linux-ext4 - Re: [patch] ext2/3: document conditions when reliable operation is possible

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Message-ID: <20090824184842.GB29763@elf.ucw.cz>
Date:	Mon, 24 Aug 2009 20:48:43 +0200
From:	Pavel Machek <pavel@....cz>
To:	Theodore Tso <tytso@....edu>, Florian Weimer <fweimer@....de>,
	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

Hi!

> > > +* 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.

Ok, something like this?

Don't damage the old data on a powerfail (ATOMIC-WRITES-ON-POWERFAIL)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Either whole sector is correctly written or nothing is written during
powerfail.


									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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