Tossing another vote in for Redis. Generally, you have to give me a reason why Redis is not the answer for a KV store situation. It can run in a cluster mode or individually, and it can choose to run only in memory and not sync to disk if you don't need persistence and you want it fast.

Some folks claim 200k inserts a second, but we topped out at around 20k inserts a second and had to start digging into more advanced configurations and whatnot. Eventually we sharded and sent some requests to one pool of redis and other requests to a different pool based on consistent hashing that we set up. But even then, I don't think most shops need to worry about that scale.