I love lmdb and mmap, but I wish there was prefix compression for keys. I'm on the fence between rocksdb and lmdb, both using mmap and will likely support both. My database file was so much smaller with rocksdb because it compresses segments, but lmdb is so much more concise and will never do background jobs like rocksdb. One can use different subtables in lmdb, but the amount must be set up front with mdb_env_set_maxdbs and it says that there is a linear search involved.