System.setProperty("javamelody.storage-directory", "/" + new File(Hudson.getInstance().getRootDir(), "monitoring").getAbsolutePath());

sets up this directory for storage, but apparently never trims old records. This is pretty bad since the directory can get quite large and consume a lot of disk space. Suggest creating an AsyncPeriodicWork to clean out old records (say, those over a month old) on a regular basis (say, daily).