RMAN uses two types of block compression when creating backup sets:
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Unused Block Compression (Supports disk backup and Oracle Secure Backup tape backup)
Null Block Compression (Supports all backups)
Although it is referred to as block compression, it might be helpful to think of block compression as block skipping. Rather than compressing the data in the blocks, RMAN completely eliminates the blocks from the backup. RMAN reads blocks from a data file and writes them to a backup set. If unused block compression is enabled, RMAN reads only those blocks of the data file in an extent that currently belongs to a database segment. Unused block compression can only be employed when backing up tablespaces whose space is locally managed. When not employing unused block compression, RMAN reads every block of the data file. Before RMAN writes a block to the backup set, it applies the null block test, and if the block has never been used, it does not write that block to the backup set
