Re: [Qemu-devel] [PATCH] Revert block-qcow2.c to kvm-72 version due to corruption reports

[Jamie Lokier <jamie@shareable.org>]

Dor Laor wrote:
> The solution is to find the real cause to the corruption.

I agree, if someone is able to do that, great, but if not and
practical reality results in these choices:

    1. Ship the current code which results in corruption on Windows
       2000 and 2003 guests (and who knows what else), and by the way
       is unlikely to have anything to do with device emulation.

    2. Revert to (nearly) kvm-72 code which appears to fix the
       majority of those corruption cases, although there is still
       something rare, which may be a different bug.

Which is the best choice?

>From a QA POV, I would revert the known bug until someone has a fix,
then reinstate everything after it which is thought to be good.

> Jamie Lokier wrote:
>      Anthony Liguori wrote:
>                Simply reverting the qcow2 code appears to fix
>                those problems, so it
>                needn't hold up cutting a release.  That's what I
>                recommend.
>           Send some patches.
>      I did already.
> 
>      Here it is again.  This should fix my bug and Marc's bug according to
>      his report that reverting qcow2.c fixes it.
> Going back to kvm-72 is not good also.

> First, there were qcow2 corruptions before it, they were very rare but still
> exist.

That's true.  But they were noticably rarer - to the point that people
clearly are using kvm-72 with qcow2 and not reporting many problems.

Ubuntu 8.10 shipped kvm-72, and that coincided with their announcement
that they're supporting KVM as their official virtualisation solution.
I imagine kvm-72 is getting a fair bit of usage because of that.

Of course they could be having rare problems and think it's a bug in
the guest or its applications :-)

> Not long ago we did not know even that qcow2 is the faulty.

Worrying, isn't it.  Does qcow2 get any rigorous testing?  Should that
be added - a blockdev test suite?

There hasn't been a complete lack of bug reports about qcow2, but
maybe they aren't getting to the right places, and maybe they're too
difficult to reproduce and easy to workaround ("my guest occasionally
shows random corruption", "don't use KVM for that guest", "I switch to
raw and it went away")

I very luckily discovered it prevented one of my VMs from booting, as
soon as I upgraded from kvm-72 (shipped with Ubuntu) to something
newer.  If it hadn't prevented it from booting, just occasional rare
corruption, I might not have realised it was qcow2 at all.  Guest
corruption can occur for many reasons, and -win2k-hack implies that
the IDE emulation is not quite right in some way.

-- Jamie