Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755282Ab1FOWoX (ORCPT ); Wed, 15 Jun 2011 18:44:23 -0400 Received: from mail-yw0-f46.google.com ([209.85.213.46]:50949 "EHLO mail-yw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753668Ab1FOWoV (ORCPT ); Wed, 15 Jun 2011 18:44:21 -0400 Message-ID: <4DF935C1.4020000@codemonkey.ws> Date: Wed, 15 Jun 2011 17:44:17 -0500 From: Anthony Liguori User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110424 Lightning/1.0b2 Thunderbird/3.1.10 MIME-Version: 1.0 To: Alexander Graf CC: Prasad Joshi , Pekka Enberg , Avi Kivity , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Andrew Morton , Linus Torvalds , Ingo Molnar , Sasha Levin , Cyrill Gorcunov , Asias He , Jens Axboe Subject: Re: [ANNOUNCE] Native Linux KVM tool v2 References: <1308153214.7566.6.camel@jaguar> <4DF8DE26.1070301@redhat.com>

<4DF92C80.3030106@codemonkey.ws> <7A30A509-47AA-4E72-ABF3-937005900F9D@suse.de> <4DF93010.1040006@codemonkey.ws> In-Reply-To: <4DF93010.1040006@codemonkey.ws> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2880 Lines: 88 On 06/15/2011 05:20 PM, Anthony Liguori wrote: > On 06/15/2011 05:07 PM, Alexander Graf wrote: >> >> On 16.06.2011, at 00:04, Anthony Liguori wrote: >> >>> On 06/15/2011 03:13 PM, Prasad Joshi wrote: >>>> On Wed, Jun 15, 2011 at 6:10 PM, Pekka Enberg >>>> wrote: >>>>> On Wed, Jun 15, 2011 at 7:30 PM, Avi Kivity wrote: >>>>>> On 06/15/2011 06:53 PM, Pekka Enberg wrote: >>>>>>> >>>>>>> - Fast QCOW2 image read-write support beating Qemu in fio >>>>>>> benchmarks. See >>>>>>> the >>>>>>> following URL for test result details: >>>>>>> https://gist.github.com/1026888 >>>>>> >>>>>> This is surprising. How is qemu invoked? >>>>> >>>>> Prasad will have the details. Please note that the above are with Qemu >>>>> defaults which doesn't use virtio. The results with virtio are little >>>>> better but still in favor of tools/kvm. >>>>> >>>> >>>> The qcow2 image used for testing was copied on to /dev/shm to avoid >>>> the disk delays in performance measurement. >>>> >>>> QEMU was invoked with following parameters >>>> >>>> $ qemu-system-x86_64 -hda -hdb >>>> /dev/shm/test.qcow2 -m 1024M >>> >>> Looking more closely at native KVM tools, you would need to use the >>> following invocation to have an apples-to-apples comparison: >>> >>> qemu-system-x86_64 -drive >>> file=/dev/shm/test.qcow2,cache=writeback,if=virtio >> >> Wouldn't this still be using threaded AIO mode? I thought KVM tools >> used native AIO? > > Nope. The relevant code is: > >> /* blk device ?*/ >> disk = blkdev__probe(filename, &st); >> if (disk) >> return disk; >> >> fd = open(filename, readonly ? O_RDONLY : O_RDWR); >> if (fd < 0) >> return NULL; >> >> /* qcow image ?*/ >> disk = qcow_probe(fd, readonly); >> if (disk) >> return disk; >> >> /* raw image ?*/ >> disk = raw_image__probe(fd, &st, readonly); >> if (disk) >> return disk; > > It uses a synchronous I/O model similar to qcow2 in QEMU with what I > assume is a global lock that's outside of the actual implementation. > > I think it lacks some of the caching that Kevin's added recently though > so I assume that if QEMU was run with cache=writeback, it would probably > do quite a bit better than native KVM tool. > > It also turns out that while they have the infrastructure to deal with > FLUSH, they don't implement it for qcow2 :-/ > > So even if the guest does an fsync(), it native KVM tool will never > actually sync the data to disk... > > That's probably why it's fast, it doesn't preserve data integrity :( Actually, I misread the code. It does unstable writes but it does do fsync() on FLUSH. Regards, Anthony Liguori -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/