linux-kernel - Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO

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Message-ID: <4F01D8C5.1060300@redhat.com>
Date:	Mon, 02 Jan 2012 17:18:13 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Stefan Hajnoczi <stefanha@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Chris Wright <chrisw@...s-sol.org>,
	Jens Axboe <axboe@...nel.dk>,
	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to
 BIO

On 01/02/2012 05:15 PM, Christoph Hellwig wrote:
> >  When QEMU uses O_DIRECT, the guest should not use QUEUE_FLAG_NONROT
> >  unless it is active for the host disk as well.  (In doubt, as is the
> >  case for remote hosts accessed over NFS, I would also avoid NONROT
> >  and allow more coalescing).
>
> Do we have any benchmark numbers where QUEUE_FLAG_NONROT makes a
> difference?

Not that I know of.

> I tried a few times, and the only constant measureable
> thing was that it regressed performance when used for rotating devices
> in a few benchmarks.

Were you trying with cache=none or writeback?  For cache=none, that's 
exactly what I'd expect.  cache=writeback could be more interesting...

Paolo
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