In "mail.ru" we wrote and widely use Tarantool/Box key-value database. It's key properties include:
1) all data is maintained in RAM
2) data persistence is implemented using a Write Ahead Log and snapshotting
3) supports asynchronous replication and hot standby
4) uses coroutines and asynchronous I/O to implement high-performance lock-free access to data
5) available on Linux, FreeBSD, Mac OS X
6) stored procedures in Lua provide high level of customization. In particular, the message queuing API is a set of stored procedures running on top of the basic functions to store, modify and retrieve tuples.
...