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Assuming something reasonable like 20TB Toshiba MG10 HDDs and 64GB DDR4 ECC RAM, quick googling suggests that 1TB of disk space uses about 0.2-0.4W of power (0.2 in idle, 0.4 while writing), 5GB of RAM about 0.3-0.5W. So your break even on power is a bit earlier depending on the access pattern, but in the same ball park.

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What about rack space?

Not just rack space. At a certain amount of disks you also need to get a separate server (chassis + main board + cpu + ram) to host the disks. Maybe you need that for performance reasons any way. But saving disk space and only paying for it with some ram sounds cost effective.

That works out only as long as you don’t have to replace the whole machine (motherboard & possibly CPU/CPUs) to be able to add more RAM… So essentially the same problem as with disks.

In the end it all comes down to: there are a whole lot of trade-offs that you have to take into account, and which ceilings you hit first depends entirely on everyone’s specific situation.

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