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Date: Mon, 01 Dec 2008 11:14:08 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] qemu-img commit -- is there a limit on file sizes?
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To: qemu-devel@nongnu.org
Cc: Avi Kivity <avi@qumranet.com>, kvm-devel <kvm@vger.kernel.org>

walt wrote:
> Some background for my question:  I've been trying to install
> and then update Windows Vista using kvm.  Everything works great
> until I use 'qemu-img commit' to apply all the Windows Updates
> to my original base install of Vista.
>
> After doing the qemu-img commit step, the backing file is now
> corrupt, 100% reproducibly.  I don't have the same problem with
> Windows XP, however, and I wondered if the problem is caused by
> the sheer size of the commit that Vista requires.
>
> When I install XP, then windows-update, and then qemu-img commit
> the updates, I'm committing about 1GB of updates to a 3GB backing
> file.
>
> When I install Vista and then later commit the Vista updates, I'm
> committing a 3GB file to a 6GB backing file, and that's when the
> corruption happens every time.
>
> So I tried an experiment with Vista -- I deliberately limit the
> number of windows updates I allow at any one time, and then use
> qemu-img commit after each small update.  Voila, everything now
> works perfectly -- no file corruption!

We've started getting some reports of corruption on "commit" in KVM.  
There is a long standing disk corruption issue too that is very 
difficult to reproduce.  The thinking is that there is a bug somewhere 
in the qcow2 code.

Is anyone actively looking into this?

Regards,

Anthony Liguori

> And that's why I suspect there is a functional limit to the size
> of each commit I can do with qemu-img.
>
> Any thoughts or possible diagnostic maneuvers to be tried?
>
> Thanks!
>
> (BTW, I get the same results using 32-bit linux and 64-bit linux
> on the same amd64 machine, using both gcc3 and gcc4.)
>
>
>