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

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Date:	Mon, 28 May 2007 12:57:53 +1000
From:	Neil Brown <neilb@...e.de>
To:	David Chinner <dgc@....com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, linux-raid@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>,
	Phillip Susi <psusi@....rr.com>,
	Stefan Bader <Stefan.Bader@...ibm.com>,
	Andreas Dilger <adilger@...sterfs.com>,
	Tejun Heo <htejun@...il.com>
Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

On Monday May 28, dgc@....com wrote:
> On Mon, May 28, 2007 at 11:30:32AM +1000, Neil Brown wrote:
> > 
> > Thanks everyone for your input.  There was some very valuable
> > observations in the various emails.
> > I will try to pull most of it together and bring out what seem to be
> > the important points.
> > 
> > 
> > 1/ A BIO_RW_BARRIER request should never fail with -EOPNOTSUP.
> 
> Sounds good to me, but how do we test to see if the underlying
> device supports barriers? Do we just assume that they do and
> only change behaviour if -o nobarrier is specified in the mount
> options?
> 

What exactly do you want to know, and why do you care?

The idea is that every "struct block_device" supports barriers.  If the
underlying hardware doesn't support them directly, then they get
simulated by draining the queue and issuing a flush.
Theoretically there could be devices which have a write-back cache
that cannot be flushed, and you couldn't implement barriers on such a
device.  So throw it out and buy another?

As far as I can tell, the only thing XFS does differently with devices
that don't support barriers is that it prints a warning message to the
kernel logs.  If the underlying device printed the message when it
detected that barriers couldn't be supported, XFS wouldn't need to
care at all.

NeilBrown
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