linux-kernel - Re: Help with implementing some form of barriers in 3.0 kernels.

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Message-ID: <20110908082229.GB14549@infradead.org>
Date:	Thu, 8 Sep 2011 04:22:29 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>, jbeulich@...ell.com,
	linux-kernel@...r.kernel.org, JBeulich@...e.com
Subject: Re: Help with implementing some form of barriers in 3.0 kernels.

On Wed, Sep 07, 2011 at 05:31:37PM -0400, Vivek Goyal wrote:
> I have a basic question. In old guest why BARRIER handling on request
> queue is not sufficient for sequencing and ordering of requests and why
> xen-blkfront and qemu have to do something about it.
> 
> I am also wondering if virtio-blk have similar issues?

Mainline xen-blkfront was using ORDERED_TAG before, which meant
bypassing the sequencer entirely, as well as not beeing able to
implement plain cache flushes as used by ->fsync.

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