Fast Startup Options
There are a few things you can do that can decrease the amount of time it take Nagios to startup (or restart). These speedups involve easing some of the burden involved in processing your configuration files.
Starting with Nagios Core 4, most of the manual optimizations described in this page are no longer necessary. The configuration processing and verification is now significantly faster. You can still use the -s option to measure your startup times to verify that this is the case.
Documentation - Fast Startup Options
Final Thoughts
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