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