Perhaps try to redefine the problem a bit. For example, if it's an activation code then you can make it out of two parts: one sequential part, and a random part. If the sequential part is unique, the activation code will be unique. Of course the drawback is that you can issue less activation codes but I do not imagine that you want to activate 2^64 whatevers :)

Check out RRDtool http://oss.oetiker.ch/rrdtool/[^] It's exactly what you need, it's a round robin database (i.e. when it has reached the end, it overwrites the first entries) and it is specifically designed for graphing sensors from various sources (server load, network traffic, temperature sensors, etc). Good luck!