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SubjectRe: [ANNOUNCE] Native Linux KVM tool v2
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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