We are working on a SSD-backed key-value solution with the following properties:
NuDB is specifically designed for your use-case. It features O(1) insertion and lookup, no matter how big the database gets. Currently it is serving the needs of rippled with a 9TB (9 terabytes) data file. The library is open source, header-only, and requires only C++11 https://github.com/CPPAlliance/NuDB