> From: bad@flatlin.ka.sub.org (Christoph Badura)
> Subject: Re: NCR Driver Problems
> To: bwalker@musings.com (Brad Walker)
> Date: Wed, 24 Jan 1996 02:23:26 +0100 (MET)
> Cc: current-users@netbsd.org
> 
> > The biggest win with tagged commands is that you can
> > issue multiple commands to a target and have the target
> > do the ordering. The target will do the ordering to
> > optimize the operations as it's doing head seeks and
> > crossing platters thus yielding a higher throughput
> > and minimizing the time due to seeking..
> 
> Except the buffer cache and the disksort routine are supposed to do
> that already.
> 

But, they don't optimize for head actuator position. Which is best
left to the SCSI target to do. Hence the use of SCSI tagged commands.

There is a good example in SCSI-3, Interlocked Protocol, X3T9.2/91-11
Rev.1 in Appendix D.


> It could well be that the target gets it wrong and the performance
> gets worse.  There is a CSRG paper about such an experience with a
> non-SCSI controller that did it's own ordering.
> 

Quote your source.. And it is worth mentioning that this is paper is
most likely quite a few years old. Before current technology.

-brad w.