From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [ANNOUNCE] Native Linux KVM tool v2 Date: Wed, 15 Jun 2011 17:20:00 -0500 Message-ID: <4DF93010.1040006@codemonkey.ws> References: <1308153214.7566.6.camel@jaguar> <4DF8DE26.1070301@redhat.com> <4DF92C80.3030106@codemonkey.ws> <7A30A509-47AA-4E72-ABF3-937005900F9D@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit 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 To: Alexander Graf Return-path: Received: from mail-pv0-f174.google.com ([74.125.83.174]:49683 "EHLO mail-pv0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752362Ab1FOWUF (ORCPT ); Wed, 15 Jun 2011 18:20:05 -0400 In-Reply-To: <7A30A509-47AA-4E72-ABF3-937005900F9D@suse.de> Sender: kvm-owner@vger.kernel.org List-ID: 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 :( Regards, Anthony Liguori > > > Alex >