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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support
Date: Wed, 29 Apr 2009 12:37:20 +0100
References: <20090427082606.GA32604@lst.de>
	<200904291211.20374.paul@codesourcery.com>
	<20090429112130.GA11241@lst.de>
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To: Christoph Hellwig <hch@lst.de>
Cc: kvm@vger.kernel.org, Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org, Christian Borntraeger <borntraeger@de.ibm.com>, Hannes Reinecke <hare@suse.de>

On Wednesday 29 April 2009, Christoph Hellwig wrote:
> On Wed, Apr 29, 2009 at 12:11:19PM +0100, Paul Brook wrote:
> > Is this actually measurably faster, or just infinitesimally faster in
> > theory?
>
> On normal disks it's rather theoretical.  On high-end SSDs and arrays the
> impact is noticeable, mostly due to the additional latency.

How exactly does it introduce additional latency? A scsi command block is 
hardly large or complicated. Are you suggesting that a 16/32byte scsi command 
takes significantly longer to process than a 16byte virtio command 
descriptor? I'd expect any extra processing to be a small fraction of the 
host syscall latency, let alone the latency of the physical host adapter. It 
probably even fits on the same CPU cache line.

Paul