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Re: [Xen-users] dom0 hangs when doing heavy I/O on domU

On Thursday 21 April 2011 12:59:17 Florian Heigl wrote:
> Hi Chang,
> 2011/4/20 Kiefer Chang <zapchang@xxxxxxxxx>:
> > Hi all,
> > We are using XEN as hypervisor to setup our private cloud.
> > The framework is Eucalyptus and using CentOS 5.4 as dom0 OS.
> > Sometimes we find some machines' dom0 become unresponsive, the symptoms
> > are: (1) We can't log into dom0 via ssh. After typing password, it just
> > stops there.
> > (2) We can ping dom0 successfully.
> > (3) We can log into domU without problem.
> > The unresponsive dom0 eventually "alive" after a period of time. Maybe
> > half hour or even several hours.
> So one of your domUs is trashing the disks and dom0 can't get enough
> performance, right?
> - are they sharing a disk?
> - can you check what I/O scheduler you are using?
>   (with cfq you can then use ionice to lower prio on all blkback
> threads a little. that way dom0 will "win the race")
> In general, your dom0 is privileged in terms of IO access rights, but
> not in IO peformance. So if one domU goes crazy, it will affect
> anything.
> ... until you take measures :)
> I'd suggest you switch to deadline scheduler and re-test.
> dom0 on a different disk media is also very advisable imho.

Another possible cause for this is if dom0 is not pinned to a "private" cpu-
I noticed similar issues before I pinned dom0 to cpu0 and configured all the 
other domUs to use the other 3 cores.


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