Messages in this thread | | | Date | Wed, 15 Jun 2011 17:20:00 -0500 | From | Anthony Liguori <> | Subject | Re: [ANNOUNCE] Native Linux KVM tool v2 |
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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 :(
Regards,
Anthony Liguori
> > > Alex >
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