linux-kernel - Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk

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Date:	Tue, 19 Jun 2012 09:21:19 +0300
From:	Dor Laor <dlaor@...hat.com>
To:	Asias He <asias@...hat.com>
CC:	Sasha Levin <levinsasha928@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk

On 06/19/2012 05:51 AM, Asias He wrote:
> On 06/18/2012 07:39 PM, Sasha Levin wrote:
>> On Mon, 2012-06-18 at 14:14 +0300, Dor Laor wrote:
>>> On 06/18/2012 01:05 PM, Rusty Russell wrote:
>>>> On Mon, 18 Jun 2012 16:03:23 +0800, Asias He<asias@...hat.com> wrote:
>>>>> On 06/18/2012 03:46 PM, Rusty Russell wrote:
>>>>>> On Mon, 18 Jun 2012 14:53:10 +0800, Asias He<asias@...hat.com> wrote:
>>>>>>> This patch introduces bio-based IO path for virtio-blk.
>>>>>>
>>>>>> Why make it optional?
>>>>>
>>>>> request-based IO path is useful for users who do not want to bypass
>>>>> the
>>>>> IO scheduler in guest kernel, e.g. users using spinning disk. For
>>>>> users
>>>>> using fast disk device, e.g. SSD device, they can use bio-based IO
>>>>> path.
>>>>
>>>> Users using a spinning disk still get IO scheduling in the host though.
>>>> What benefit is there in doing it in the guest as well?
>>>
>>> The io scheduler waits for requests to merge and thus batch IOs
>>> together. It's not important w.r.t spinning disks since the host can do
>>> it but it causes much less vmexits which is the key issue for VMs.
>>
>> Is the amount of exits caused by virtio-blk significant at all with
>> EVENT_IDX?
>
> Yes. EVENT_IDX saves the number of notify and interrupt. Let's take the
> interrupt as an example, The guest fires 200K request to host, the
> number of interrupt is about 6K thanks to EVENT_IDX. The ratio is 200K /
> 6K = 33. The ratio of merging is 40000K / 200K = 20.
>

In this case, why don't you always recommend bio over request based?
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