linux-kernel - Re: [Xen-devel] Re: [PATCH] xen block backend driver. - proper flush/barrier/fua support missing.

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Message-ID: <20110502190843.GA29913@dumpdata.com>
Date:	Mon, 2 May 2011 15:08:43 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Daniel Stodden <daniel.stodden@...rix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"jaxboe@...ionio.com" <jaxboe@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"konrad@...nel.org" <konrad@...nel.org>
Subject: Re: [Xen-devel] Re: [PATCH] xen block backend driver. - proper
 flush/barrier/fua support missing.

On Fri, Apr 22, 2011 at 05:09:04AM -0400, Christoph Hellwig wrote:
> On Thu, Apr 21, 2011 at 12:14:37PM -0700, Daniel Stodden wrote:
> > > There is a huge userbase of guests out there that does rely on it.
> > 
> > Which ones? Old blkfront would have make a difference back then when
> > barriers used to be an option, but it never actually declared it, right?
> 
> Pre-Linux 2.6.37 guests using reiserfs actually relied on the queue
> flushing.  This includes a lot of SLES installation which are still
> in common use.  There's only two options to make sure they work:
> 
>  (1) keep the original barrier semantics and flush the queue
>  (2) do not advertize "barrier" support at all, and make sure to submit
>    every I/O we get with the FUA bit.
> 
> In practice (2) is going to be faster for most real-life workloads.  So
> maybe you should just drop the old "barrier" support and just send
> requests with the FUA bit set for now, until you have proper flush
> and fua support in the protocol.

Let me play with both options and see how they pan out.

Christoph,

Is there a good tool to unmask barrier bugs? I found this one:
http://lwn.net/Articles/283169/

but not sure if that still applies to this case? Or does
running bonnie++ expose the potential issues?
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