linux-kernel - Re: [PATCH V4 3/3] virtio-blk: Add bio-based IO path for virtio-blk

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Message-ID: <20120730134247.GA6041@lst.de>
Date:	Mon, 30 Jul 2012 15:42:47 +0200
From:	Christoph Hellwig <hch@....de>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, Asias He <asias@...hat.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	Christoph Hellwig <hch@....de>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH V4 3/3] virtio-blk: Add bio-based IO path for virtio-blk

On Mon, Jul 30, 2012 at 11:25:51AM +0930, Rusty Russell wrote:
> I consider this approach a half-way step.  Quick attempts on my laptop
> and I couldn't find a case where the bio path was a loss, but in theory
> if the host wasn't doing any reordering and it was a slow device, you'd
> want the guest to do so.
> 
> I'm not sure if current qemu can be configured to do such a thing?


The host kernel will do the I/O scheduling for you unless you explicitly
disable it.  And we should be able to assume an administrator will only
disable it when they have a reason for it - if not they'll get worse
performance for non-virtualized workloads as well.

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