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Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old

From: Marc Bevand
Subject: Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change
Date: Thu, 12 Feb 2009 23:50:30 -0800

Forwarding a post I sent to the kvm ML:

I have got a massive KVM installation with hundreds of guests runnings
dozens of different OSes, and have also noticed multiple qcow2
corruption bugs. All my guests are using the qcow2 format, and my
hosts are running vanilla linux 2.6.28 x86_64 kernels and use NPT
(Opteron 'Barcelona' 23xx processors).

My Windows 2000 guests BSOD just like yours with kvm-73 or newer. I
have to run kvm-75 (I need the NPT fixes it contains) with
block-qcow2.c reverted to the version from kvm-72 to fix the BSOD.

kvm-73+ also causes some of my Windows 2003 guests to exhibit this
exact registry corruption error:
This bug is also fixed by reverting block-qcow2.c to the version from kvm-72.

I tested kvm-81 and kvm-83 as well (can't test kvm-80 or older because
of the qcow2 performance regression caused by the default writethrough
caching policy) but it randomly triggers an even worse bug: the moment
I shut down a guest by typing "quit" in the monitor, it sometimes
overwrite the first 4kB of the disk image with mostly NUL bytes (!)
which completely destroys it. I am familiar with the qcow2 format and
apparently this 4kB block seems to be an L2 table with most entries
set to zero. I have had to restore at least 6 or 7 disk images from
backup after occurences of that bug. My intuition tells me this may be
the qcow2 code trying to allocate a cluster to write a new L2 table,
but not noticing the allocation failed (represented by a 0 offset),
and writing the L2 table at that 0 offset, overwriting the qcow2

Fortunately this bug is also fixed by running kvm-75 with
block-qcow2.c reverted to its kvm-72 version.

Basically qcow2 in kvm-73 or newer is completely unreliable.


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