUP DATABASE	The users tablespace is backed up.	Wednesday backup

The backups on Tuesday, Wednesday, and Thursday back up the offline users tablespace to satisfy the condition that three backups must exist (one more than redundancy setting). The Friday and Saturday backups do not back up the users tablespace because of backup optimization. The Tuesday backup of users is obsolete beginning on Thursday.

On Sunday, you delete all obsolete backups, which removes the Tuesday backup of users. The Tuesday backup is obsolete because of the retention policy setting. The whole database backup on Monday then backs up the users tablespace to satisfy the condition that three backups must exist (one more than redundancy setting). In this way, you can recycle your tapes over time.

See Also:

"Backup Optimization and the CONFIGURE command"

Configuring Backup Optimization

By default, backup optimization is configured to OFF. You can use the SHOW BACKUP OPTIMIZATION command to view the current settings of backup optimization.

To configure backup optimization:

1. Start RMAN and connect to a target database and a recovery catalog (if used).

2. Run the SHOW BACKUP OPTIMIZATION command to determine whether optimization is currently enabled.

   For example, enter the following command:

   SHOW BACKUP OPTIMIZATION;
   

   Sample output for SHOW BACKUP OPTIMIZATION follows:

   RMAN configuration parameters for database with db_unique_name PROD1 are:
   CONFIGURE BACKUP OPTIMIZATION OFF;
   

3. Enable backup optimization by running the following command:

   CONFIGURE BACKUP OPTIMIZATION ON;
   

参考至:http://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmconfb.htm#BRADV138

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