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SubjectRe: iowait problems on 2.6, not on 2.4
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On Fri, 2004-05-28 at 18:45, Andrew Morton wrote:
> Antonio Larrosa Jiménez <antlarr@tedial.com> wrote:
> >
> > On Thursday 27 May 2004 05:52, you wrote:
> > > Antonio Larrosa Jiménez <antlarr@tedial.com> wrote:
> > > > My next test will be to do the "dd tests" on one of the internal hard
> > > > disks and use it for the data instead of the external raid.
> > >
> > > That's a logical next step. The reduced read bandwith on the raid array
> > > should be fixed up before we can go any further. I don't recall any
> > > reports of qlogic fc-scsi performance regressions though.
> >
> > Ok, let's analyze that first.
> >
> > The dd tests gave the following results:
>
> Let me cc linux-scsi.
>
> Guys: poke. Does anyone know why this:
>
> The machine is a 4 cpu Pentium III (Cascades) system with four SCSI
> SEAGATE ST336704 hard disks connected to an Adaptec AIC-7899P U160/m, and
> a external RAID connected to a QLA2200/QLA2xxx FC-SCSI Host Bus Adapter.
> The machine has 1Gb RAM.
>
> got all slow at reads?

My first guess would be read ahead values. Try poking around with
those. When using a hard disk vs. a raid array, it's easier to trigger
firmware read ahead since all reads go to a single physical device and
that in turn compensates for lack of reasonable OS read ahead. On a
raid array, depending on the vendor, there may be next to no firmware
initiated read ahead and that can drastically reduce read performance on
sequential reads.


> > So yes, I suppose there's a regression on the qlogic fc-scsi module.

A regression, yes. In the qlogic-fc driver? I doubt it. Performance
tuning is what I think this basically boils down to.

But, I could be wrong. Give it a try and see what happens. In the 2.4
kernels I would tell you to tweak /proc/sys/vm/{min,max}-readahead,
don't know if those two knobs still exist in 2.6 and if they have the
same effect. Andrew?

--
Doug Ledford <dledford@redhat.com> 919-754-3700 x44233
Red Hat, Inc.
1801 Varsity Dr.
Raleigh, NC 27606


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