linux-ext4 - Re: [patch] ext2/3: document conditions when reliable operation is possible

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Message-ID: <82k50tjw7u.fsf@mid.bfk.de>
Date:	Mon, 24 Aug 2009 11:19:01 +0000
From:	Florian Weimer <fweimer@....de>
To:	Pavel Machek <pavel@....cz>
Cc:	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,
	tytso@....edu, rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [patch] ext2/3: document conditions when reliable operation is possible

* Pavel Machek:

> +Linux block-backed filesystems can only work correctly when several
> +conditions are met in the block layer and below (disks, flash
> +cards). Some of them are obvious ("data on media should not change
> +randomly"), some are less so.

You should make clear that the file lists per-file-system rules and
that some file sytems can recover from some of the error conditions.

> +* 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?

-- 
Florian Weimer                <fweimer@....de>
BFK edv-consulting GmbH       http://www.bfk.de/
Kriegsstraße 100              tel: +49-721-96201-1
D-76133 Karlsruhe             fax: +49-721-96201-99
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