2.) Unused Block Compression
There is no special command to restore database from the compressed backupsets. The restore command will be the same as with uncompressed backups.The restore from the compressed backpuset will take more time than uncompressed backupsets.
To use rman compression option, we can run the following RMAN commands to configure compression
RMAN> CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO COMPRESSED BACKUPSET;
followed by ..
RMAN> CONFIGURE COMPRESSION ALGORITHM ‘HIGH’ ;
RMAN> CONFIGURE COMPRESSION ALGORITHM ‘MEDIUM’ ;
RMAN> CONFIGURE COMPRESSION ALGORITHM ‘LOW’ ;
RMAN> CONFIGURE COMPRESSION ALGORITHM ‘BASIC’ ;
The compression levels are BASIC, LOW, MEDIUM and HIGH and each affords a trade off related to backup throughput and the degree of compression afforded. If we have enabled the Oracle Database 11g Release 2 Advanced Compression Option, then we can choose from the following compression levels :
- HIGH - Best suited for backups over slower networks where the limiting factor is network speed
- MEDIUM - Recommended for most environments. Good combination of compression ratios and speed
- LOW - Least impact on backup throughput and suited for environments where CPU resources are the limiting factor.
SQL> select * from V$RMAN_COMPRESSION_ALGORITHM;
I found a good scenario on net related to compression level having statistics about the this compression level . Here is the scenario : The environment being used was a freshly created 11g Release 2 database with some smaller tables in it. The total sum of all segments equals to 4.88 GB. All database data files excluding the temporary ones are 7.3 GB total. Excluding temporary and undo data files total size equates to 5.9 GB.
Here is the test results displays of the compression level :
If we want to test and optimize our rman backup, we basically have three major switches to play with :
- compression algorithmn
- rman parallelism and
- data transfer mechanism (SAN or Ethernet [this includes: iSCSI, NFS, CIFS, Backup to tape over Ethernet])
Enjoy J J J