Incremental Differential vs. Incremental Cumulative Backups

Incremental Differential vs. Incremental Cumulative Backups

Question:  What are RMAN differential Backups and how are they different from cumulative RMAN backups?  Are these the same as an RMAN incremental backup?

Answer:  Differential and cumulative are the two types of RMAN incremental backups.  It’s very confusing because RMAN differential backups are sometimes called "cumulative incremental backups" while RMAN incremental backups are sometimes called "differential incremental backups":

·         Differential backup (for level 1 parents only):  The default is differential backup, where RMAN looks for last level 1 or level 0 backup, and a differential backup only captures those data block changes that were made after those backups.  Differential backups are faster because there are fewer changes stored, but they take longer at recovery time.

·         Cumulative backup (for level 0 or level 1 parents):  With a cumulative backup, RMAN backups up all data block changes that are made after a level 0 backup. The main advantage of cumulative backup over differential is the fast recovery time, but at the expense of a longer daily backup window and more disk usage.

In a nutshell we see these advantages and disadvantages governing your choice of incremental cumulative vs. incremental differential backups. 

·         Recovery speed:  Cumulative backups are faster to restore from than differential backups because fewer incremental backups need to be applied during recovery.

·         Backup speed:  For daily backup speed, differential backups run faster than cumulative backups because to don’t duplicate the work done by previous backups.  However, differential backups take longer when doing a recovery.

·         Disk space usage: Cumulative backups take more disk space because they duplicate the work done by previous backups:

Essentially, it’s a tradeoff between disk cost and recovery speed.  All else being equal, the approach with the fastest recovery time is best because it minimizes unplanned downtime, and if you have plenty of disk and plenty of time to take nightly backups, you would choose incremental cumulative.   If disk space is at a premium and you have only a short window to take daily backups, you would choose incremental differential, knowing that it will take longer to recover.

Differential Incremental Backup (for level 1 parents only)

 When using differential incremental backup, RMAN looks for changed data blocks which were changed after last level 1 incremental backup. It there’s no level 1 backup made before it, it takes a backup of the changed data blocks which were made after level 0 incremental backup.

RMAN differential incremental backup.  (Source: Oracle Corporation)

Cumulative Incremental Backup (for level 0 or level 1 parent backups)

In a cumulative incremental backup RMAN takes backup of all changed data blocks after level 0 or level 1 incremental backup.  Like a differential backup, incremental backups also back up only the changed data blocks, but an incremental backup only backs up the data that has changed since the last backup. 

If the last backup was also an incremental backup, the current incremental backups only records “changes to the changes”, a much smaller set of block changes, and hence, a much smaller recovery time than a differential backup. 

RMAN Cumulative incremental backups (Source: Oracle Corporation)

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