Every now and then, some spurious peaks show up on munin graphs. The peaks are order of magnitude higher than the expected range of the data. This particularly happens with DERIVE plugins, that are notably used for network interfaces.
To ease the task of viewing the data, each machine runs munin-node, but only a couple of masters do the data collection with munin-update. This works reasonably well, except that machines monitored by more than one server need to work extra time to provide the same data to both.