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

Jamie Lokier
2009-02-11T16:48:19Z

Kevin Wolf wrote:
> Besides reviewing the code over and over again, I think the only real
> chance is that you can get a non-productive copy of your image and add
> some debug code so that we can see at least which code path is causing
> problems.

I have a copy of my image to reproduce the bug, so I can test patches
including diagnostic patches. That's what I did to narrow it down.

Being a company mail server, I can't send you the image of course.

> > Aside from logic, the code mixes signed 32-bit with unsigned 64-bit
> > with unclear naming which would make me nervous. My host is 64-bit,
> > by the way.
>
> I would suspect that simply having a 64 bit host isn't enough to trigger
> the problem. These patches were in for half a year now without anyone
> noticing such failure.

It was just for clarity. If there are any bugs it's more likely to be
truncation on a 32 bit host :-)

I didn't see any mention of "long", so the code should behave the
same on 64-bit and 32-bit hosts.

> By the way and completely off-topic: Have you already tried to use the
> VHD patches? I would really like to know if they fix your problems.

Are those patches in kvm-83? I still have the image that was causing
problems way back, and I'm converting it to raw now with kvm-83 to see
if it now matches the raw image produced by VPC's own tool.

Beyond checking that it reads ok, which it didn't before, I don't know
how to test the VPC support properly, but I can try booting the image
and see if it at least doesn't fsck^H^H^H^Hscandisk like it used to.

I'm not using VPC images any more, we just install Windows into empty
QCOW2 or raw images, like everyone else. :-)

At some point I may test the VPC support with prebuilt images
downloaded from Microsoft - you can too!

-- Jamie