In my previous blog post “RMAN Compression Algorithms in 11gR2”, I explained the different compression options and did some testing to determine how RMAN compression affects the time it takes to do a backup. As a follow-up to that, I wanted to find out how RMAN compression will affect recovery time.

For this test, I created about 19GB worth of data in the USERS tablespace, did a backup using each of the 5 available compression algorithms (including NONE) and then did a restore using each of the 5 backups. I ran through the test a couple of times to make sure the timings were consistent.

Test script:

CONFIGURE COMPRESSION ALGORITHM 'BASIC';
backup as compressed backupset database format '/tmp/basic/db%U.dbf' tag ‘basic’;
CONFIGURE COMPRESSION ALGORITHM 'LOW';
backup as compressed backupset database format '/tmp/basic/db%U.dbf' tag ‘low’;
CONFIGURE COMPRESSION ALGORITHM 'MEDIUM';
backup as compressed backupset database format '/tmp/basic/db%U.dbf' tag ‘medium’;
CONFIGURE COMPRESSION ALGORITHM 'HIGH';
backup as compressed backupset database format '/tmp/high/db%U.dbf' tag ‘high’;
CONFIGURE COMPRESSION ALGORITHM 'NONE';
backup database format '/tmp/none/db%U.dbf' tag ‘none’

sql “alter tablespace USERS offline”;
host “rm users01.dbf”;
restore tablespace USERS from tag ‘basic’;
host “rm users01.dbf”;
restore tablespace USERS from tag ‘low’;
host “rm users01.dbf”;
restore tablespace USERS from tag ‘medium’;
host “rm users01.dbf”;
restore tablespace USERS from tag ‘high’;
host “rm users01.dbf”;
restore tablespace USERS from tag ‘none’

Test Results:

Interestingly, the time to restore is pretty close to the backup time except in the case of HIGH compression where it took 88 minutes to backup and only 16 minutes to restore. The overall quickest way to backup and recover is to use LOW compression but the BASIC option still looks pretty good considering it is the one that doesn’t cost any extra. However, if you have a strict recovery SLA then using either LOW or MEDIUM may be worth the cost in order to minimize downtime.

My initial assumption was that RMAN compression would add processing time to both backup and recovery operations and would therefore be slower. In this case, that turned out not to be true; using compression makes them go faster. With a different mix of data and hardware, you could certainly get different results and I encourage you to try it out. I would be interested to hear your findings.