Date: Tue, 24 Mar 2009 15:18:25 +0000
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Theodore Tso <tytso@mit.edu>
Cc: Ingo Molnar <mingo@elte.hu>, Arjan van de Ven <arjan@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>, Nick Piggin <npiggin@suse.de>,
       Jens Axboe <jens.axboe@oracle.com>, David Rees <drees76@gmail.com>,
       Jesper Krogh <jesper@krogh.cc>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Linux 2.6.29
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> Surely the elevator should have reordered the writes reasonably?  (Or
> is that what you meant by "the other one -- #8636 (I assume this is a
> kernel Bugzilla #?) seems to be a bug in the I/O schedulers as it goes
> away if you use a different I/O sched.?")

There are two cases there. One is a bug #8636 (kernel bugzilla) which is
where things like dump show awful performance with certain I/O scheduler
settings. That seems to be totally not connected to the fs but it is a
problem (and has a patch)

The second one the elevator is clearly trying to sort out but its
behaving as if someone is writing the file starting at say 0 and someone
else is trying to write it back starting some large distance further down
the file. The elevator can only do so much then.

> Yeah, I could see that doing it.  How big is the image, and out of
> curiosity, can you run the fsync-tester.c program I posted while

150MB+ for the pnm files from gimp used as temporaries by Eve (Etch
Validation Engine), more like 10MB for xcf/tif files.

> saving the gimp image, and tell me how much of a delay you end up
> seeing?

Added to the TODO list once I can set up a suitable test box (my new dev
box is somewhere between Dell and my desk right now)

> More testing would be appreciated --- and yeah, we need to groom the
> bugzilla.  

I'm currently doing this on a large scale (closed about 300 so far this
run). Bug 8147 might be worth a look as its a case where the jbd locking
and the jbd comments seem to disagree (the comments say you must hold a
lock but we don't seem to)
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