My hybrid LSM/Ext. Hash system may be of interest actually. It's purpose built for tracking historical data similar to Datomic's data model, but emphasizing constant-time read operations for current data while keeping the read speeds for historical data reasonable. In short: 2 level LSM tree (log file, immutable page files) with a extendible hashing to index into the page files. The identity of each record is the key + tx instant (time of COMMIT, or some other monotonically increasing number that is unique to a transaction set). Optionally you can index over historical data by having a record per-key that just appends a new tx-instant + offset for better longitudinal query performance.