linux-kernel - Re: [PATCH RFC 12/12] xen-block: implement indirect descriptors

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Date:	Tue, 19 Mar 2013 10:38:25 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Roger Pau Monné <roger.pau@...rix.com>,
	martin.petersen@...cle.com
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [PATCH RFC 12/12] xen-block: implement indirect descriptors

On Mon, Mar 18, 2013 at 06:06:38PM +0100, Roger Pau Monné wrote:
> On 28/02/13 11:28, Roger Pau Monne wrote:
> > Indirect descriptors introduce a new block operation
> > (BLKIF_OP_INDIRECT) that passes grant references instead of segments
> > in the request. This grant references are filled with arrays of
> > blkif_request_segment_aligned, this way we can send more segments in a
> > request.
> > 
> > The proposed implementation sets the maximum number of indirect grefs
> > (frames filled with blkif_request_segment_aligned) to 256 in the
> > backend and 64 in the frontend. The value in the frontend has been
> > chosen experimentally, and the backend value has been set to a sane
> > value that allows expanding the maximum number of indirect descriptors
> > in the frontend if needed.
> 
> I've added some additional debugging messages in blkfront, and found out
> that the queue in blkfront is not providing request bigger than 64
> segments for read requests, or 128 segments for write requests, although
> I set:
> 
> blk_queue_max_segments(info->rq, 256);
> 
> Is there any other limit I'm missing on the number of segments per
> request a queue can provide?

Martin, any ideas?
> 
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