linux-kernel - Re: document ext3 requirements

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Message-ID: <Pine.LNX.4.64.0901042015230.31141@anakin>
Date:	Sun, 4 Jan 2009 20:21:06 +0100 (CET)
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Theodore Tso <tytso@....edu>
cc:	Duane Griffin <duaneg@...da.com>, Valdis.Kletnieks@...edu,
	Martin MOKREJŠ <mmokrejs@...osome.natur.cuni.cz>,
	Pavel Machek <pavel@...e.cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org
Subject: Re: document ext3 requirements

On Sun, 4 Jan 2009, Theodore Tso wrote:
> On Sun, Jan 04, 2009 at 02:24:43PM +0000, Duane Griffin wrote:
> > > Is there a way using md/dm/lvm etc to make the source partition R/O and
> > > replay the journal onto a CoW snapshop?  Admittedly, not easy to do inside
> > > the 'mount' command itself, but at least it might be workable for LiveCD R/O
> > > mounts and forensics work, where you can *tell* beforehand that's what you
> > > want and can jump through setup games before doing the mount...
> > 
> > Yes, something like that is best practice, as I understand it. The
> > LiveCD init scripts could check whether they are about to R/O mount an
> > ext[34] filesystem needing recovery and either refuse with a useful
> > message to the user, or even automatically create and mount a COW
> > snapshot, as you described. They'd still need to warn the user though,
> > since things like remounting R/W wouldn't work as expected.
> 
> So what's the use case where people want to be able to mount a
> filesystem needing recovery read/only without running the journal?

As mentioned before, suspending a laptop (running from hdd), running a live CD,
and expecting everything to work fine when resuming from hdd?

I think most people get shocked when they discover that mounting something
read-only may actualy write to the media. This is a bit unexpected (hey, if I
mount `read-only', I expect that no writes will happen), as it behaved
differently before the introduction of journalling.

As for mounting the root file system read-only during early boot up, and
remounting it read-write later, I guess it's quite complicated to replay the
journal (in RAM) on read-only mount, and deferring the replay writeback until
remounting read-write?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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