Restarting and Optimizing RMAN Backups

RMAN supports two distinct features by which it can back up only those files that require backups: restartable backups and backup optimization.

With the restartable backup feature, RMAN backs up only those files that were not backed up after a specified date. For example, by specifying the NOT BACKED UP SINCE TIME clause, you can direct RMAN to back up only those files that were not backed up within the last day.

With backup optimization, the BACKUP command skips the backup of a file if the identical file has already been backed up to the allocated device type. To override this behavior and back up all files whether or not they have changed, specify the FORCE option on the BACKUP command. To enable or disable backup optimization, specify ON or OFF on the CONFIGURE BACKUP OPTIMIZATION command.

Additionally, BACKUP ... PLUS ARCHIVELOG can archive unarchived online logs as well as back up archived logs.

Backing Up Files Using Backup Optimization

For backup optimization to be enabled, you must CONFIGURE BACKUP OPTIMIZATION to ON. Backup optimization is OFF by default.

To use backup optimization with a backup operation:

If you have not already enabled backup optimization, then enable it by running the CONFIGURE BACKUP OPTIMIZATION command. For example, enter:
```
RMAN> CONFIGURE BACKUP OPTIMIZATION ON;
```
Back up the desired files. The following example backs up to an sbt device any archived redo logs that either have not been already backed up, or where the existing backups do not satisfy the current duplexing setting:
```
RMAN> BACKUP DEVICE TYPE sbt ARCHIVELOG ALL;
```
RMAN does not signal an error when it skips backing up files due to backup optimization.

See Also:
"Backup Optimization" for a conceptual overview of optimization and backup retention policies

Restarting a Backup After It Partially Completes

Use the SINCE TIME parameter of the BACKUP command to specify a date after which a new backup is required. If you do not specify the SINCE parameter, then RMAN only backs up files that have never been backed up.

To only back up files that were not backed up after a specified date:

Specify a valid date in the SINCE TIME parameter. For example, this command uses the default configured channel to back up all database files and archived redo logs that have not been backed up in the last two weeks:

RMAN> BACKUP NOT BACKED UP SINCE TIME 'SYSDATE-14'
          DATABASE PLUS ARCHIVELOG;

Backups to CD, DVD and Other Disk Devices with Large Block Sizes

When backing up database files to DISK, the logical block size of the database file to be backed up must be an even multiple of the physical block size of the destination device. For example, a DISK device with a block size of 2K can only be used as a destination for backups of database files with logical block sizes of 2K, 4K, 6K and so on. In practice, most disk drives have physical block sizes of 512 bytes, so this limitation rarely affects backup to disk drives. However, you can encounter this limitation when using BACKUP ... DEVICE TYPE DISK to back your database up to a writeable CD or DVD, or some other device which has a larger physical block size.