linux-ext4 - Re: [PATCH 0/4] (RESEND) ext3[34] barrier changes

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Message-id: <20080517175958.GL3491@webber.adilger.int>
Date:	Sat, 17 May 2008 11:59:58 -0600
From:	Andreas Dilger <adilger@....com>
To:	Theodore Tso <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Sandeen <sandeen@...hat.com>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/4] (RESEND) ext3[34] barrier changes

On May 17, 2008  09:43 -0400, Theodore Ts'o wrote:
> On Fri, May 16, 2008 at 05:35:52PM -0700, Andrew Morton wrote:
> > Journal wrapping could cause the commit block to get written before its
> > data blocks.  We could improve that by changing the journal layout a bit:
> > wrap the entire commit rather than just its tail.  Such a change might
> > be intrusive though.
> 
> Or we backport jbd2's checksum support in the commit block to jbd;
> with the checksum support, if the commit block is written out-of-order
> with the rest of the blocks in the commit, the commit will simply not
> be recognized as valid, and we'll use the previous commit block as the
> last valid commit.

The jbd2 journal checksum support, along with the 64-bit journal block
layout is designed to be 100% compatible with jbd, so it would be
trivial to "back port" jbd2.  Just copy jbd2 into jbd, and add the
option to set (or set by default) the JBD_FEATURE_COMPAT_CHECKSUM
feature in the journal.

I was going to suggest complete removal of jbd2 at that point, but
the ordered-mode rewrite is still pending and should live in ext4
for a reasonably long while before going back to ext3.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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