What Backup Optimization does in RMAN backup

•You can see whether your backup optimization is on or off by issuing following commands,

RMAN> SHOW BACKUP OPTIMIZATION;
RMAN configuration parameters are:
CONFIGURE BACKUP OPTIMIZATION OFF; # default

By default it is OFF. To make it ON issue,

RMAN> CONFIGURE BACKUP OPTIMIZATION ON;

Now the question is if backup optimization is on then how RMAN behaves while taking backup. In the following section I will try to make understand.

•In a straight-forward 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. Now question is how RMAN check whether the file is identical or not?

-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.

-In case of archived redo log RMAN compares thread, sequence number, and RESETLOGS SCN and time.

-In case of backupset RMAN compares recid and stamp.

Besides these comparison RMAN further checks 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 applies to only commands,
-BACKUP DATABASE
-BACKUP ARCHIVELOG with ALL or LIKE options
-BACKUP BACKUPSET ALL

•You can override optimization at any time by specifying the FORCE option on the BACKUP command.
Like,

RMAN>BACKUP DATABASE FORCE;
RMAN>BACKUP DEVICE TYPE sbt BACKUPSET ALL FORCE;