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RE: [Xen-devel] what happens when a PoD page is touched?

To: "Paul Durrant" <Paul.Durrant@xxxxxxxxxx>
Subject: RE: [Xen-devel] what happens when a PoD page is touched?
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Mon, 16 May 2011 19:21:44 +1000
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] what happens when a PoD page is touched?
> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> > bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Tim Deegan
> > Sent: 16 May 2011 09:39
> > To: James Harper
> > Cc: George Dunlap; xen devel
> > Subject: Re: [Xen-devel] what happens when a PoD page is touched?
> >
> > At 11:16 +0100 on 15 May (1305458171), James Harper wrote:
> > > I'm finding that the time of boot and hibernation of Windows under
> > > xen-4.0.2-rc3 when maxmem is set is a big problem - 40 seconds to
> > > balloon down 512MB on my system. Hibernation is even worse with
> > delays
> > > of minutes or hours.
> > >
> Are you sure that your hibernation slowdown is not caused by your
> retrying failed block writes? Since hibernation knows nothing of what
> allocated to the balloon it may try to dump ballooned out pages
> because they border pages containing useful info) which the storage
> will fail to grant map and the blkif request will be errored. If you
> then clearly you'll just get another error so if you have some sort of
> retry/timeout mechanism in your hiber driver it may will explain the
> you are seeing.

I'm almost absolutely positive. What you are describing happens during
crash dumps and I allow for a small number of such errors (although not
enough for ballooned domains probably), but the hibernation file is
compressed so pages are never actually written directly, only the
compression buffers. I imagine that the delay is while it sweeps the
ballooned out area - the hibernate progress never appears to increases
during this time because 3GB of 0's most likely compresses remarkably
well and there is nothing to write to disk


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