Bingo, the -B option did the trick.

yf

Date:    Tue, 21 Mar 2000 18:58:04 EST
To:      Yoseff Francus <francus@yossi.com>
From:    Dave Archer <dmarcher@pobox.com>
Subject: Re: Configuring a SEAGATE DAT tape drive

Ah, what Jochen said in the message I see after yours is a good point. 
By default, dump wants to switch tapes based on whatever the default
density is which is based on some really old tape format and is indeed
much much smaller than 2GB.  You'll want to set it to the theoretical
maximum or higher since you really just want dump to ignore how much
it's written to the tape and just change tapes when it hits an end of
tape.    I use -B4194304 ( 4*(1024^2) ).   I'm not sure why Jochen uses
a smaller number unless perhaps he's paranoid about the end of tape
which historically hasn't always worked too well or just figures he's
not going to get any compression out of the hardware.

Dave


Yoseff Francus wrote:
> 
> Dave,
> 
> That makes sense.
> 
> However, I was getting far far less than 2GB on a when using dump to create
> the tape.
> What is even odder is that while dump does not work very well I was
> able to tar a 1.7GB filesystem to the tape without a problem and without
> the message that only one density is supported. I wonder why dump and tar
> give different behaviors.
> 
> yf
> 
> Date:    Tue, 21 Mar 2000 16:50:52 EST
> To:      Yoseff Francus <francus@yossi.com>
> From:    Dave Archer <dmarcher@pobox.com>
> Subject: Re: Configuring a SEAGATE DAT tape drive
> 
> DAT drives use hardware compression and thus don't have a fixed density
> in terms of the bits sent from the computer since this depends on the
> compression rate.  The drive you have is a DDS-1 drive which physically
> holds 2GB on a 90m tape, so you can do the math to work out what the
> uncompressed density is.  Typically they're advertised as 4GB with the
> assumption of 2:1 compression, but if the files you're backing up are
> mostly already compressed, you'll get closer to 2GB.
> 
> Dave
> 
> Yoseff Francus wrote:
> >
> > Looking at the tz.c code it looks like only one density is supported
> > for PYTHON tape drives. WHat is the denisty that is supported?
> >
> > yf
> >
> > Date:    Tue, 21 Mar 2000 14:43:07 EST
> > To:      netbsd-users@netbsd.org
> > From:    Yoseff Francus <francus@yossi.com>
> > Subject: Configuring a SEAGATE DAT tape drive
> >
> > I have a Seagate STD64000N-SB external 4GB DAT drive. When I attempt
> > backups I am not getting anywhere near that. I have a feeling that
> > I need to somehow configure different densities for it but have no idea
> > where to even begin. Is anyone using this model (Scorpion family)?
> > I am running 1.4.1 on a DECstation 5000/120 (3MIN).
> >
> > yf