One power supply for the whole rack may very well work, but not for the whole datacenter. If you start trying to send that much power to all those servers at 12V DC, you're going to be pushing a LOT of amps and require very thick wires to avoid transmission losses. And if you're not useing low voltage DC, you're going to need some sort of PSU at the server level anyway.

Not true, we have health care! :D

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