[2063] in linux-scsi channel archive
Re: compression mode for HP1533A streamer
daemon@ATHENA.MIT.EDU (Pete Popov)
Tue Jun 24 13:31:08 1997
To: ClimServ <climserv@info13.polytechnique.fr>
cc: Uwe Schmeling <uschmeli@gibson.csa.de>, linux-scsi@vger.rutgers.edu
In-reply-to: Your message of "Tue, 24 Jun 1997 13:01:30 +0200."
<33AFA90A.560C7FEE@info13.polytechnique.fr>
Date: Tue, 24 Jun 1997 10:26:34 -0700
From: Pete Popov <pete@jones.asd.sel.sony.com>
Hi,
> 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.
The 2GB spec refers to 2,000,000,000 bytes of data, which is the
nominal drive capacity. However, one of the ways DAT drives
ensure data integrity is by using a Read-After-Write (RAW) algorithm.
Because the DAT drive's head drum assembly has 4 heads, the drive
can read its own data on the fly, and determine if the data needs
to be rewritten. If the data needs to be rewritten, the drive
does NOT reposition, instead it rewrites the data over again. If
the RAW rate is "normal", the drive will still achieve the
2GB spec. However, if the media is old, worn out, or the drive's
head is dirty, or the drive is out of alignment ...etc, the RAW
rate can indeed reduce the nominal capacity.
>You can record only a little bit
> more than 2 GB of random binary data in compressed mode.
True, binary data will not compress much at all. In fact, if the
data is truly random, it will expand.
> I think the annouced 2GB native is a raw capacity, which does not
> take into account the blocking factor, file gaps, and so on.
Yes, it is raw _nominal_ capacity.
> 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.
Not true. The 2X is the "average" compression ratio of the DCLZ
algorithm that DDS DAT drives use. This number is the average of
the compression ratio of text files, binary files, graphic image
files, etc. It's true that binary files will not compress much,
but other files, such as text files, will compress a lot, so
you don't need "highly redundant data" in order to achieve the
2X compression ratio.
> 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.
I'm not sure that gzip is so much more efficient, if it's more
efficient at all, to make it worth increasing the backup time
by so much.
Pete
