Writing a DB

Objective: write fast and simple database program. Interaction with external applications through socket.
What's the best way to organise memory usage and file storage? I've come up with reading file and decomposing it into array elements. But there can be problems as file size limit and memory usage restrictions.
Any ideas?

Are you planning on storing such massive amounts of data? Do you want to distribute it across servers? You'll have to be more specific.