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

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Date:	Mon, 2 Jan 2012 11:15:18 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
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,
	Christoph Hellwig <hch@...radead.org>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO

On Mon, Jan 02, 2012 at 05:12:00PM +0100, Paolo Bonzini wrote:
> On 01/01/2012 05:45 PM, Stefan Hajnoczi wrote:
> >By the way, drivers for solid-state devices can set QUEUE_FLAG_NONROT
> >to hint that seek time optimizations may be sub-optimal.  NBD and
> >other virtual/pseudo device drivers set this flag.  Should virtio-blk
> >set it and how does it affect performance?
> 
> By itself is not a good idea in general.
> 
> 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?  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.

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