linux-kernel - Re: ext3 IO latency measurements (was: Linux 2.6.29)

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Message-ID: <alpine.LFD.2.00.0903271508520.3994@localhost.localdomain>
Date:	Fri, 27 Mar 2009 15:10:56 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Theodore Tso <tytso@....edu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>
Subject: Re: ext3 IO latency measurements (was: Linux 2.6.29)



On Fri, 27 Mar 2009, Jan Kara wrote:
>
>   Doable but not fairly simple ;) Firstly you have to restart a transaction
> when you've used up all the credits you originally started with (easy),
> secondly ext3 uses lock order PageLock -> "transaction start" which is
> unusable for the scheme you suggest. So we'd have to revert that - which
> needs larger audit of our locking scheme and that's probably the reason
> why noone has done it yet.

It's also not clear that ext3 can really do much better than the regular 
generic_writepages() logic. I mean, seriously, what's there to improve on? 
The transaction code is all normally totally pointless, and I merged the 
patch that avoids it when not necessary. 

It might be different if more people used "data=journal", but I don't 
doubt that is very common. For data=writeback and data=ordered, I bet 
generic_writepages() is as good as anything ext3-specific could be.

			Linus
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