This is a simple python script I put together for updating the MIN and MAX values of pre-existing RRD files. This script will go through a specified directory tree and update DS0 Min and/or Max values on all matching RRDs. At the moment Zenoss only creates one datapoint per RRD file, and that datapoint should always be DS0, so this script only deals with DS0. This is useful for going back and updating those thresholds to sane values after the RRDs have been in use for a while. Adjusting MIN/MAX values on COUNTER datapoints is mostly useful in helping RRDTool to recognize invalid data (graph spikes) when devices reboot and a datapoint counter resets to zero. It will do some basic safety and sanity checking, but as always your mileage may vary.
More information describing the problem and potential solutions is available in the RRDTool documentation here and here.
Please let me know if you have any suggestions for improving it.