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

[Kevin Wolf]

Am 24.08.2010 13:02, schrieb Stefan Hajnoczi:
> On Tue, Aug 24, 2010 at 11:40 AM, Kevin Wolf <address@hidden> 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.
> 
> Sometimes an improvement has a side effect and it makes sense to hold
> back the improvement until the side effect can be resolved.  The
> period of time in which users could rely on qcow2 data integrity is
> small to none, I feel reverting the commit makes sense.

Right, that's the vague feeling I have, too.

> QEMU 0.12.5 has qcow2 sync metadata writes in commit
> 37060c28e522843fbf6f7e59af745dfcb05b132c.  Was the performance
> regression spotted on 0.12.5 or 0.13?

Both. You mean we should consider a 0.12.6 if we decide to revert? I
think so far 0.12.5 was planned to be last 0.12.x release.

Kevin