linux-kernel - Re: writing file to disk: not as easy as it looks

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Message-ID: <20081202191038.GE29091@vanheusden.com>
Date:	Tue, 2 Dec 2008 20:10:42 +0100
From:	Folkert van Heusden <folkert@...heusden.com>
To:	Theodore Tso <tytso@....edu>, Pavel Machek <pavel@...e.cz>,
	mikulas@...ax.karlin.mff.cuni.cz, clock@...ey.karlin.mff.cuni.cz,
	kernel list <linux-kernel@...r.kernel.org>, aviro@...hat.com
Subject: Re: writing file to disk: not as easy as it looks

> > If disk looses data after acknowledging the write, all hope is lost.
> > Else I expect filesystem to preserve data I successfully synced.
> >      (In the b-tree split failed case I'd expect transaction commit to
> >      fail because new data could not be weitten; at that point
> >      disk+journal should still contain all the data needed for
> >      recovery of synced/old files, right?)
> 
> Not necessarily.  For filesystems that do logical journalling (i.e.,
> xfs, jfs, et. al), the only thing written in the journal is the
> logical change (i.e., "new dir entry 'file_that_causes_the_node_split'").

> The transaction commits *first*, and then the filesystem tries to
> write update the filesystem with the change, and it's only then that
> the write fails.  Data can very easily get lost.

> Even for ext3/ext4 which is doing physical journalling, it's still the

So do I understand this right that ext3/4 are more robust?


Folkert van Heusden

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