RMAN uses two types of block compression when creating backup sets:
1. Unused Block Compression (Supports disk backup and OSB tape backup)
2. Null Block Compression (Supports all backups)
Unused Block Compression
During unused block compression, RMAN does not check each block. Instead,
RMAN reads the bitmaps that indicate what blocks are currently allocated and then
only reads the blocks that are currently allocated.
Unused block compression is turned on automatically when all of the following five
conditions are true:
1. The COMPATIBLE initialization parameter is set to 10.2 or higher.
2. There are currently no guaranteed restore points defined for the database.
3. The datafile is locally managed.
4. The datafile is being backed up to a backup set as part of a full backup or a level 0 incremental backup.
5. The backup set is created on disk or Oracle Secure Backup is the media manager.
Null Block Compression
During null block compression, RMAN checks every block to see if it has ever
contained data. Blocks that have never contained data are not backed up. Blocks that
have contained data, either currently or at some point in time in the past, are backed up.
Binary Compression for Backup Sets
RMAN supports binary compression of backup sets. Backup compression is only
enabled when you specify AS COMPRESSED BACKUPSET in BACKUP command, or
one-time with: CONFIGURE DEVICE TYPE
You have two binary compression options:
You can use the BASIC compression algorithm which does not require the
Advanced Compression Option. This setting offers a compression ratio
comparable to MEDIUM, at the expense of additional CPU consumption.