Subject: Re: scsi DLT tape drives
To: der Mouse <>
From: Andy Ruhl <>
List: port-i386
Date: 07/07/2004 10:20:02
On Wed, 7 Jul 2004 13:02:09 -0400 (EDT), der Mouse
<> wrote:
> >>> You REALLY don't want to be running a tape drive and a disk drive
> >>> on the same channel.  If your card has another channel, use it.  If
> >>> not, invest in another card.
> >> I can't see any reason for this unless the tape drive doesn't
> >> support disconnect/reconnect, which anything even vaguely modern
> >> should.  [...]  Andy, am I missing something?
> > That sounds a little bit like a challenge or something, but I'll
> > deflect that.
> :-)  Perhaps a bit.  In a case like that, it's entirely possible I am
> missing something - but I also might not be, and I regularly see
> baseless advice passed around as gospel.
> > It's generally not a good idea to use devices of widely varying speed
> > on the same bus, especially if those devices are expected to talk
> > directly to each other.
> Well, in this case, NetBSD doesn't support any effective topology but
> the star, with the host at the centre of the star, as far as I can
> tell, so every transfer has the host on one end of it.  Which means
> that talking directly to another (non-host) device is a non-issue.
> If by "widely varying speed" you mean media speed, then I strongly
> doubt that there is any real reason behind it.  If you mean bus speed,
> then there may be some sense behind it, but it is probably inapplicable
> here: a tape drive can be expected to eat the whole transfer fast then
> sit there doing nothing on the bus for a comparatively long time while
> it performs the write (mutatis mutandis for read).  This is whence the
> remark about disconnect/reconnect, because that long delay will be a
> performance killer unless the drive disconnects during it to let
> something else use the bus.
> Even if you mean bus speed, I can't see any reason for it - but I know
> I'm no SCSI guru, which is why I asked if I were missing something.

I could be wrong, not sure. Probably am actually. I'm coming from the
"old days" when you don't want to mix devices of varying speeds on the
same bus. If there has been a solution for this, then that's great and
I'll update the foggy areas of my brain on the subject. I'm not a scsi
guru either but at one time I did have to read up on it. But it's been
a while.