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

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Message-ID: <4DF92C80.3030106@codemonkey.ws>
Date:	Wed, 15 Jun 2011 17:04:48 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Prasad Joshi <prasadjoshi124@...il.com>
CC:	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 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

It doesn't appear that writes are stable by default with native KVM 
tools.  They are stable by default in QEMU because since many guests 
simply do not inject FLUSH's reliably.

cache=writeback with qcow2 will use the same mode that native KVM tools 
is using, unstable writes for data with metadata consistency preserved.

This is almost certainly while you're seeing such high performance btw.

You should also advertise WCE=1 to the guest from a correctness 
perspective.  You aren't doing that right now.

Regards,

Anthony Liguori
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