[2058] in linux-scsi channel archive
Re: compression mode for HP1533A streamer
daemon@ATHENA.MIT.EDU (ClimServ)
Tue Jun 24 08:20:16 1997
Date: Tue, 24 Jun 1997 13:01:30 +0200
From: ClimServ <climserv@info13.polytechnique.fr>
To: Uwe Schmeling <uschmeli@gibson.csa.de>
CC: linux-scsi@vger.rutgers.edu
Uwe Schmeling wrote:
>
> Hi!
> does anyone know how to get the HP1533 into compression mode?
> I've tried the following:
> 1.)Option switches 1&2 to on -> Compression enabled at power on, with
> host control
> 2.)mt datcompression /dev/st0 -> says "Compression on."
> 3.)with a 90m cartridge the HP1533 says "device full" after backing up
> about 2 GB
>
> Uwe
Hi !
You _are_ in compressed mode !
My own experience is that you cannot record much more than 1.7 GB
on a "2 GB" DAT, with compression off. You can record only a little bit
more than 2 GB of random binary data in compressed mode.
I think the annouced 2GB native is a raw capacity, which does not
take into account the blocking factor, file gaps, and so on.
A 2X compression factor may be reached when you compress very
redundant data, with long series of identical bytes. this is generally
not the case with binary files.
When I have to shrink 2.5 GB of binary, I do not use the internal
compression scheme of the HP1533. I use
...|gzip -9|dd of=<my_device> bs=<my_blocksize> conv=sync .
It takes a lot of cpu, but it is efficient in terms of compression
ratio.
jlm
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Jean-Louis Monge tel. (33.1)69 33 45 35
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