Disk write performance will be more accurate if iperf3 uses fflush() after each block to eliminate disk cache effects.

fflush() is for stdio and the -F stuff does not use stdio, it's doing read()/write(). So this particular cache effect does not apply. There are of course other caches in the pipeline.

Maybe fsync() is what we want? I'll give that a try.

I think I meant 'fsync':

http://linux.die.net/man/2/fsync