Oracle RMAN backup to disk is fast but slow to tape when the database is large and mostly empty. (4045883)

Date - 11/2007
Affected Product & Version - All Support Version
Affected Module & Version - APM for Oracle (Version 5.0), Oracle RMAN APM 4.5
OS Version - All Support Version
Application Information - Oracle 10.2.0.2 and Up

Description:
Oracle RMAN backup to disk is fast but slow to tape when the database is large and mostly empty.

Symptoms:
When using RMAN to backup to disk, the backup finishes at the expect time. However when backing with to tape (sbt_tape) using either CLI or GUI, backup takes a longer than expected time to finish.
Also job status is "Writing to Media" but there is no writing activities on the tape for a period of time.

The backup validate command can be use to verify the extra datafile read when the backup is to tape.

For example:

For disk backup

Run {
allocate channel c1 device type disk;
backup full validate
(datafile "DATAFILE_LOCATION_AND_NAME");
release channel c1
}

For Tape backup

Run {
allocate channel c1 device type 'sbt_tape';
backup full validate
(datafile "DATAFILE_LOCATION_AND_NAME");
release channel c1
}

The finish time for the tape is significantly higher if the datafile is largely empty.

From 10gR2, Oracle introduce a feature call "Unused Block Compression" which allows RMAN to skip other datafile blocks that do not currently contain data during backup. However this feature only works if the backup is writing to disk or using the Oracle Secure Backup product, and other criteria relating to this feature. For more information regarding this feature, please refer to your Oracle documentation.

The only way to speed up the backup is to reduce the datafile so it does not take time to read the complete datafile while backing up to tape.