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

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Message-ID: <4663C1C1.5000009@gmail.com>
Date:	Mon, 04 Jun 2007 16:39:45 +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.

Jens Axboe wrote:
> On Sat, Jun 02 2007, Tejun Heo wrote:
>> 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?
> 
> Yeah, that's what the zero-length barrier implementation I posted does.
> Not sure if you have a question beyond that, if so fire away :-)

I thought you were talking about adding BIO_RW_ORDERED instead of
exposing zero length BIO_RW_BARRIER.  Sorry about the confusion.  :-)

-- 
tejun
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