[Beowulf] How to configure a cluster network

[Mark Hahn]

> Optical links may bring things back toward larger switches.

optical increases costs, though.  we just put in a couple long DDR
runs using Intel Connects cables, which work nicely, but are 
noticably more expensive than copper ;)

although I give DDR and QDR due respect, I don't find many users
(we have a lot, and varied) who are overly concerned about bandwidth,
even as core-per-node increases.  yes, most of them would notice if
we downgraded to gigabit, but myri 2g seems to be enough for 4c/node.
those that notice the difference between our myri 2g and quadrics 
clusters are probably responding to latency more than BW anyway.