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

From: Andreas Dilger
Date: Sat May 17 2008 - 14:00:29 EST

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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