Wednesday, October 3, 2012

Unused Blocks in RMAN backupsets (Null compression, Unused Block compression and Backupset compression)

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

Null 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 

RMAN> configure device type disk backuptype to compressed;

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