On 30 Jan 2002 at 16:07, Wholey, Joseph (TGA\MLOL) wrote: > With regard to compressing data twice, I disagree. There's >something very wrong with it. That's why it is strongly >recommended not to do it. (not just with TSM, but with all >data) Some data that goes thru multiple compression routines >can "blow up" to 2x the size the file started out as.
This absolutely true, but is not the whole story. Modern compression systems (programs and chips) can detect the condition of compressing uncompressable data. While It's been years, I remember pkzip detecting this condition and just storing the file - giving up any compression on the file. The unix compress utility is NOT smart. It will happily compress uncompressable data, making the file bigger. Sony uses a IBM compression chip in AIT tape drives. I once found a IBM web site that went into much detail on various IBM compression chip - this chip was included. It stated that the chip could detect when it was attempting to compress uncompressable data, and would stop compression. What is hard to find is a statement from tape drive vendors that their hdwr compression is this smart. So, at least for AIT drives, set them to hdwr compression and forget about what you send them. Rick
