munin-update is the primary Munin component. It is run from the munin-cron script.
This script is responsible for contacting all the agents (munin-nodes) and collecting their data. Upon fetching the data, munin-update stores everything in RRD files - one RRD files for each field in each plugin.
Running munin-update with the –debug flag will often give plenty of hints on what might be wrong.
munin-update is a component in the Munin server.
Use <file> as the configuration file. [/etc/munin/munin.conf]
If set, log debug messages. Can be negated with –nodebug [–nodebug]
If set, will fork off one process for each host. Can be negated with –nofork [–fork]
Limit fetched data to those from <host<gt>. Multiple –host options may be supplied. [unset]
Limit fetched data to those of <service>. Multiple –service options may be supplied. [unset]
Set the network timeout to <seconds>. 
Print the help message then exit.
Print version information then exit.