Nagios Support Forum

While setting up for SNMP monitoring, there is virtual memory usage and physical memory usage. Do I check virtual memory usage only for my virtual servers and physical memory usage only for my physical servers? Or do i just check both for my virtual and physical servers .

Thanks,

Not exactly:
http://www.tldp.org/LDP/sag/html/vm-intro.html

Linux supports virtual memory, that is, using a disk as an extension of RAM

In a nutshell, the virtual memory lives in the swap file on disk most of the time, while the physical memory is "I am a stick of RAM that lives somewhere holding your data". There's different reasons to monitor both of those things.