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

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Date:	Wed, 04 Jul 2012 12:10:42 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Asias He <asias@...hat.com>
Cc:	Sasha Levin <levinsasha928@...il.com>, dlaor@...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 Tue, 03 Jul 2012 08:39:39 +0800, Asias He <asias@...hat.com> wrote:
> On 07/02/2012 02:41 PM, Rusty Russell wrote:
> > Sure, our guest merging might save us 100x as many exits as no merging.
> > But since we're not doing many requests, does it matter?
> 
> We can still have many requests with slow devices. The number of 
> requests depends on the workload in guest. E.g. 512 IO threads in guest 
> keeping doing IO.

You can have many requests outstanding.  But if the device is slow, the
rate of requests being serviced must be low.

Am I misunderstanding something?  I thought if you could have a high
rate of requests, it's not a slow device.

Cheers,
Rusty.
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