From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44721) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YrTo6-0000tE-J9 for qemu-devel@nongnu.org; Sun, 10 May 2015 12:10:23 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YrTo5-0006T7-OW for qemu-devel@nongnu.org; Sun, 10 May 2015 12:10:22 -0400 Message-ID: <554F82E0.2000005@redhat.com> Date: Sun, 10 May 2015 18:10:08 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1430971496-32659-1-git-send-email-phoeagon@gmail.com> <1431011818-15822-1-git-send-email-phoeagon@gmail.com> <554CB6C6.3060809@redhat.com> <20150508135512.GJ4318@noname.redhat.com> <554D2A03.3080201@weilnetz.de> <554F72E6.5060001@redhat.com> <554F8117.1070800@weilnetz.de> In-Reply-To: <554F8117.1070800@weilnetz.de> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v4] block/vdi: Use bdrv_flush after metadata updates List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Weil , phoeagon , Kevin Wolf , Max Reitz Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org On 10/05/2015 18:02, Stefan Weil wrote: >> Since the default qemu-img convert case isn't slowed down, I >> would think that correctness trumps performance. Nevertheless, >> it's a huge difference. > > I doubt that the convert case isn't slowed down. The *default* convert case isn't slowed down because "qemu-img convert" defaults to the "unsafe" cache mode. The *non-default* convert case with flushes was slowed down indeed: 2x in total (if you include the final flush done by bdrv_close), and 10x if you only consider the sequential write part of convert. Paolo > Writing to a tmpfs as it was obviously done for the test is not a > typical use case. > With real hard disks I expect a significant slowdown. > > Stefan