There's currently an issue with the latest release of munin-node that can cause munin to omit disk names containing a dash between name and numerical prefix. E.g., dm-0, dm-1, etc.

A bug against munin-node's iostat plugin was filed on launchpad in 2009:
https://bugs.launchpad.net/ubuntu/+source/munin/+bug/493982

There's no assignee or milestone, so this may well never be fixed upstream, but it does include a workaround to the plugin. Therefore for MMS users, it might be good document the existence of the bug in our munin setup section.

Note: MMS users actually bumping into this limitation is rare - I've only known 1 or maybe 2 customers to have device names with the affected naming pattern.