When backing up your Oracle database to tape, always use hardware compression if your tape devices support it. This reduces backup time because more data can be written to a single volume. Tape units with hardware compression are now industry-standard. The compression method used is normally based on the Lempel-Ziv algorithm. A few operating systems also support hardware compression for disk.

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, but this can vary as follows:

• 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, you must compress the affected tablespaces again. If a database file has no essential changes in two consecutive compression runs, you can consider the compression rate to be constant. You only need check the compression rate again after a longer period (for example, after a year). 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. You can use parallel compressions to determine the compression rates (that is, without starting a backup).