In an online backup ( backup_type = online_split) the system remains available the whole time. The tablespaces to be backed up are merely set to BACKUP status temporarily while the mirror disks are split. In comparison to the normal online backup, in which the tablespaces retain this status during the whole backup period, the extent of the redo information is significantly reduced.
Online backup in the split mirror disks configuration contains the following steps:

1. Start BRBACKUP on the backup host.
2. BRBACKUP sets the tablespaces to be backed up to the status ‘BACKUP’
   (ALTER TABLESPACE.....BEGIN BACKUP).
3. Splitting of the mirror disks ( split_cmd ).
4. BRBACKUP resets the tablespaces to the normal status
   (ALTER TABLESPACE....END BACKUP).
5. Backup of the mirror disks.
6. Synchronization of the mirror disks with the originals ( resync_cmd ). The resynchronization process requires no CPU on the productive machine if, for example, you use an EMC disk system.

Split Mirror Backup: Software Configuration

Description of Parameters