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Date: Tue, 05 May 2009 21:18:49 +0300
From: Avi Kivity <avi@redhat.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] Strange virtio regression on mainline and stable-0.10
References: <4A000C74.5020907@redhat.com> <4A0066D9.6030008@redhat.com>
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To: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>

Avi Kivity wrote:
> Avi Kivity wrote:
>> Running the Fedora 10 installer on a virtio disk on current master 
>> and on v0.10.3 will cause the installer to complain when mounting the 
>> freshly formatted filesystems.
>
> The problem is that qcow2 does a read-modify-write on 
> non-cluster-aligned writes.  So the following sequence triggers the bug:

No, that's not the problem.

Now I think the problem occurs if the guest does two non-overlapping 
writes in parallel that hit the same cluster.  Initially the cluster is 
not allocated, so the two writes will go into two newly allocated 
clusters.  When qcow2 tries to update the block pointers, something 
strange happens.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.