[mikepurvis]

I've never done DC ops, but I bet fan failure is a factor too— basically there'd be a benefit to centralizing all the cooling for N racks in 2-3 large redundant pumps rather than having each node bringing its own battalion of fans that are all going to individually fail in a bell curve centered on 30k hours of operation, with each failure knocking out the system and requiring hands-on maintenance.

[jabl]

A cool (ha ha!) solution was the old Cray XT3/4 supercomputers, which were air cooled. But instead of a battalion of tiny fans, each cabinet had a single huge fan at the bottom, blowing air vertically through the cabinet (the boards were mounted vertically). No redundancy, sure, but AFAIU it was reliable enough to not be a problem in practice.

[mikepurvis]

That’s a similar design principle to the Mac Pro trashcan, I guess, which also pulled air through a central column alongside vertical PCBs/heatsinks.