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) See my additional notes on Incremental RMAN backups. |
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