RE: DASD Performance

["Pytel, Alexei" <pytela1@xxxxxxxxxxxxxxxx>]

Very strange rumour indeed !

AS/400 disks were always SCSI - from day one.
While it's technically true, that at any one moment in time SCSI bus can
talk only to one SCSI device, it still does not mean much.
First, actual time spend on bus exchanges is just a fraction of the
whole time between system requests data from disk and receives it.
Second, there are more than one SCSI bus in AS/400 system, and certainly
they can work in parallel.

There are many factors to take into consideration, when estimating
performance of DASD subsystem:
        - number of I/O buses in AS/400 box
        - number of IOPs and their placement on I/O buses
        - number of disk drives per IOP and their placement on IOP SCSI
buses
Of course, SCSI bus performance should be on this list, but somewhere at
the bottom.

The valid point in this rumour is that for a SMALL AS/400 system, which
has one DASD IOP and all HDs on a single SCSI bus, this COULD be a
problem, if workload is particlularly I/O bound.
But again, not because of SCSI bus limitations, but more likely because
of single IOP bottleneck.

To recapitalize, number of arms IS much more important factor.

Hope this clarifies it a bit.



          Alexei Pytel



> -----Original Message-----
> From: DAsmussen@aol.com [SMTP:DAsmussen@aol.com]
> Sent: Thursday, October 01, 1998 8:36 PM
> To:   midrange-l@midrange.com
> Subject:      DASD Performance
> 
> Hey Folks!
> 
> I heard a _NASTY_ rumor last week, and wanted to confirm it here.
> While we've
> had our tuning differences here in the past (I prefer manual tuning,
> others
> prefer QPFRADJ set on), one thing we've always agreed upon is that the
> number
> of disc arms is important.  Now I hear that arms are irrelevant on the
> new
> SCSI-based boxes, because the SCSI architecture will only access one
> drive at
> a time.  Is this true?  If so, how does one tune hardware under this
> new
> paradigm?  All the books still talk about arms!!!
> 
> TIA,
> 
> Dean Asmussen
> Enterprise Systems Consulting, Inc.
> Fuquay-Varina, NC  USA
> E-Mail:  DAsmussen@aol.com
> 
> "The two hardest things to deal with in life are failure and success."
> --
> Unknown
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