How is the TMC volume record COMPRES value calculated?
Release : 14.0
Component : CA 1 Tape Management
During CLOSE-output processing, we get the RBL (Read Buffered Log) and take the channel-bytes-written (CHAN) and bytes-stored values (COMP).
You can collect these values if you issue the IBM TS7700 Library Request LVOL command with INFO keyword (see SIZE and COMP values )
COMPRES = (CHAN - COMP) / CHAN
Before APAR SO12740 was written, if the obtain compression ratio is 99.5 or above, the
compression ratio is rounded up to 100 producing invalid results when reports are generated or the data is viewed through Vantage.
With APAR SO12740 we stopped "rounding up" if the calculated compression ratio was 99.5 or above. So, instead of rounding 99.6 compression to 100 we leave it as 99 compression. While technically 100% is closer to the truth; a compression ratio of 100% means that nothing was written (which is not true). And since our compression ratio is only done as an actual numeric value (0-100) we changed to reflect it as 99 compression.