[Beowulf] Reasonable upper limit in kW per rack for air cooling?

[David Mathog]

There are a series of white papers by APC here:

  http://www.apc.com/tools/mytools/index.cfm?action=wp

where they discuss various power and cooling factors.  They note
a disconnect between the higher densities achieved by blades and
similar high density racks and the practicality of actually
cooling these beasts.  Basically it comes down to you save space
on the rack and then give it all back on the cooling system.  Think
of it minimally in these terms - to move enough cfm at less than 30
feet per minute starts to require a duct larger than the rack itself!

In terms of TCO, at the moment, APC rejects the notion that
these ultra high density machines are cost effective because they
are so very difficult to cool.

It seems to me that at a certain power point the racks are going to
have to resort to water cooling.  Long ago the ECL mainframes were
cooled this way, but it's been a long time since most of us have
seen water pipes running into the computers in a machine room. 

Cooling a 10 kW rack well looks to be extremely tough with air,
and going much above that would seem to require something approaching
a dedicated wind tunnel.  Any opinions on how high the power
dissipation in racks will go  before the manufacturers throw
in the air cooling towel and start shipping them with water
connections?

If you were designing a computer room today (which I am) what would
you allow for the maximum power dissipation per rack _to_be_handled_
by_the_room_A/C.  The assumption being that in 8 years if somebody
buys a 40kW (heaven forbid) rack it will dump its heat through
a separate water cooling system. 

Thanks,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech