"Bitcask keeps all keys in memory at all times, this means that your system must have enough memory to contain your entire keyspace"
I read this in the description of Bitcask, and I'm worried the same issue may affect Riak, as Riak is implemented on Bitcask. Is this the case?
If you use the Bitcask backend, yes.
Bitcask is the defualt backend, but should you expect your list of keys to exceed your server's memory, you can use the eleveldb backend instead.