RMAN Backup Optimization

Backup optimization specifies that files can be omitted from the backup in some circumstances if an identical copy of the file has already been backed up.

Backup optimization is an RMAN parameter which is specified using the CONFIGURE parameter.

RMAN> CONFIGURE BACKUP OPTIMIZATION ON;

Backup optimization can be used with following operations:

BACKUP DATABASE
BACKUP ARCHIVELOG ALL
BACKUP ARCHIVELOG LIKE
BACKUP BACKUPSET ALL
BACKUP RECOVERY AREA
BACKUP RECOVERY FILES
BACKUP DATAFILECOPY

Only one type of channel can be allocated - disk and tape channels cannot be specified in the same backup command.

RMAN determines that a file is identical to one that has already been backed up based on the following rules:

Datafile - Must have same DBID, checkpoint SCN, creation SCN, RESETLOGS SCN and RESETLOGS time as a data file that has already been backed up. The datafile must be offine-normal, read-only or closed normally.
Archived log - Must have same DBID, thread, sequence number, RESETLOGS SCN and RESETLOGS time. as an archived log that has already been backed up.
Backup set - Must have same DBID, backup set record ID and stamp as a backup set that already exists

In addition RMAN considers both the retention policy and backup duplexing when determining if a file can be skipped.

Backup optimization can be overridden using the FORCE option. For example:

RMAN> BACKUP DATABASE FORCE;
RMAN> BACKUP ARCHIVELOG ALL FORCE;