Databases Reference
In-Depth Information
Making a Compressed Backup
Problem
You want to compress RMAN backups to save storage space.
Solution
Specify the
as compressed backupset
option with your
backup
command to direct RMAN to produce a binary
compressed backup set, as shown in the following example:
RMAN> backup
2> as compressed backupset
3> database plus archivelog;
2> 3>
Starting backup at 21-SEP-12
current log archived
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=52 device type=DISK
skipping archived logs of thread 1 from sequence 29 to 59; already backed up
channel ORA_DISK_1: starting compressed archived log backup set
...
Starting backup at 21-SEP-12
using channel ORA_DISK_1
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
The previous command will back up all data files and the archived redo log files as a compressed backup set.
The backup may be made to disk or tape, depending on which one you configured as the default backup destination.
How It Works
RMAN's compression capabilities are especially useful when you're backing up to disk and confront a tight disk space
situation. Just make sure you schedule the compressed backups during a low database usage period, because of the
higher CPU overhead for compression.
You don't need to explicitly uncompress a compressed backup during recovery. RMAN recommends that you not
use RMAN's backup set compression feature if you're backing up to a tape device and the media manager is using its
own compression capability.
You don't have to worry about any extra work during the recovery of a compressed backup—you restore
a compressed backup the same way as an uncompressed backup set.
By using what's called
unused block compression
, Oracle reads only the currently allocated blocks to a database
table and doesn't write the unused blocks to the backup. If the database satisfies the following five conditions, RMAN
automatically applies the
unused block compression
feature.
compatible
initialization parameter must be set to 10.2 or higher.
•
The
•
There are no guaranteed restore points in the database.
•
The data files are locally managed.
•
You must either back up to disk, or use Oracle Secure Backup as the media manager.
•
You're backing up the data file to a backup set, either as part of a full backup or a level 0
incremental backup.
