RMAN compression

RMAN Compression:

There are 3 three types of compression by default:

1.      Null compression

·        Usages only till Oracle version 10.1 by default

·        RMAN skips the block that have never been used when backing up datafiles into backup sets.

·        E.g. let’s tablespace having once datafile of size 100MB and out of 100 MB only 50 is used. Then RMAN will backup only 50 MB.

2.      Unused block compression

·        Oracle version 10.2 forward, uses ‘Null’ and ‘unused block’ compression.

·        More compact backups of datafiles by skipping datafile blocks that are not currently used to store data.

·        In previous release, RMAN only supported NULL compression, which skipped space in datafiles that had never been allocated.

Example

§ User01 – 100MB out of 100MB, 50 MB is used by user tables.

Then user dropped a table belonging to that tablespace which was of 25MB, with the new unused block compression on 25MB of the files is backed up.

§ In this case if null compression is used then it would have backed up 50MB because null compression will consider the blocks that are formatted/ever used.

·        Unused block compression is done, if all the conditions met:

ü  The compatible initialization parameter is set to 10.2

ü  There are currently no restore points defined for the database

ü  The datafile is locally managed

ü  The datafile is being backed up to a backup set as part of a full backup or a level  0 incremental backup

ü  The backup set is being created on DISK or backup is done to tape using “OSB” (oracle secure backup)

Note:

o   Unused block compression is NOT used if backup done to tape using a THIRD PARTY BACKUP SOFTWARE

o   Unused block compression is ONLY used in Enterprise Edition.

3.      Binary compression

Binary compression can be done by specifying “AS COMPRESSED” clause in backup command, this compression is called as binary compression.

This binary compression algorithm can greatly reduce the space required for disk backup storage. It is typically 2x to 4x and greater for text-intensive database.

The command to take the compressed backup:

RMAN> backup as compressed backupset database;

Ø  There is some CPU overhead associated with compressing backup sets. If the database being backup up is running at or near its maximum load, may find the overhead from using AS COMPRESSED BACKUPSET unacceptable. In most other circumstances, compressing backupsets saves enough disk space to be worth the CPU overhead. 

Ø  There is no special command to restore database from compressed backupsets, the restore command will be same as with uncompressed backups. 

Ø  The restore from the compresses backupset will take more time than uncompressed backupsets. 

Ø  In addition to existing binary compression of backup in ORACLE 10G, RMAN 11G executable offers a wider range of compression levels with the Advanced Compress Option (ACO). The default compression algorithm setting is BASIC and does not require the Advanced Compression Option. 

Ø  If , however , enabled the Oracle Database 11g Release 2 Advanced Compression Option  , can choose from the following compression levels:

o   LOW: Least compression. Performance of database and backup is good. Advanced Compression License required. 

o   HIGH: Highest Compression. Performance of database and backup is low. Advanced Compression License required. 

o   MEDIUM: Recommended. Good Combination of Ratio and Performance. Advanced Compression License required.

RMAN compress the backupset contents before writing to disk. No extra decompression steps are required during recovery for rman compressed backup:

To configure the compression algorithm:

RMAN> CONFIGURE COMPRESSEION ALGORITHM <’<alg_name>’;

 Various Compression Types:

SQL> 

select ALGORITHM_NAME, ALGORITHM_DESCRIPTION, ALGORITHM_COMPATIBILITY from

V$RMAN_COMPRESSION_ALGORITHM;      

            

ALGORITHM_NAME	ALGORITHM_DESCRIPTION	ALGORITHM_COMPATIB
BZIP2	good compression ratio	9.2.0.0.0
BASIC	good compression ratio	9.2.0.0.0
LOW	maximum possible compression speed	11.2.0.0.0
ZLIB	balance between speed and compression ratio	11.0.0.0.0
MEDIUM	balance between speed and compression ratio	11.0.0.0.0
HIGH	maximum possible compression ratio	11.2.0.0.0

RMAN Compressed Backup Command

Oracle 10g

RMAN> BACKUP AS COMPRESSED BACKUPSET DATABASE PLUS ARCHIVELOG;

Oracle 11g/12c

Without Advanced Compression

RMAN > CONFIGURE COMPRESSION ALGORITHM 'BASIC';

RMAN> BACKUP AS COMPRESSED BACKUPSET DATABASE PLUS ARCHIVELOG;

With Advanced Compression

RMAN > CONFIGURE COMPRESSION ALGORITHM 'HIGH';

RMAN> BACKUP AS COMPRESSED BACKUPSET DATABASE PLUS ARCHIVELOG;

As mentioned earlier, with Advanced Compression can be 'HIGH', 'MEDIUM', 'LOW'.

Pre-compression Block Processing

It helps in finding the block suitable for binary compression. Consolidates free space in each block and setting that free space into binary zeroes.

The OPTIMIZE FOR LOAD option controls pre-compression processing.

By specifying the default, OPTIMIZE FOR TRUE, we ensure that RMAN optimizes CPU usage and avoids pre-compression block processing.

By OPTIMIZE FOR LOAD FALSE, RMAN uses additional CPU resources to perform pre-compression block processing.

Undo Block Compression/Optimization

Starting 11g, RMAN performs undo block optimization. In backup undo optimization, RMAN excludes undo not needed for recovery a backup, that is, for transactions that have been committed. Undo optimization is only possible if:

·        This is backup set backup

·        Full or incremental level 0

·        Not a validate

·        Backup piece version is 11.0 or above

· User has not disabled undo optimization with hidden parameter: _undo_block_compression=FALSE

·        Backup is going to DISK or OSB tape

·        No Guaranteed Restore Point (The check is enabled from 11.2 onward)