Well, the Unix v6 filesystem implemented
in-order writes, as did 4.x BSD and the
other pre-journaled filesystems. POSIX allows
reordering to make coalesence easy, as
a lot of research was being done at that
time to get better performance.
A colleague at ICL (hi, Ian!) did his
masters at UofT on that, and found you
could get a performance improvement
and still preserve correctness by using
what I'd characterize as tsort(1), which
worked better than BSD/Solaris soft updates.