On Fri, Sep 09, 2005 at 11:07:16AM +0100, Rodrigo Ventura wrote:
> 
> Hum, I'm a bit confused here... I read somewhere that it was not
> trivial to disable hardware compression. I have an autoloader with a
> DAT 72 drive, connected to a linux server. The command mt
> datcompression 0 Compression off does not seem to work.

A relevant question is how are you detecting the compression state
of the compression drive before and after the mt command?

> 
> I read that the hardware compression flag was written on the tape, and
> to disable it I had to "clear" that bit in all tapes. Is this true?
> How can I do that?

See regular postings by Gene Heskett where he details his procedure.

I don't think it is actually a 'flag', but merely the presence of HW
compressed data at the front of the tape.  The compression setting
is only relevant when writing tapes.  Reading switches automatically
based on the type of data (HW compressed or not) written before.
And that "auto switch" is what causes the problem.  You set the
tape to compression off, read a bit of compressed data (like the
amanda label at the start of the tape previously written with
compression on) and the drive "auto switches" to compression on.

To clear this state for amanda you have to write a bunch of data
at the beginning of the tape, after setting compression off, and
be sure to not read anything between the two.

> 
> I also read that there was non-documentated dip switch settings on the
> drive that could disable hardware compression. I have a HP DAT
> 72x6. The manuals just states the correct dip switch settings for each
> operating system. Any way I could get the info on what exactly each
> dip switch does?

Oh, it is well documented.  One thing HP does pretty good is document.
Go to their support website for storage, tape drives.  Locate your
model (may have to get to some legacy section, maybe not).  There
will be a listing for user manuals or docs or somesuch.  Download
not just the user manuals, but the technical docs too.

I have a dds2 drive and a dds3 autoloader.  Recently I glanced at
the docs for dds4 & 5 drives.  Switch is still there from what I saw.

On my dds2 drive I have to take the cover off to access the switch
from the bottom.

On the bottom of my dds3 6x24 autoloader there is a 3 cm cave about
5 cm deep exposing the dip switch.

Two of the dip switches are relevant.  I don't know their numbers.
One says, when powered up, what state is compression set to, on or off.
The second says can this state be changed by software commands, yes or no.
I.e. whether you can change the power up state with your mt command.

For my two drives the manufacturer's defaults were on and yes.

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
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