Due to the behaviour of ext3, to write the metadata changes to disk (creation of "file.new" and rename of "file.new" to "file"), it was necessary for the file's blocks to be written out to disk so the previous contents won't be available. This is almost but not quite the same as journalling data too (it won't protect against partial writes if you cut power at the wrong time).
With ext4's delayed allocation, the metadata changes can be journalled without writing out the blocks. So in case of a crash, the metadata changes (that were journalled) get replayed, but the data changes don't.
If you journal data changes, presumably this won't happen on either ext3 or ext4. That is likely to give a performance hit though.
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