The size need only be proportional to the amount of hot data. And realistically, it need only be proportional to the amount of hot data that's randomly accessed

The size need only be proportional to the amount of hot data. And realistically, it need only be proportional to the amount of hot data that's randomly accessed in small chunks, because that's where the bulk of the improvements will come from. I wonder why the performance ain't so great. I bet 32GBytes is in principle more than sufficient.