My layman's guess is something that combines the two. larger immutable storage using B-Tree's (or similar), and more live data using log structured stuff. Wit

My layman's guess is something that combines the two. larger immutable storage using B-Tree's (or similar), and more live data using log structured stuff. With semi-regular promotion of data from log to b-tree and eventual clean-up of data no longer referencable in the b-tree.