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

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. For an example, see the Cacti project.