How Pinterest reduced memory usage by 4.5% by adopting #LMDB https://medium.com/pinterest-engineering/how-optimizing-memory-management-with-lmdb-boosted-performance-on-our-api-service-f85fa7d1626d
PS: on the topic of "stories you will never see" - you will never see a story about a project improving its efficiency by adopting RocksDB.