linux-ext4 - Journal async commit broken for data=ordered?

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Date:	Tue, 14 Feb 2012 16:55:36 +0100
From:	Jan Kara <jack@...e.cz>
To:	linux-ext4@...r.kernel.org
Cc:	Ted Tso <tytso@....edu>
Subject: Journal async commit broken for data=ordered?

  Hello,

  I've just realized that JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT breaks
guarantees of data=ordered mode in ext4. The problem is that async commit
code assumes that when a checksum of a transaction in the journal matches,
all necessary data is on disk. This is true for metadata but need not be so
for data - the whole transaction may be correctly on pernament storage
while some data is still sitting in drive's caches. Thus if a power failure
happens at that moment, we have broken guarantees of data=ordered mode.
Seeing that async commit code isn't used by default anyway (I remember
there used to be some problems with it), shouldn't we just rip it out?

								Honza
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