From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=40250 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OnrGd-0004Ct-D6 for qemu-devel@nongnu.org; Tue, 24 Aug 2010 07:02:12 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OnrGZ-0005CG-0b for qemu-devel@nongnu.org; Tue, 24 Aug 2010 07:02:11 -0400 Received: from mail-qw0-f45.google.com ([209.85.216.45]:42094) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OnrGY-0005CC-PG for qemu-devel@nongnu.org; Tue, 24 Aug 2010 07:02:06 -0400 Received: by qwh5 with SMTP id 5so6108812qwh.4 for ; Tue, 24 Aug 2010 04:02:06 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1282646430-5777-1-git-send-email-kwolf@redhat.com> References: <1282646430-5777-1-git-send-email-kwolf@redhat.com> Date: Tue, 24 Aug 2010 12:02:04 +0100 Message-ID: From: Stefan Hajnoczi Content-Type: text/plain; charset=ISO-8859-1 Subject: [Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes" List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: avi@redhat.com, mjt@tls.msk.ru, qemu-devel@nongnu.org, hch@lst.de On Tue, Aug 24, 2010 at 11:40 AM, Kevin Wolf 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