[Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"

[Juan Quintela <quintela@redhat.com>]

Kevin Wolf <kwolf@redhat.com> wrote:
> This reverts commit 8b3b720620a1137a1b794fc3ed64734236f94e06.
>
> This fix has caused severe slowdowns on recent kernels that actually do flush
> when they are told so. Reverting this patch hurts correctness and means that we
> could get corrupted images in case of a host crash. This means that qcow2 might
> not be an option for some people without this fix. On the other hand, I get
> reports that the slowdown is so massive that not reverting it would mean that
> people can't use it either because it just takes ages to complete stuff. It
> probably can be fixed, but not in time for 0.13.0.
>
> Usually, if there's a possible tradeoff between correctness and performance, I
> tend to choose correctness, but I'm not so sure in this case. I'm not sure with
> reverting either, which is why I post this as an RFC only.
>
> I hope to get some more comments on how to proceed here for 0.13.

I think that we have to revert also.

It is a thoug decission so.  This patch makes things more "correct",
but it makes some load really slow, for instance, and install went from
15m to 50m on my hardware :(

My point here is that with the patch, qcow2 becomes almost unusable, so
... what to do?

Later, Juan.