That sounds very interesting. I could only find RRDTool which seems like a combination of a RRD and a graphing tool. Further some vague references about RRD in PostgreSQL. Am I missing something?
RRDTool is what you're looking for. The graphing portion is because you probably want to display the data. "rrdtool dump" will export to XML. You can use the RRD files without using the graphing utility.