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Re: [Xen-devel] Re: [PATCH] xen block backend driver. - proper flush/bar

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] xen block backend driver. - proper flush/barrier/fua support missing.
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Mon, 2 May 2011 15:08:43 -0400
Cc: "jaxboe@xxxxxxxxxxxx" <jaxboe@xxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "konrad@xxxxxxxxxx" <konrad@xxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Daniel Stodden <daniel.stodden@xxxxxxxxxx>
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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.


Is there a good tool to unmask barrier bugs? I found this one:

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

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