About RMAN Backups

Database backups created by RMAN are stored as image copies or backup sets.

Image copies are exact byte-for-byte copies of files. You can create an image copy by copying a file at the operating system level. Unlike copying files at the operating system level, however, image copies created through RMAN or Database Control are recorded in the RMAN repository so that RMAN can use these copies during database restore operations and recovery. RMAN can restore files only if they are recorded in the RMAN repository. RMAN can create image copies only on disk.

Backup sets are logical entities produced by the RMAN BACKUP command. This command can produce one or more backup sets on disk or media management devices. RMAN can write backup sets only to a media manager.

Each backup set contains several physical files called backup pieces. A backup piece stores the backup of one or more database files in a compact RMAN-specific format. One advantage of backup sets is that RMAN uses unused block compression to save space in backing up data files. Only those blocks in the data files that have been used to store data are included in the backup set.

RMAN depends on server sessions, processes that run on the database server, to create backups and restore them. Each server session, in turn, corresponds to an RMAN channel, representing one stream of data to or from a backup device. Channels are either of type disk or type SBT (tape).

RMAN supports parallelism, which is the use of multiple channels and server sessions to perform the work of one backup or recovery task. Proper use of parallelism can greatly increase performance on backup and recovery tasks.