linux-kernel - Re: document ext3 requirements

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Message-ID: <20090104001859.GH1666@elf.ucw.cz>
Date:	Sun, 4 Jan 2009 01:19:00 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Martin MOKREJŠ <mmokrejs@...osome.natur.cuni.cz>
Cc:	Duane Griffin <duaneg@...da.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, tytso@....edu,
	mtk.manpages@...il.com, rdunlap@...otime.net,
	linux-doc@...r.kernel.org
Subject: Re: document ext3 requirements

On Sun 2009-01-04 00:01:58, Martin MOKREJŠ wrote:
> 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?
> > ...basically treating it like an ext2?
> > 
> > ...ok, that will present "old" version of the filesystem to the
> > user... violating fsync() semantics.
> 
> Hmm, so if my dual-boot machine does not shutdown correctly and I boot
> accidentally in M$ Win where I use ext2 IFS driver and modify some
> stuff on the ext3 drive, after a while reboot to linux and the journal
> get re-played ... Mmm ...

ext2 driver should refuse to mount dirty ext3 filesystem. (Linux ext2
driver does that).

> > Still handy for recovering badly broken filesystems, I'd say.
> 
> Me as well. How about improving you doc patch with some summary of
> this thread (although it is probably not over yet)? ;-) Definitely,
> a note that one can mount it as ext2 while read-only would be helpful
> when doing some forensics on the disk.

No, you can't mount unclean ext3 as an ext2; patch to do that would be
possible but...

I believe the patch is correct & useful.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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