Databases Reference
In-Depth Information
CONFIGURE DEVICE TYPE 'SBT_TAPE' BACKUP TYPE TO COMPRESSED BACKUPSET PARALLELISM 3;
new RMAN configuration parameters are successfully stored
RMAN>
Both in the case of disk and tape backups, you can revert to the default noncompressed backup format by
omitting the keyword
compressed
in the two commands shown in this solution.
Starting backup at 07-AUG-12
using channel ORA_DISK_1
channel ORA_DISK_1: starting compressed full datafile backup set...
How It Works
RMAN uses binary compression to produce compressed backup sets. Since a compressed backup means fewer bytes
are transmitted across the network, it makes it a lot easier for you to safely schedule a daily backup of the database
without adversely affecting other users of your network. Of course, even compression may not permit you to back up a
very large database during the backup window.
When you restore a compressed backup set, RMAN can read the backup set directly, without having to first
uncompress it, thus saving you a considerable amount of time. If you compress backup sets through some other
means, such as the Unix/Linux
tar
command, then you'll incur significant overhead in time and in disk space when
uncompressing them.
When using the RMAN compression feature, you can choose among different compression algorithms. You can
query the view V$RMAN_COMPRESSION_ALGORITHM to view the compression algorithms available to you, as
shown here:
RMAN> select algorithm_name,algorithm_description, is_default
2* from v$rman_compression_algorithm;
ALGORITHM_NAME ALGORITHM_DESCRIPTION IS_
BZIP2 good compression ratio NO
BASIC good compression ratio YES
LOW maximum possible compression speed NO
ZLIB balance between speed and compression ratio NO
MEDIUM balance between speed and compression ratio NO
HIGH maximum possible compression ratio NO
RMAN>
The ZLIB compression algorithm offers speed but not the best compression ratio. The alternate compression
algorithm, BZIP2, is slower but provides a better compression ratio. In Oracle Database 12
c,
both the ZLIB and the
BZIP2 compression algorthims are deprecated. A new compression algorithm named BASIC is the default starting
with Oracle Database 12c. The BASIC algorithm replaces the BZIP2 algorithm.
These three compression algorithms—BZIP2, ZLIB, and BASIC—are available to you out of the box and with
no additional cost. Starting with the Oracle Database 11g Release 2, you can choose from among three additional
compression levels, by enabling the Advanced Compression option. As with the free compression options, the
Advanced Compression option offers three different compression levels: HIGH, MEDIUM, and LOW.
If your network speed is the limting factor, you may want to look at the HIGH compression level. For most
environments, Oracle recommends the MEDIUM level, because it offers an ideal compromise between speed and
compression ratios. The LOW compression level has the least effect on backup throughput. Note that the MEDIUM
compression algorithm is a replacement for the ZLIB, which is deprecated starting with the Oracle Database 12.1.2 release.
