Ok, I finally found this statement in SG24-6946, which is an old book. Can't find anything that explains how it achieves the result:
"The 3592 Model J1A Tape Drive has ..... (snip) It uses an optimal dynamic compression method called byte level compression scheme swapping, which is designed to achieve maximum data compression, and unlike other tape drive compression methods, it is designed to prevent data expansion." -----Original Message----- From: ADSM: Dist Stor Manager [mailto:ads...@vm.marist.edu] On Behalf Of Richard Sims Sent: Thursday, December 09, 2010 1:36 PM To: ADSM-L@VM.MARIST.EDU Subject: Re: [ADSM-L] ULTRIUM4C but not 1.6TB On Dec 9, 2010, at 12:26 PM, Scott McCambly wrote: > ... > > I always assumed that the hardware compression mechanism would have > something equivalent to "CompressAlways=NO" and detect already > compressed data in its input buffer, however we backup a number of > already compressed file formats and often see FULL tapes with estimated > capacities from 5 to 20% less than the stated native (uncompressed) > capacity. Technote 1267820 at least contributes to explaining this, where the effects are familiar to old MVS programmers using Fixed Block processing and dealing with short blocks. In this context, consider the impact that in-Aggregate reclamation will contribute. Richard Sims http://people.bu.edu/rbs