linux-kernel - Re: Linux 2.6.29

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Message-ID: <49CACC4A.2080600@redhat.com>
Date:	Wed, 25 Mar 2009 20:28:58 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jeff Garzik <jeff@...zik.org>, Theodore Tso <tytso@....edu>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

Linus Torvalds wrote:
> On Wed, 25 Mar 2009, Ric Wheeler wrote:
>   
>> One concern with doing this above the file system is that you are not in the
>> context of a transaction so you have no clean promises about what is on disk
>> and persistent when. Flushing the cache is primitive at best, but the way
>> barriers work today is designed to give the transactions some pretty critical
>> ordering semantics for journalling file systems at least.
>>
>> I don't see how you could use this approach to make a really robust, failure
>> proof storage system, but it might appear to work most of the time for most
>> people :-)
>>     
>
> You just do a write barrier after doing all the filesystem writing, and 
> you return with the guarantee that all the writes the filesystem did are 
> actually on disk.
>
>   
In this case, you have not gained anything  - same number of barrier 
operations/cache flushes and looser semantics for the transactions?
> No gray areas. No questions. No "might appear to work". 
>
> Sure, there might be other writes that got flushed _too_, but nobody 
> cares. If you have a crash later on, that's always true - you don't get 
> crashes at nice well-defined points.
>
> 			Linus
>   
This is pretty much how write barriers work today - you carry down other 
transactions (even for other partitions on the same disk) with you...

ric

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