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

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Message-ID: <1340019575.22848.2.camel@lappy>
Date:	Mon, 18 Jun 2012 13:39:35 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	dlaor@...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 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?

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