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Re: [Qemu-devel] Relative/Absolute timing snapshot problem

From: Clemens Kolbitsch
Subject: Re: [Qemu-devel] Relative/Absolute timing snapshot problem
Date: Mon, 28 Mar 2011 09:25:57 -0700
User-agent: KMail/1.13.5 (Linux/2.6.35-28-generic; KDE/4.5.1; x86_64; ; )

> On 03/18/11 21:39, Clemens Kolbitsch wrote:
> > Hi list,
> > 
> > strange situation: When I create a snapshot using Qemu 0.14.0 stable,
> > everything works smoothly and resuming the CPU takes about 1-2 seconds.
> > If I don't use the snapshot file for some time, the time it takes to
> > resume grows by 2-3 seconds per day. At the moment, I'm looking at a
> > snapshot file from last week and it takes nearly 30 seconds to load.
> > 
> > Funny thing about it: if I turn my system time back to the date when the
> > snapshot was created (or before that), resuming CPU works within the
> > expected 1-2 seconds. I have _very briefly_ looked into it and it seems
> > like Qemu spends an aweful long amount of time catching up with timer
> > execution -- is it possible that these are stored using absolute time
> > instead of relative timing?
> > 
> > I am using qcow2 file format, because I absolutely rely on CPU-snapshots
> > and support for base-files. I have read here and there that it is more
> > or less broken (or at least very slow), but with the correct
> > cache-options it works for me (except for this bug, of course).
> > 
> > Has anyone encountered this or should I start looking into it (although I
> > have some experience with the core source, I'm not very experienced with
> > the snapshotting code).
> Hi Clemens,
> Could you clarify what you are doing, when you say snapshot do you mean
> a savevm operation (ie. checkpoint) or a disk snapshot?
> Cheers,
> Jes

I Jes,

sorry for not being clear: I use a savevm operation.

Following setup: I have a base file (qcow2 or qew format) and a snapshot file 
(generated via 'qemu-img create -b <basefile> -f qcow2 <snapshotfile>') and 
boot the snapshot file. Once the system (WinXP SP3 guest) has fully started, I 
create a snapshot (savevm <foo>) and exit the emulator. Later, I resume the 
snapshot by starting Qemu with the snapshotfile and say 'loadvm <foo>'.

Actually, I found that the times I gave in my last email were way 
underestimated. The time difference is MUCH larger than 2-3 seconds per day. A 
week-old snapshot can take minutes to load on a reasonably idle host machine.


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