linux-kernel - Re: [patch] ext2/3: document conditions when reliable operation is possible

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Message-ID: <20090824135035.GI23677@mit.edu>
Date:	Mon, 24 Aug 2009 09:50:35 -0400
From:	Theodore Tso <tytso@....edu>
To:	Florian Weimer <fweimer@....de>
Cc:	Pavel Machek <pavel@....cz>,
	Goswin von Brederlow <goswin-v-b@....de>,
	Rob Landley <rob@...dley.net>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [patch] ext2/3: document conditions when reliable operation is
	possible

On Mon, Aug 24, 2009 at 11:19:01AM +0000, Florian Weimer wrote:
> > +* don't damage the old data on a failed write (ATOMIC-WRITES)
> > +
> > +	(Thrash may get written into sectors during powerfail.  And
> > +	ext3 handles this surprisingly well at least in the
> > +	catastrophic case of garbage getting written into the inode
> > +	table, since the journal replay often will "repair" the
> > +	garbage that was written into the filesystem metadata blocks.
> 
> Isn't this by design?  In other words, if the metadata doesn't survive
> non-atomic writes, wouldn't it be an ext3 bug?

So I got confused when I quoted your note, which I had assumed was
exactly what Pavel had written in his documentation.  In fact, what he
had written was this:

+Don't damage the old data on a failed write (ATOMIC-WRITES)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Either whole sector is correctly written or nothing is written during
+powerfail.
+
+....

So he had explicitly stated that he only cared about the whole sector
being written (or not written) in the power fail case, and not any
other.  I'd suggest changing ATOMIC-WRITES to
ATOMIC-WRITE-ON-POWERFAIL, since the one-line summary, "Don't damage
the old data on a failed write", is also singularly misleading.

    	     	  	 	    	 	    - Ted
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