linux-kernel - Re: [ANNOUNCE] Native Linux KVM tool v2

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Date:	Wed, 15 Jun 2011 17:44:17 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Alexander Graf <agraf@...e.de>
CC:	Prasad Joshi <prasadjoshi124@...il.com>,
	Pekka Enberg <penberg@...nel.org>, Avi Kivity <avi@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Sasha Levin <levinsasha928@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Asias He <asias.hejun@...il.com>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [ANNOUNCE] Native Linux KVM tool v2

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@...nel.org>
>>>> wrote:
>>>>> On Wed, Jun 15, 2011 at 7:30 PM, Avi Kivity<avi@...hat.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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