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

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Message-Id: <20090328124303.d902fb57.akpm@linux-foundation.org>
Date:	Sat, 28 Mar 2009 12:43:03 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, 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 15:10:56 -0700 (PDT) Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 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? 

- opening a single transaction for many pages in the cases when a
  transaction _is_ needed.

- single large BIO versus zillions of single-page BIOs.

Relatively minor benefits, but it's a bit odd that we never got around
to doing it.

It just got quite a bit harder to do, so I expect we won't be doing it.

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