Rasberry pi - high density

Brandon Martin lists.nanog at monmotha.net
Mon May 11 22:50:59 UTC 2015

On 05/11/2015 06:21 PM, Randy Carpenter wrote:
> That is .8-1.6A at 5v DC. A far cry from 120V AC. We're talking ~5W versus ~120W each.
> Granted there is some conversion overhead, but worst case you are probably talking about 1/20th the power you describe.

His estimates seem to consider that it's only 5V, though.  He has 825 
Pis per rack at ~5-10W each is call it ~8kW on the high end.  8kW is 
2.25 tons of refrigeration at first cut, plus any power conversion 
losses, losses in ducting/chilled water distribution, etc.  Calling for 
at least 3 tons of raw cooling capacity for this rack seems reasonable.

8kW/rack is something it seems many a typical computing oriented 
datacenter would be used to dealing with, no?  Formfactor within the 
rack is just a little different which may complicate how you can deliver 
the cooling - might need unusually forceful forced air or a water/oil 
type heat exchanger for the oil immersion method being discussed 
elsewhere in the thread.

You still need giant wires and busses to move 800A worth of current.  It 
almost seems like you'd have to rig up some sort of 5VDC bus bar system 
along the sides of the cabinet and tap into it for each shelf or 
(probably the approach I'd look at first, instead) give up some space on 
each shelf or so for point-of-load power conversion (120 or 240VAC to 
5VDC using industrial "brick" style supplies or similar) and 
conventional AC or "high voltage" (in this context, 48 or 380V is 
"high") DC distribution to each shelf.  Getting 800A at 5V to the rack 
with reasonable losses is going to need humongous wires, too.  Looks 
like NEC calls for something on the order of 800kcmil under rosy 
circumstances just to move it "safely" (which, at only 5V, is not 
necessarily "effectively") - yikes that's a big wire.
Brandon Martin

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