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SubjectRe: Higher than expected disk write(2) latency
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On Thu, 2008-07-10 at 07:01 -0700, Arjan van de Ven wrote:
> On Thu, 10 Jul 2008 15:41:46 +0200
> Martin Lucina <mato@kotelna.sk> wrote:
>
> > chris.mason@oracle.com said:
> > > Is NCQ enabled on the drive? The basic way to fix this is to have
> > > multiple requests in flight, which isn't going to happen on sata
> > > with the cache off and with ncq off.
> >
> > ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
> > ata1.00: ATA-7: Hitachi HDS721616PLA380, P22OAB3A, max UDMA/133
> > ata1.00: 321672960 sectors, multi 0: LBA48 NCQ (depth 31/32)
> > ata1.00: configured for UDMA/133
> >
> > I'm assuming the above indicates NCQ is enabled.
> >
> > We'll try running the tests tomorrow with different i/o schedulers and
> > report back.
>
>
> another thing to try is using AHCI mode instead; AHCI tends to be
> higher performance
>
> (and.. for fun try to run latencytop during a run, to see if maybe
> there are unsuspected delay causes)
>
Any latencies during submit (at least for kernel aio) are probably from
get_request_wait, but the numbers so far look like they are only seeing
one request in flight at a time.

-chris




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