[Beowulf] Opteron cooling specifications?

[Mark Hahn]

> 50u racks are also available (I've seen them in colo facilities)... 
> personally I don't like racking things that far over my head.

I don't really care about that, myself - racking any serious number
is a two-person job anyway.  but taller racks make some sense to me,
since open space above the rack is pretty much wasted and/or 
a design flaw.  (that is, you want your hot-air plenum to seal 
the top of the rack, which is most easily done with a drop-ceiling 
that meets the top.)

density is an interesting question - if nodes are 350W, does it make 
sense to have 50 of them in a rack?  17.5 KW requires 5 tons of cooling,
and as a rule of thumb 5 perf tiles.  you can probably get by with 
2-ish high-perf tiles if your air pressure is good.  but that means 
your cold rows will be at least 4 tiles wide, possibly more.
oops, wasn't density the argument for taller racks?

> We've being switching cabinets in one of our datacenters to 220volt 
> service to support the sort of density we're seeing without running new 
> conductors.

sure, there's no downside to 220 afaikt.  one thing I don't fully 
understand is where the 80% figure comes from.  just a fudge-factor,
and what if you wind up always using that extra capacity?
presumably an L6-30 circuit is actually safe to run at 30...