A very nice bit of work, my only questions is if you ever feel motivated to repeat this test, it would be fun to do it with ext3 (or ext4) using the stride= parameter. I did limited testing and it really seemed to help, but nothing remotely as format as your test.
Speaking about which, it would probably be good to adjust a little how the filesystem is created and mounted (both in xfs and ext3/4 cases). E.g. lazy-count=1 is still not the default last time I checked mkfs.xfs. And even ext4.txt from kernel documentation recommends mounting it with data=writeback,nobh when doing comparison with metadata journaling filesystems (the same would go for ext3).
Along with different journal sizes, keeping an eye on stripe & stripe-width, and other settings that might be of interest.
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