On May 17, 2008  09:43 -0400, Theodore Ts'o wrote:

On Fri, May 16, 2008 at 05:35:52PM -0700, Andrew Morton wrote:

quoted

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.