linux-kernel - Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

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Date:	Thu, 31 May 2007 08:24:04 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Phillip Susi <psusi@....rr.com>
Cc:	device-mapper development <dm-devel@...hat.com>,
	David Chinner <dgc@....com>, Tejun Heo <htejun@...il.com>,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Andreas Dilger <adilger@...sterfs.com>,
	Stefan Bader <Stefan.Bader@...ibm.com>
Subject: Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices,  filesystems, and dm/md.

On Wed, May 30 2007, Phillip Susi wrote:
> >That would be the exactly how I understand Documentation/block/barrier.txt:
> >
> >"In other words, I/O barrier requests have the following two properties.
> >1. Request ordering
> >...
> >2. Forced flushing to physical medium"
> >
> >"So, I/O barriers need to guarantee that requests actually get written
> >to non-volatile medium in order." 
> 
> I think you misinterpret this, and it probably could be worded a bit 
> better.  The barrier request is about constraining the order.  The 
> forced flushing is one means to implement that constraint.  The other 
> alternative mentioned there is to use ordered tags.  The key part there 
> is "requests actually get written to non-volatile medium _in order_", 
> not "before the request completes", which would be synchronous IO.

No Stephan is right, the barrier is both an ordering and integrity
constraint. If a driver completes a barrier request before that request
and previously submitted requests are on STABLE storage, then it
violates that principle. Look at the code and the various ordering
options.

-- 
Jens Axboe

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