Messages in this thread | | | Subject | Re: iowait problems on 2.6, not on 2.4 | From | Doug Ledford <> | Date | Sat, 29 May 2004 20:46:31 -0400 |
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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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