Subject: Re: NCR Driver Problems
To: Christoph Badura <>
From: None <Chris_G_Demetriou@NIAGARA.NECTAR.CS.CMU.EDU>
List: current-users
Date: 01/29/1996 20:03:50
> > 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.
> 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.


hardware wins significantly.

directory in almost reverse order.)

Consider also the case where you're talking about a RAID4 controller that
appears as a LUN on a SCSI bus.  How can you _hope_ to get the maximum
bandwidth that it provides if you've only got one outstanding request
at a time, because it's going to have several spindles completely idle!

While i believe that disksort etc., do the right thing, you cannot get
maximum bandwidth out of modern disks and disk subsystems if you don't
expose concurrency of access to the disks.