Always use this option if your tape devices support hardware compression. This reduces back up time because more data can be written to a single volume. Tape units with hardware compression are very common. The compression method used is normally based on the Lempel-Ziv algorithm.

The amount of data that can actually be written to a tape depends on the compression rate. The average compression rate is between 3 and 5, however this can vary for several reasons:

It is lower if

• the data is mostly already compressed. the compression rate does not improve if the files are compressed again.

It is higher if

• New or relatively empty database files are compressed.

After a large data transfer or a reorganization of a tablespace, the affected tablespaces must be compressed again. If a database file has no essential changes in two consecutive compression runs, this compression rate can also be seen as a constant. A further check of the compression rate must only be carried out after a longer period (for example, after a year).

BRBACKUP can optimize a backup on tape units with hardware compression if the current compression rates are known before starting the backup. To do this, there is a BRBACKUP option which approximates compression rates: brbackup -k only. Repeat this activity at least once a month to update the compression rates.

Compression can take a very long time when large databases are involved (around 100 hours for 300-400 GB) and do not result in much change in compression rates. You can exclude these files from regular compression since the compression rate stays constant. To reduce compression time for large databases you can reduce the amount of data by compressing database files individually, or excluding them from compression. You can also run multiple compressions in parallel. Parallel compressions to determine the compression rates (without starting a backup) are used.