ADSM-L

OT? - DLT hardware compression

2003-04-09 15:10:48
Subject: OT? - DLT hardware compression
From: Simeon Johnston <simeonuj AT INDIVISUALLEARNING DOT COM>
To: ADSM-L AT VM.MARIST DOT EDU
Date: Wed, 9 Apr 2003 14:10:22 -0500
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

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