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

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Message-ID: <4660B21D.9030801@tmr.com>
Date:	Fri, 01 Jun 2007 19:56:13 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Neil Brown <neilb@...e.de>
CC:	David Chinner <dgc@....com>, Phillip Susi <psusi@....rr.com>,
	Jens Axboe <jens.axboe@...cle.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.

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.

My impression is that the sync will return when the i/o has been 
delivered to the device, and will get special treatment by the elevator 
code (I looked quickly, more is needed). I'm sore someone will tell me 
if I misread this. ;-)

-- 
bill davidsen <davidsen@....com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

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