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

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Message-ID: <4FF3017C.4020605@redhat.com>
Date:	Tue, 03 Jul 2012 17:28:12 +0300
From:	Dor Laor <dlaor@...hat.com>
To:	Ronen Hod <rhod@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>, Asias He <asias@...hat.com>,
	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 07/03/2012 05:22 PM, Ronen Hod wrote:
> On 06/18/2012 02:14 PM, 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.
>
> Does it make sense to use the guest's I/O scheduler at all?

That's the reason we have a noop io scheduler.

> - It is not aware of the physical (spinning) disk layout.
> - It is not aware of all the host's disk pending requests.
> It does have a good side-effect - batching of requests.
>
> Ronen.
>
>>
>>>
>>> Cheers,
>>> Rusty.
>>> _______________________________________________
>>> Virtualization mailing list
>>> Virtualization@...ts.linux-foundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/virtualization
>>
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