When backup devices with hardware compression are used, BRBACKUP requires the current compression rates of the database files in order to determine the quantity of data to be saved after the hardware compression. Only in this manner can BRBACKUP ensure that the specified tape size is not exceeded and that the database files are correctly divided between the tapes.
BRBACKUP can only estimate the quantity of data that can be written to the volume, because BRBACKUP cannot directly determine the compression rates for hardware compression. BRBACKUP uses the software compression rates as an estimate. SAP therefore assumes that hardware and software compression return similar results. See compress_cmd.
Before the first backup using tape devices with hardware compression, you should start a compression run to determine the compression rates: brbackup -k only. This call does not actually start a backup. It only determines the compression rates. The database files are only compressed (not saved) and the determined compression rates are stored in table SDBAD and in a detail log.
For more information, see Logs for BR*Tools.
Repeat this activity at least once a month to update the compression rates.
Also repeat this activity after a reorganization or after the loading of a large amount of data.