linux-ext4 - Re: Bug#615998: linux-image-2.6.32-5-xen-amd64: Repeatable "kernel BUG at fs/jbd2/commit.c:534" from Postfix on ext4

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Message-ID: <20110627160140.GC2729@thunk.org>
Date:	Mon, 27 Jun 2011 12:01:40 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	Jan Kara <jack@...e.cz>,
	"Moffett, Kyle D" <Kyle.D.Moffett@...ing.com>,
	Sean Ryle <seanbo@...il.com>,
	"615998@...s.debian.org" <615998@...s.debian.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Sachin Sant <sachinp@...ibm.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: Bug#615998: linux-image-2.6.32-5-xen-amd64: Repeatable "kernel
 BUG at fs/jbd2/commit.c:534" from Postfix on ext4

On Mon, Jun 27, 2011 at 05:30:11PM +0200, Lukas Czerner wrote:
> > I've found some. So although data=journal users are minority, there are
> > some. That being said I agree with you we should do something about it
> > - either state that we want to fully support data=journal - and then we
> > should really do better with testing it or deprecate it and remove it
> > (which would save us some complications in the code).
> > 
> > I would be slightly in favor of removing it (code simplicity, less options
> > to configure for admin, less options to test for us, some users I've come
> > across actually were not quite sure why they are using it - they just
> > thought it looks safer).

Hmm...  FYI, I hope to be able to bring on line automated testing for
ext4 later this summer (there's a testing person at Google is has
signed up to work on setting this up as his 20% project).  The test
matrix that I have him included data=journal, so we will be getting
better testing in the near future.

At least historically, data=journalling was the *simpler* case, and
was the first thing supported by ext4.  (data=ordered required revoke
handling which didn't land for six months or so).  So I'm not really
that convinced that removing really buys us that much code
simplification.

That being siad, it is true that data=journalled isn't necessarily
faster.  For heavy disk-bound workloads, it can be slower.  So I can
imagine adding some documentation that warns people not to use
data=journal unless they really know what they are doing, but at least
personally, I'm a bit reluctant to dispense with a bug report like
this by saying, "oh, that feature should be deprecated".

Regards,

					- Ted
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