Subject: Re: NCR Driver Problems
To: Christoph Badura <firstname.lastname@example.org>
From: None <Chris_G_Demetriou@NIAGARA.NECTAR.CS.CMU.EDU>
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.
People who say "use disksort" are missing something: concurrency.
There's a lot of literature out there, that's been put out recently,
about the fact that exposing I/O concurrency to the OS, and to the
hardware wins significantly.
In the recent SOSP paper that my group did, we saw some occasionally
quirky -- but always better -- performance by exposing more concurrency
to the drive(s). (The one particular case i can think of, that i
think was mentioned in that paper was a case where we were doing grep
string * for the files in one manual section, and disclosing our future
file accesses to the OS via hints, so that it would prefetch the data.
w/ one request outstanding at the drive, we got horrible performance,
with many, we got great performance... turns out that the layout
of the files on-disk -- done by the FFS -- was EXACTLY OPPOSITE
optimal, because of the fact that the files were added to the
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.