Ask HN: How do you store thousands of quadrillions of keys?

It has to be fast, reliable. Let's not worry about backups, etc. :) It's just a hypothetical thought experiment. How much would it cost?

I don't know much about databases, but for certain applications, maybe it'd be OK to sacrifice certain ACID requirements. For example, maybe make it less than totally isolation. Some of the data could be changed sometime, because with these many keys, how many of them change often?

So how do we do it now? Is it even possible?