Re: [patch] ext2/3: document conditions when reliable operation ispossible

From: Theodore Tso
Date: Mon Aug 24 2009 - 09:51:18 EST

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

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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