Messages in this thread |  | | | Date | Wed, 15 Jun 2011 17:44:17 -0500 | | From | Anthony Liguori <> | | Subject | Re: [ANNOUNCE] Native Linux KVM tool v2 |
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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<penberg@kernel.org> >>>> wrote: >>>>> On Wed, Jun 15, 2011 at 7:30 PM, Avi Kivity<avi@redhat.com> 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<disk image on hard disk> -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
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