Subject: Re: NCR Driver Problems
To: None <Chris_G_Demetriou@NIAGARA.NECTAR.CS.CMU.EDU>
From: Gordon W. Ross <email@example.com>
Date: 01/23/1996 10:23:53
> From: Chris_G_Demetriou@NIAGARA.NECTAR.CS.CMU.EDU
> Cc: firstname.lastname@example.org
> X-Copyright: Copyright 1995, Christopher G. Demetriou. All rights reserved.
> X-Notice: Duplication and redistribution prohibited without consent of
> the author.
> Date: Mon, 22 Jan 96 23:58:09 EST
> Sender: email@example.com
> Precedence: list
> X-Loop: current-users@NetBSD.ORG
> > Why do you need tagged commands anyway?
(I was going to ask the same question. 8^)
> "highest performance." the SCSI code should support them,
> hopefully in a hba-independent way.
The "highest performance" argument is questionable.
Using tagged commands on your SCSI bus eliminates the
bus arbitration and selection phases between commands,
and just keeps the SCSI bus busy, proceeding directly
into the next comand phase. The time saved is only a
few microseconds per command (unless your driver does
a horrible job implementing arbitration/selection).
Furthermore, keeping the SCSI bus busy between commands
is not necessarily desirable. If you have reselecting
targets, they will have to wait longer to "get in" and
finish the commands you have already issued to them, so
those jobs will take MUCH longer to finish.
I'm NOT saying "tagged commands are always bad" but
they are not necessarily always better.