>From quantum's web site:

Compression Algorithms
One last compression related issue: how is compression accom-plished?
In most cases, compression is done by the tape drive itself.
Several different compression technologies are used. Quantum
DLTtape systems use the LZ (Lempel Ziv) compression algorithm. <SNIP>

So, basically, yes, the tape drive runs a compression algorithm within the
drive.  And it's not so much "accelerated" in the hardware as it offloads
the work of doing compression on your computer itself.

Alex Paschal
Freightliner, LLC
(503) 745-6850 phone/vmail


-----Original Message-----
From: Simeon Johnston [mailto:simeonuj AT INDIVISUALLEARNING DOT COM]
Sent: Wednesday, April 09, 2003 12:10 PM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: OT? - DLT hardware compression


This isn't really a TSM specific question.
I have several DLT 4000 drives here.  2 are used by Retrospect and one
will eventually be used by our TSM server.
What I'm wondering is what exactly happens when the data is compressed
in hardware by the drives?  At one point I read somewhere that when
compression is on it uses 2 writing heads instead of one (that doesn't
make sense, why not use 2 heads all the time then?), from someone else I
heard that the tape was just slowed down and that's how it was
compressed.
>From someone else I was told it was like software compression, i.e. if
the file is already compressed, hardware compression won't compress it
further (so jpg's and other compressed files are not compressed).  This
doesn't make any sense as it would require the tape drive to have some
kind of standard software compression that is accelerated by being done
in hardware (maybe that last sentence isn't quite clear, but I don't
know how to say it any better ATM).

What exactly is the deal with DLT hardware compression?  And I do
realize that it is probably slightly different for different depending
on who made it, but since DLT IV tapes can always to ~20GB/~40GB
compressed I don't see how much different it could possibly be.

sim