Re: Disk I/O stuck with KVM - no clue how to solve that

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On Sun, Nov 7, 2010 at 4:07 PM, Hermann Himmelbauer <dusty@xxxxxxx> wrote:
> Am Samstag 06 November 2010 20:58:12 schrieb Stefan Hajnoczi:
>> On Fri, Nov 5, 2010 at 5:16 PM, Hermann Himmelbauer <dusty@xxxxxxx> wrote:
>> > I experience strange disk I/O stucks on my Linux Host + Guest with KVM,
>> > which make the system (especially the guests) almost unusable. These
>> > stucks come periodically, e.g. every 2 to 10 seconds and last between 3
>> > and sometimes over 120 seconds, which trigger kernel messages like this
>> > (on host and/or guest):
>> >
>> > INFO: task postgres:2195 blocked for more than 120 seconds
>>
>> The fact that this happens on the host too suggests there's an issue
>> with the host software/hardware and the VM is triggering it but not
>> the root cause.
>>
>> Does dmesg display any other suspicious messages?
>
> No, there's anything that can be seen via dmesg. I at first suspected the
> hardware, too. I can think of the following reasons:
>
> 1) Broken SATA cable / Harddisks - I changed some cables, no change, thus this
> is probably ruled out. I also can't see anything via S.M.A.R.T. Moreover, the
> problem is not bound to a specific device, instead it happens on sda - sdd,
> so I doubt it's harddisk related.
>
> 2) Broken Power Supply / Insufficient Power - I'd expect either a complete
> crash or some error messages in this case, so I'd rather rule that out.
>
> 3) Broken SATA-Controller - I cannot think of any way to check that, but I'd
> also expect some crashes or kernel messages. I flashed the board to the
> latest BIOS version, no change either.
>
> However, it seems no one except me seems to have this problem, so I'll buy a
> new, similar but different mainboard (Intel instead of Asus), hopefully this
> solves the problem.
>
> What do you think, any better idea?

If you have the time, you can use perf probes to trace I/O requests in
the host kernel.  Perhaps completion interrupts are being dropped.
You may wish to start by tracing requests issued and completed by the
SATA driver.

Stefan
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