Scale definitely matters. You want your caches to be hot, but not too hot. If your primary keys are sequential you will probably have great performance, but wil

Scale definitely matters. You want your caches to be hot, but not too hot. If your primary keys are sequential you will probably have great performance, but will have a hard time sharing when you need to. Although that can probably be solved by shuffling a bit of the high entropy bits to the front of the shard key. Ideally you would be able to dynamically adjust the number of bits shuffled to get the right balance of hot but not overloaded caches. On the other hand if you were using random IDs no amount of shuffling is going to make them group into nice high-hit-rate slices.