On Fri, Sep 05, 2014 at 04:39:51PM +0100, Stefan Hajnoczi wrote:
> 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.

At your prompting, I've done a bit more investigation.

I was basing my observations on qemu 2.1.0.  However I tried my test
against qemu from git today and the bug has gone.  Good!

For my entertainment, I bisected the problem, and the commit which
*fixes* it is:

  commit 91af7014125895cc74141be6b60f3a3e882ed743
  Author: Max Reitz <mreitz@redhat.com>
  Date:   Fri Jul 18 20:24:56 2014 +0200

    block: Add bdrv_refresh_filename()

I didn't believe this either, but I have checked the result manually
and I'm pretty sure that whatever this commit does, it does end up
fixing the lazy_refcounts problem as a side-effect.

----------------------------------------------------------------------

I've updated the test script (attached), so you can now run it against
your own qemu, and also to improve the output.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/