rman>backup full database format = 'd:test.bak';
Unused Block Compression Of Datafile Backups to Backup Sets
When backing up datafiles into backup sets, RMAN does not back up the contents of data blocks that have never been allocated. (In previous releases, this behavior. was referred to as NULL compression.)
RMAN also skips other datafile blocks that do not currently contain data, if all of the following conditions apply:
■ The COMPATIBLE initialization parameter is set to 10.2
■ There are currently no guaranteed restore points defined for the database
■ The datafile is locally managed
■ The datafile is being backed up to a backup set as part of a full backup or a level 0 incremental backup
■ The backup set is being created on disk.
Skipping unused data blocks where possible enables RMAN to back up datafiles using less space, and can make I/O more efficient.
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