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

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Message-ID: <Pine.LNX.4.64.0705311220080.17389@asgard.lang.hm>
Date:	Thu, 31 May 2007 12:21:31 -0700 (PDT)
From:	david@...g.hm
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Phillip Susi <psusi@....rr.com>, David Chinner <dgc@....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>,
	Tejun Heo <htejun@...il.com>
Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems,
  and dm/md.

On Thu, 31 May 2007, Jens Axboe wrote:

> On Thu, May 31 2007, 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?
>>   They need to be two completely different flags which you can choose
>> to combine, or use individually.
>
> If you have a use case for that, we can easily support it as well...
> Depending on the drive capabilities (FUA support or not), it may be
> nearly as slow as a "real" barrier write.

true, but a "real" barrier write could have significant side effects on 
other writes that wouldn't happen with a synchronous wrote (a sync wrote 
can have other, unrelated writes re-ordered around it, a barrier write 
can't)

David Lang
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