RMAN backup optimization is another feature to minimize backup size. Enabling this feature, RMAN BACKUP will skip backup of files that comes in certain condition or exactly identical to the file already backup up.

As the name indicates the feature is for optimizing the RMAN backups. When you turn on this backup optimization parameter, RMAN skips the backing up the files which has been already been backed up to the same device. The main benefit out of this is it will reduce the unwanted space usage.

This is how the RMAN determines the identical files for different categories. Below mentioned are the only applicable categories of files.

Datafile: File must be offline-normal or read-only or closed normally; with same DBID, checkpoint SCN, creation SCN and resetlogs SCN and time as a datafile already in the backup.

Archived Log: the file must be with same DBID, thread, sequence number and resetlogs SCN and time.

Backup Set: The file must be with same DBID, backupset record ID and time stamp.

Below are the criteria when RMAN check which files can be skipped from the backup.

  1. In case of datafile RMAN compares the DBID, checkpoint SCN, creation SCN, and RESETLOGS SCN and time as a datafile already in a backup. If they are identical then skip to take backup again.
  2. In case of archived redo log RMAN compares thread, sequence number, and RESETLOGS SCN and time.
  3. In case of backupset RMAN compares recid and stamp.

By default the backup optimization will be turned off. You have to use following command to enable the same.

You can override the optimization at any time by using FORCE option on the backup command.

  • BACKUP DATABASE FORCE;
  • BACKUP ARCHIVELOG ALL FORCE;
  • BACKUP BACKUPSET ALL;

By Default it is set to OFF:

Example:

Initiating archivelog backup

In second backup it will skip the files which are already backed up.

The force option will take the backup of all available archivelog files even if it is backed up already.

Source

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