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

From: Gleb Natapov
Subject: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change
Date: Sun, 15 Feb 2009 12:57:18 +0200

> > 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 header.
> > 
> > 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.
> > 
> > -marc
> I think the corruption is a completely unrelated bug. I would suspect it
> was introduced in one of Gleb's patches in December. Adding him to CC.
I am not able to reproduce this. After more then hundred boot linux; generate
disk io; quit loops all I've got is an image with 7 leaked blocks and
couple of filesystem corruptions that were fixed by fsck.


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