LMDB is an ultra-fast, ultra-compact key-value data
store developed by Symas for the OpenLDAP Project.

It uses memory-mapped files, so it has the read
performance of a pure in-memory database while still
offering the persistence of standard disk-based
databases, and is only limited to the size of the
virtual address space, (it is not limited to the
size of physical RAM). LMDB was originally called
MDB, but was renamed to avoid confusion with other
software associated with the name MDB.
