Date: Sun, 5 Jan 2003 22:01:26 -0500 From: Bruce Campbell <bruce@engmail.uwaterloo.ca> To: freebsd-hardware@FreeBSD.ORG Cc: freebsd-questions@FreeBSD.ORG Subject: Re: ata "fallback to PIO mode" on dual processor AMD systems Message-ID: <1041822086.3e18f1868e32c@www.nexusmail.uwaterloo.ca> In-Reply-To: <1041371397.3e121105cdf30@www.nexusmail.uwaterloo.ca> References: <1041368236.3e1204ac45da5@www.nexusmail.uwaterloo.ca> <025701c2b112$ddfbf580$1200a8c0@gsicomp.on.ca> <1041371397.3e121105cdf30@www.nexusmail.uwaterloo.ca>
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Quoting Bruce Campbell <bruce@engmail.uwaterloo.ca>:
> Quoting Matthew Emmerton <matt@gsicomp.on.ca>:
>
> > [ cc'ing Soren since he's the ATA guru ]
> >
> > > Dec 30 23:27:00 ecserv13 /kernel: ad0: trying fallback to PIO mode
> > > Dec 30 23:27:00 ecserv13 /kernel: ata0: resetting devices .. done
> > >
> > > The test continues to run with the ata controller in PIO mode, with
> > > slower performance, and higher load average.
> > >
> > > Once the master drops to PIO, attempts to access the slave then cause
> > > it to drop to PIO.
> >
> > Are you using 80-conductor cables on all your drives? These are required
> to
> > get consistent high throughput, and running without them may cause the
> > problems you're seeing.
>
> Thanks for the information about the design of IDE etc, and the suggestion
> about the cables. I was about to shuffle things to get the disks
> onto separate channels, but I now see that would be a mistake as my
> CD drive would share a cable with a disk.
ps. As an aside, I have since determined that putting a PIO device and
a UDMA device on the same channel does not affect the performance
of the UDMA device, unless the PIO device is in use. So, sharing
a low use CD rom drive with a disk wouldn't be so bad.
I am puzzled about the fallback to PIO concept. If a disk has
gives some sort of timeout error or whatever, why would trying
PIO correct the problem ? That seems equivalent to asking the
disk to do the same thing, just more slowly.
In my case, some sort of timeout error occurs on ad0, so
it falls back to PIO, and works. A later access to ad1
also yields a timeout error, and then it drops to PIO,
and works too. I'm fairly confident both disks did not
experience media errors at the same time, which suggests
a problem with the onboard IDE controller, or a driver bug.
Tests continue...
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