linux-kernel - Re: 2.6.36 io bring the system to its knees

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Date:	Thu, 11 Nov 2010 10:36:48 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Theodore Tso <tytso@....EDU>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, dave b <db.pub.mail@...il.com>,
	Sanjoy Mahajan <sanjoy@...n.edu>,
	Jesper Juhl <jj@...osbits.net>,
	Chris Mason <chris.mason@...cle.com>,
	Ingo Molnar <mingo@...e.hu>, Pekka Enberg <penberg@...nel.org>,
	Aidar Kultayev <the.aidar@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Corrado Zoccolo <czoccolo@...il.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Steven Barrett <damentz@...il.com>
Subject: Re: 2.6.36 io bring the system to its knees

On Wed, Nov 10, 2010 at 09:33:29AM -0500, Theodore Tso wrote:
> 
> On Nov 9, 2010, at 8:32 PM, Dave Chinner wrote:
> 
> > 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.
> 
> This is strictly speaking not true.  Using data=writeback will not
> cause you to lose any data --- at least, not any more than you
> would without the feature.   If you have applications that write
> files in an unsafe way, that data is going to be lost, one way or
> another.  (i.e., with XFS in a similar situation you'll get a
> zero-length file)   The difference is that in the case of a system
> crash, there may be unwritten data revealed if you use
> data=writeback.  This could be a security exposure, especially if
> you are using your system in as time-sharing system, and where you
> see the contents of deleted files belonging to another user.

In theory, that's all that is _supposed_ to happen. However, my
recent experience is that massive ext3 filesystem corruption occurs
in data=writeback mode when the system crashes and that does not
happen in ordered mode.

Why do you think i posted the patches to change the default back to
ordered mode a few months back? I basically trashed the root ext3
partitions on three test machines (to the point where >5000 files
across /sbin, /bin, /lib and /usr were corrupted or missing and I
had to reinstall from scratch) when I'd forgotten to set the
ordered-is-defult config option in the kernel i was testing.  And
that is when the only thing being written to the root filesystems
was log files...

The worst part about this was that I also had ext3 filesystems
corrupted by crashes in such a way that e2fsck didn't detect it but
they would repeatedly trigger kernel crashes at runtime....

> So it is not an "eat your data" situation,

My experience says otherwise....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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