A full backup of a datafile is a backup that includes every used data block in the file. If a full datafile backup is created as an image copy, the entire file contents are reproduced exactly. However, If backing the datafile up to a backup set, then unused blocks may be skipped as discussed below.
1. Null Compression
2. Unused Block compression
3. Binary Backupset compression
In Oracle 9i, RMAN would not backup empty blocks above the high watermark. This is called Null compression.
Unused Block compression:
In Oracle 10g Release 2 RMAN would not backup unused blocks orempty blocks below the High watermark
Binary Backupset compression:
As of Oracle10g, rman can compress backupsets that significantly reduces the space needed for the backup up to one fifth of the size comparing to uncompressed backup.
Backup set compression can be enabled as below