Munin and Nagios are really different tools.
From the official Munin website:
Munin is a networked resource monitoring tool that can help analyze
resource trends and “what just happened to kill our performance?”
problems. It is designed to be very plug and play. A default
installation provides a lot of graphs with almost no work.
Nagios is a monitoring (alerting) tool. Munin could be considered a replacement for Cacti.
We use both of them: Nagios and Munin.
- Nagios tell us in real time if something is wrong: like web server down, database load average, etc.
- Using Munin you can see the trends and the history about why that happenend.