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From: "Nicholas A. Bellinger" <nab@linux-iscsi.org>
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Date: Thu, 14 Jun 2012 13:45:37 -0700
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Subject: Re: [Qemu-devel] IO performance test on the tcm-vhost scsi
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: target-devel@vger.kernel.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@gmail.com>, linuxram@us.ibm.com, qemu-devel@nongnu.org, Cong Meng <mc@linux.vnet.ibm.com>, Anthony Liguori <anthony@codemonkey.ws>, Asias He <asias@redhat.com>

On Thu, 2012-06-14 at 14:27 +0200, Paolo Bonzini wrote:
> Il 14/06/2012 14:07, Stefan Hajnoczi ha scritto:
> > Perfect, thanks.  virtio-scsi userspace is much better than virtio-blk
> > here.  That's unexpected since they both use the QEMU block layer.  If
> > anything, I would have expected virtio-blk to be faster!
> 
> Yes, I would have expected something similar.  A blktrace would be
> useful here because Asias measured the opposite---virtio-scsi being much
> slower than virtio-blk.
> 
> > The second question is why is tcm_vhost faster than virtio-scsi
> > userspace.
> 
> I would expect a difference on more high-end benchmarks (i.e. lots of
> I/O to lots of disks), similar to vhost-blk.  In this simple case I
> wonder how much it is due to the vagaries of the I/O scheduler, or even
> statistical noise.
> 

Mmmm, good point wrt to the I/O scheduler.

I wondering if the discrepancy between the two tests might be attributed
one of the tests using noop (or something else..?) with virtio guest
LUNs, or even the block scheduler used per default with the SCSI LUNs
serving as the backends here..

--nab