Re: [Qemu-devel] qcow2, lazy_refcounts and killing qemu

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Sat, Aug 30, 2014 at 03:53:13PM +0100, Richard W.M. Jones wrote:
> I found out a few days ago that if you:
> 
> (1) Open a qcow2 file that has lazy_refcounts = on and a backing file, and
> 
> (2) Write lots of stuff, and
> 
> (3) Kill qemu with SIGTERM [which I believed, maybe incorrectly, is a
> "nice" way to kill qemu]
> 
> .. then you can end up with a corrupt qcow2 file.  In particular the
> qcow2 file sometimes forgot that it had a backing file, but I suspect
> this was just a symptom and in fact the qcow2 file header wasn't being
> written to disk correctly.
> 
> Is it correct that sending SIGTERM to qemu should kill it cleanly, or
> is that no longer the case, or is lazy_refcounts a special case, or
> have I found a bug?
> 
> I can reproduce this easily, although of course the reproducer will
> involve libguestfs.

That is very interesting, thanks for posting.

Did you try older QEMU versions?  I'm curious if this is something that
crept in later or is fundamentally broken in lazy_refcounts=on.

Stefan

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