How can I make the creation of an efficient keystore (like memcache) / simple database?

For several projects that I am working on, I need a persistent key store (something similar to memcache). It works perfectly as a server; it must be really effective. I know memcachedb exists, but I would like to write it myself as there will be many custom functions that I will need to include later. I'll probably write this in C ++ (or perhaps C or Java if there is every reason to do so).

Should I be looking for a database implementation (B-trees, indexes, etc.) or is it unnecessary for this kind of work? What would be a good way to keep most of the content on disk, but have quick access to it using memory for caching?

Thanks.