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

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Message-ID: <46613666.7010800@gmail.com>
Date:	Sat, 02 Jun 2007 18:20:38 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	David Chinner <dgc@....com>, david@...g.hm,
	Phillip Susi <psusi@....rr.com>, Neil Brown <neilb@...e.de>,
	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>
Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems,
  and dm/md.

Hello,

Jens Axboe wrote:
>> Would that be very different from issuing barrier and not waiting for
>> its completion?  For ATA and SCSI, we'll have to flush write back cache
>> anyway, so I don't see how we can get performance advantage by
>> implementing separate WRITE_ORDERED.  I think zero-length barrier
>> (haven't looked at the code yet, still recovering from jet lag :-) can
>> serve as genuine barrier without the extra write tho.
> 
> As always, it depends :-)
> 
> If you are doing pure flush barriers, then there's no difference. Unless
> you only guarantee ordering wrt previously submitted requests, in which
> case you can eliminate the post flush.
> 
> If you are doing ordered tags, then just setting the ordered bit is
> enough. That is different from the barrier in that we don't need a flush
> of FUA bit set.

Hmmm... I'm feeling dense.  Zero-length barrier also requires only one
flush to separate requests before and after it (haven't looked at the
code yet, will soon).  Can you enlighten me?

Thanks.

-- 
tejun

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