How do you know if your server needs more RAM

To find out if you need more RAM for your Linux server, run:

vmstat -m 2

(which updates itself every 2 seconds) while the server is busy. The "swpd" column shows how much swap (in Megs) is being used. Ideally this should be zero or very close to zero. Also watch the columns "si" and "so" under "swap", which stand for "swap in" and "swap out." These show how much RAM the system is having to swap in/out from the hard drive. These should be zero or very close to zero.

If any of these values are high, your system has run out of physical RAM and is using swap, which tends to make things run very slowly. Particularly watching si and so, you can tell how hard your system is swapping if these values are constantly large and fluxuating.