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

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Date:	Fri, 23 Dec 2011 08:41:35 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Stefan Hajnoczi <stefanha@...il.com>
Cc:	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 Thu, Dec 22, 2011 at 12:57:40PM +0000, Stefan Hajnoczi wrote:
> On Wed, Dec 21, 2011 at 1:00 AM, Minchan Kim <minchan@...nel.org> wrote:
> > This patch is follow-up of Christohp Hellwig's work
> > [RFC: ->make_request support for virtio-blk].
> > http://thread.gmane.org/gmane.linux.kernel/1199763
> >
> > Quote from hch
> > "This patchset allows the virtio-blk driver to support much higher IOP
> > rates which can be driven out of modern PCI-e flash devices.  At this
> > point it really is just a RFC due to various issues."
> 
> Basic question to make sure I understood this series: does this patch
> bypass the guest I/O scheduler (but then you added custom batching
> code into virtio_blk.c)?

Right.

> 
> If you're stumped by the performance perhaps compare blktraces of the
> request approach vs the bio approach.  We're probably performing I/O
> more CPU-efficiently but the I/O pattern itself is worse.

You mean I/O scheduler have many techniques to do well in I/O pattern?
That's what I want to discuss in this RFC.

I guess request layer have many techniques proved during long time
to do well I/O but BIO-based drvier ignores them for just reducing locking
overhead. Of course, we can add such techniques to BIO-batch driver like 
custom-batch in this series. But it needs lots of work, is really duplication,
and will have a problem on maintenance.

I would like to listen opinions whether this direction is good or bad.

> 
> Stefan

-- 
Kind regards,
Minchan Kim
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