Date: Wed, 10 Nov 2010 12:32:55 +1100
From: Dave Chinner <david@fromorbit.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jens Axboe <axboe@kernel.dk>, dave b <db.pub.mail@gmail.com>,
        Sanjoy Mahajan <sanjoy@olin.edu>, Jesper Juhl <jj@chaosbits.net>,
        Chris Mason <chris.mason@oracle.com>, Ingo Molnar <mingo@elte.hu>,
        Pekka Enberg <penberg@kernel.org>,
        Aidar Kultayev <the.aidar@gmail.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Nick Piggin <npiggin@suse.de>,
        Arjan van de Ven <arjan@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, "Ted Ts'o" <tytso@mit.edu>,
        Corrado Zoccolo <czoccolo@gmail.com>,
        Shaohua Li <shaohua.li@intel.com>, Steven Barrett <damentz@gmail.com>
Subject: Re: 2.6.36 io bring the system to its knees
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On Sun, Nov 07, 2010 at 07:50:13AM -0800, Linus Torvalds wrote:
> On Sun, Nov 7, 2010 at 4:08 AM, Jens Axboe <axboe@kernel.dk> wrote:
> >
> > As already mentioned, ext3 is just not a good choice for this sort of
> > thing. Did you have atimes enabled?
> 
> At least for ext3, more important than atimes is the "data=writeback"
> setting. Especially since our atime default is sane these days (ie if
> you don't specify anything, we end up using 'relatime').
> 
> If you compile your own kernel, answer "N" to the question
> 
>   Default to 'data=ordered' in ext3?
> 
> at config time (CONFIG_EXT3_DEFAULTS_TO_ORDERED), or you can make sure
> "data=writeback" is in the fstab (but I don't think everything honors
> it for the root filesystem).

Don't forget to mention data=writeback is not the default because if
your system crashes or you lose power running in this mode it will
*CORRUPT YOUR FILESYSTEM* and you *WILL LOSE DATA*. Not to mention
the significant security issues (e.g stale data exposure) that also
occur even if the filesystem is not corrupted by the crash. IOWs,
data=writeback is the "fast but I'll eat your data" option for ext3.

So I recommend that nobody follows this path because it only leads
to worse trouble down the road.  Your best bet it to migrate away
from ext3 to a filesystem that doesn't have such inherent ordering
problems like ext4 or XFS....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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