linux-kernel - Re: Linux 2.6.29

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Date:	Tue, 24 Mar 2009 13:52:49 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Theodore Tso <tytso@....edu>
Cc:	Ingo Molnar <mingo@...e.hu>,
	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>,
	Jens Axboe <jens.axboe@...cle.com>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

> They don't solve the problem where there is a *huge* amount of writes
> going on, though --- if something is dirtying pages at a rate far

At very high rates other things seem to go pear shaped. I've not traced
it back far enough to be sure but what I suspect occurs from the I/O at
disk level is that two people are writing stuff out at once - presumably
the vm paging pressure and the file system - as I see two streams of I/O
that are each reasonably ordered but are interleaved.

> don't get *that* bad, even with ext3.  At least, I haven't found a
> workload that doesn't involve either dd if=/dev/zero or a massive
> amount of data coming in over the network that will cause fsync()
> delays in the > 1-2 second category.  Ext3 has been around for a long

I see it with a desktop when it pages hard and also when doing heavy
desktop I/O (in my case the repeatable every time case is saving large
images in the gimp - A4 at 600-1200dpi).

The other one (#8636) seems to be a bug in the I/O schedulers as it goes
away if you use a different I/O sched.

> solve.  Simply mounting an ext3 filesystem using ext4, without making
> any change to the filesystem format, should solve the problem.

I will try this experiment but not with production data just yet 8)

> some other users' data files.  This was the reason for Stephen Tweedie
> implementing the data=ordered mode, and making it the default.

Yes and in the server environment or for typical enterprise customers
this is a *big issue*, especially the risk of it being undetected that
they just inadvertently did something like put your medical data into the
end of something public during a crash.

> Try ext4, I think you'll like it.  :-)

I need to, so that I can double check none of the open jbd locking bugs
are there and close more bugzilla entries (#8147)

Thanks for the reply - I hadn't realised a lot of this was getting fixed
but in ext4 and quietly

Alan
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