Hello,

I'm setting up Nagios as my monitoring/alerting system and want to monitor the memory usage of a few specific processes.
I've read the ps man page as well as lots of online docs about the differences between virtual and resident memory.
I'm still unsure about which metric I should use to monitor though.
VSZ obviously includes RSS along with other shared things (libraries, devices, etc) so it seems that if I monitor VSZ, I'll find out quickly enough if any processes leak memory.

Does this sound reasonable, or should I only really be concerned with the Resident memory usage?

I'm running a Linux Debian Etch system.

Thanks for the suggestions.

There is no good memory usage metric in Linux. Out of those two, RSS would be your best bet, but includes (in storage) pages for shared libraries.
Recently a new metric PSS was added to smaps to apportion the shared pages more "fairly" - FSVO fairly. If you have access to that, use it instead. Merely makes the numbers less "rubbery", not correct.