qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_syn


From: Stefan Hajnoczi
Subject: [Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"
Date: Tue, 24 Aug 2010 12:02:04 +0100

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.

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

Stefan



reply via email to

[Prev in Thread] Current Thread [Next in Thread]