linux-kernel - Re: Linux 2.6.29

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Message-ID: <49CA9346.6040108@garzik.org>
Date:	Wed, 25 Mar 2009 16:25:42 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	unlisted-recipients:; (no To-header on input)
CC:	Linus Torvalds <torvalds@...ux-foundation.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

Jens Axboe wrote:
> On Wed, Mar 25 2009, Jeff Garzik wrote:
>> Stating "fsync already does that" borders on false, because that assumes
>> (a) the user has a fs that supports barriers
>> (b) the user is actually aware of a 'barriers' mount option and what it  
>> means
>> (c) the user has turned on an option normally defaulted to off.
>>
>> Or in other words, it pretty much never happens.
> 
> That is true, except if you use xfs/ext4. And this discussion is fine,
> as was the one a few months back that got ext4 to enable barriers by
> default. If I had submitted patches to do that back in 2001/2 when the
> barrier stuff was written, I would have been shot for introducing such a
> slow down. After people found out that it just wasn't something silly,
> then you have a way to enable it.
> 
> I'd still wager that most people would rather have a 'good enough
> fsync' on their desktops than incur the penalty of barriers or write
> through caching. I know I do.

That's a strawman argument:  The choice is not between "good enough 
fsync" and full use of barriers / write-through caching, at all.

It is clearly possible to implement an fsync(2) that causes FLUSH CACHE 
to be issued, without adding full barrier support to a filesystem.  It 
is likely doable to avoid touching per-filesystem code at all, if we 
issue the flush from a generic fsync(2) code path in the kernel.

Thus, you have a "third way":  fsync(2) gives the guarantee it is 
supposed to, but you do not take the full performance hit of 
barriers-all-the-time.

Remember, fsync(2) means that the user _expects_ a performance hit.

And they took the extra step to call fsync(2) because they want a 
guarantee, not a lie.

	Jeff



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