linux-kernel - Re: document ext3 requirements

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Message-ID: <20090106100634.GA6700@merlin.emma.line.org>
Date:	Tue, 6 Jan 2009 11:06:34 +0100
From:	Matthias Andree <matthias.andree@....de>
To:	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: document ext3 requirements

On Sat, 03 Jan 2009, Pavel Machek wrote:

> On Sat 2009-01-03 22:17:15, Duane Griffin wrote:
> > [Fixed top-posting]
> > 
> > 2009/1/3 Martin MOKREJŠ <mmokrejs@...osome.natur.cuni.cz>:
> > > Pavel Machek wrote:
> > >> readonly mount does actually write to the media in some cases. Document that.
> > >>
> > > Can one avoid replay of the journal then if it would be unclean?
> > > Just curious.
> > 
> > Nope. If the underlying block device is read-only then mounting the
> > filesystem will fail. I tried to fix this some time ago, and have a
> > set of patches that almost always work, but "almost always" isn't good
> > enough. Unfortunately I never managed to figure out a way to finish it
> > off without disgusting hacks or major surgery.
> 
> Uhuh, can you just ignore the journal and mount it anyway?

An ext3 file system that needs journal recovery sets one of the ext2
incompatible flags to prevent just that.

> ...basically treating it like an ext2?
> 
> ...ok, that will present "old" version of the filesystem to the
> user... violating fsync() semantics.
> 
> Still handy for recovering badly broken filesystems, I'd say.

While you cannot have that, you'll need to dump the file system
(possibly with dd_rescue) to another medium and work on the copy.
That's what you should do anyways. ;-)

I think if you really want to mount the file system without journal
replay, you need to clear the needs-recovery "incompat" flag (on the
copy, obviously).

-- 
Matthias Andree
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