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Let's say I'm trying to monitor a box with Nagios, but there's a runaway process that's stealing nearly 100% of the computer's CPU and RAM. How does Nagios continue to alert me about the status of the box, in this case? What's to stop the runaway process from blocking Nagios from running as well?

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Do you mean Nagios doesn't keep sending alert when you have a little free RAM? How about your config? –  quanta Nov 24 '11 at 6:40

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up vote 6 down vote accepted

If the monitored server is overloaded, Nagios or any other monitoring program is unable to work at that server.

But no worries, you will receive your precious alerts: you will recognize the situation about the fact that every single Nagios test monitoring the server will timeout.

Though if you meant you have everything, including Nagios, running on the same server, then your Nagios is vulnerable for sudden death. Always have a separate monitoring server or monitoring will be useless: it will go down on the very moment you actually would need it!

You might also consider putting up some informative monitors, such as load average. Make Nagios warn you if load average gets past some critical point.

Graphing various server aspect such as CPU, memory and disk usage with MRTG, Cacti or Nagiosgraph is also helpful and makes it easier for you to keep an eye on trends.

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+2 if I could for the point that the monitoring server should be separate from everything else; it's not just another function that should be shared with the mail server, or the web server, or indeed the coffee machine. And if you decide to go for a parallel quantitative monitoring solution, as Janne suggests, munin is worth a look as it specifically integrates with NAGIOS for notification. –  MadHatter Nov 24 '11 at 7:09
    
Also having a failover/hot-standby set up (Merlin is what I use, there are probably other options). We use a dedicated Nagios VM but have a second instance running as a hot standby on an app server, so if our main Nagios goes down we still get alerts (Also the hot-standby will alert us of issues with the monitoring server itself) –  sam Nov 24 '11 at 8:25
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@sam: That's a good idea, too. Also worth noting is that Nagios supports distributed monitoring with multiple Nagios servers: library.nagios.com/library/specialtopics/distributedmonitoring –  Janne Pikkarainen Nov 24 '11 at 8:28

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