scsi tape curiousity

[Dan Nelson]

In the last episode (May 28), David Bear said:
> On Tue, May 27, 2003 at 11:43:48PM -0500, Dan Nelson wrote:
> > In the last episode (May 27), David Bear said:
> > > I called the cybernetics people (maker of the tape unit) and
> > > their recommendation was to put the tape unit on a separate scsi
> > > controller from the hard drives.  I didn't want to do this since
> > > but did anyway. It seems to have fixed the problem.  I can now
> > > use the tape unit.  The question is
> > 
> > Most likely bad termination.  Make sure all the cables are seated
> > well, make sure you're got active terminators, and make sure that
> > if all your devices are LVD, you have LVD terminators.
> 
> so the guy from cybernetics was feeding me BS?  ie you can safely run
> tapes on hard drives on the same scsi chain?

Of course.  The main issue is that since each device on SCSI can
negotiate its own speed with the controller, an ancient SCSI tape drive
with a 5MB/sec write speed that negotiated a 10MB/sec bus speed with
the controller could tie up the SCSI bus 50% of the time just to
transfer data.  If the server is trying to do other stuff while a
backup is running it can degrade performance noticeably.

If your tape drive really is LVD-capable, it's not an issue.  Even a
15MB/sec drive will only tie up 20% of the SCSI bus bandwidth at full
speed, assuming it negotiated an LVD-80 connection to the server.
 
> I think all drives are lvd AND I know I have an active lvd terminator
> -- it cost me $30!  But, I do have cdrom ron the 50 pin bus.  It is
> terminated using the cdrom internal terminator.  I wonder...

On the 29160 cards, the internal SE plugs are searated from the LVD/SE
plugs by a bridge chip, so your cdrom should not affect your other
devices' ability to negotiate LVD mode on their segment of the cable. 
You may want to explicitly tell your controller to terminate the top 8
bits of the bus only; sometimes the autodetect setting doesn't work.

Running "camcontrol inq da0" (then cd0, then sa0) will tell you what
speed each device negotiated with the controller.

-- 
	Dan Nelson
	dnelson at allantgroup.com