Databases Reference
In-Depth Information
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Backup sets are the only choice you have for creating level 1 incremental backups to either a tape device or
a disk device.
Note
Since a level 1 incremental backup backs up only the changed blocks, it tends to be faster than a level 0 backup in
most cases. You can make backups of the backup set type only when making a level 1 incremental backup.
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Note
RMAN makes differential incremental backups by default if you don't specify the incremental backup type.
Cumulative Incremental Backups
A
cumulative incremental backup
is an incremental backup of
all
data blocks that changed subsequently to the most
recent level 0 incremental backup. The following command shows how to make a cumulative incremental backup of
a database:
RMAN> backup incremental level 1 cumulative database;
The previous command backs up all data blocks that have changed since the last
level 0
backup.
How It Works
An incremental backup is designed to make shorter and faster backups of your data files by backing up only changed
data blocks instead of all the data blocks in a data file. RMAN uses the SCNs present in each of Oracle's data blocks in
every data file as the basis of its incremental backup policy. If the SCN of a data block in the data file that's a backup
candidate is the same or greater than the SCN of the parent incremental backup, RMAN will back up that data block.
Otherwise, RMAN will exclude that data block from the incremental backup.
The basis for all incremental backups is the parent backup, also called a
level 0
backup. A level 0 backup includes
all the data blocks in all the datafiles and serves as the base or foundation for future incremental backups. Note that
even though a full backup also includes all data blocks, it can't serve as the basis for future incremental backups—you
can use a level 0 backup only as the parent for incremental backups.
Often the choice between a cumulative differential and incremental differential backup comes down to a trade-off
between space and recovery time. If you use cumulative backups, you'll use more storage space, but you can recover
faster, since you'll need to apply fewer incremental backups. Differential incremental backups, on the other hand, take
less space to store, but you'll take more time to recover with them, because in most cases you'll need to apply a lot more
of these than the cumulative differential backups.
When you issue the following command to perform a differential incremental backup, if neither a level 1 nor a level
0 incremental backup is available, RMAN will back up all blocks changed since the creation of that data file and save the
backup as a level 1 backup (for database compatibility greater than or equal to 10.0.0).
RMAN> backup incremental level 1 database;
Here's an example of how the default differential incremental backup works in an Oracle database:
1.
Let's say you take an incremental level 0 backup on a Sunday night. This backup will
include all the blocks in the database that were used and will serve as the foundation for
future incremental backups.
2.
On Monday, you take a differential incremental level 1 backup that backs up all changed
blocks since the level 0 backup on Sunday.
