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

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Date:	Tue, 25 Aug 2009 11:42:44 +0200
From:	Pavel Machek <pavel@....cz>
To:	Theodore Tso <tytso@....edu>, Ric Wheeler <rwheeler@...hat.com>,
	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, corbet@....net
Subject: Re: [patch] ext2/3: document conditions when reliable operation is
	possible

On Mon 2009-08-24 20:08:42, Theodore Tso wrote:
> On Tue, Aug 25, 2009 at 01:00:36AM +0200, Pavel Machek wrote:
> > Then to answer your question... ext2. You expect to run fsck after
> > unclean shutdown, and you expect to have to solve some problems with
> > it. So the way ext2 deals with the flash media actually matches what
> > the user expects. (*)
> 
> But if the 256k hole is in data blocks, fsck won't find a problem,
> even with ext2.

True.

> And if the 256k hole is the inode table, you will *still* suffer
> massive data loss.  Fsck will tell you how badly screwed you are, but
> it doesn't "fix" the disk; most users don't consider questions of the
> form "directory entry <precious-thesis-data> points to trashed inode,
> may I delete directory entry?" as being terribly helpful.  :-/

Well it will fix the disk in the end. And no, "directory entry
<precious-thesis-data> points to trashed inode, may I delete directory
entry?" is not _terribly_ helpful, but it is slightly helpful and
people actually expect that from ext2.

> Maybe this came as a surprise to you, but anyone who has used a
> compact flash in a digital camera knows that you ***have*** to wait
> until the led has gone out before trying to eject the flash card.  I
> remember seeing all sorts of horror stories from professional
> photographers about how they lost an important wedding's day worth of
> pictures with the attendant commercial loss, on various digital
> photography forums.  It tends to be the sort of mistake that digital
> photographers only make once.

It actually comes as surprise to me. Actually yes and no. I know that
digital cameras use VFAT, so pulling CF card out of it may do bad
thing, unless I run fsck.vfat afterwards. If digital camera was using
ext3, I'd expect it to be safely pullable at any time.

Will IBM microdrive do any difference there?

Anyway, it was not known to me. Rather than claiming "everyone knows"
(when clearly very few people really understand all the details), can
we simply document that?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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