Yet I wonder what really has changed that makes it commercially viable now. Pumping fluids around isn't a technology that has significantly advanced, after all. Is it just that Moore's law has ceased to apply to heat and power usage? Is it that CPU manufacturers finally got around to testing and falsifying the long-assumed-true hypothesis that servers need to be kept in ultra cool rooms to begin with?

I spent some time reflecting on this, and I think it's because what the bottleneck is has changed over the years. For a while, I/O was the real challenge for servers. But hard drives have improved a ton, and many-drive systems have really improved a lot. Memory had its day, but the price has plummeted.

Today, the limiting factor everyone is focusing on is core density, and cores generally produce more heat than any other component aside from high-end graphics and displays.

Of course, power consumption and thus heat has been continually improving, but not by orders of magnitude. Core density, on the other hand, has been going up by a few orders.

As a result, you've got to move more heat, and because density is what's going up, you've also impaired airflow.

Take this with a grain of salt though, it's just speculation based on what I've seen & heard.