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

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Date:	Fri, 1 Jun 2007 08:11:06 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Neil Brown <neilb@...e.de>
Cc:	David Chinner <dgc@....com>, Phillip Susi <psusi@....rr.com>,
	david@...g.hm, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	linux-raid@...r.kernel.org, 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 Fri, Jun 01 2007, Neil Brown wrote:
> On Friday June 1, dgc@....com wrote:
> > On Thu, May 31, 2007 at 02:31:21PM -0400, Phillip Susi wrote:
> > > David Chinner wrote:
> > > >That sounds like a good idea - we can leave the existing
> > > >WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED
> > > >behaviour that only guarantees ordering. The filesystem can then
> > > >choose which to use where appropriate....
> > > 
> > > So what if you want a synchronous write, but DON'T care about the order? 
> > 
> > submit_bio(WRITE_SYNC, bio);
> > 
> > Already there, already used by XFS, JFS and direct I/O.
> 
> Are you sure?
> 
> You seem to be saying that WRITE_SYNC causes the write to be safe on
> media before the request returns.  That isn't my understanding.
> I think (from comments near the definition and a quick grep through
> the code) that WRITE_SYNC expedites the delivery of the request
> through the elevator, but doesn't do anything special about getting it
> onto the media.
> It essentially say "Submit this request now, don't wait for more
> request to bundle with it for better bandwidth utilisation"

That is exactly right. WRITE_SYNC doesn't give any integrity guarentees,
it's just makes sure it goes straight through the io scheduler.

-- 
Jens Axboe

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