One last note: doing fdatasync once, before fadvise+fsync, not after every 
write is ~10x faster:

`
FDIO:      30 writes,  3763601 total bytes in 0.003558 secs
 FDIO:       1 allocs,  3763601 total bytes in 0.000024 secs
 FDIO:       1 dsyncs,        0 total bytes in 0.074081 secs
 FDIO:       1  syncs,        0 total bytes in 0.005785 secs
`

That makes the best case ~17x slower than using the kernel caches. A cost of 
17x is almost tolerable in RPM for the benefits of having files written to disk 
and not blowing out the kernel caches.

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