I am looking for a good book or online article on analysis and tuning Linux system performance.
So far I've found openSUSE 12.2 System Analysis and Tuning Guide which looks promising. Any other recommendations?
Without delving into details of applications running on the system which is where normally performance tuning would start you can start looking at system performance through:
This list is by no means complete and by no means likely to give you answers on what you might be doing. Given that you are running multiple virtual machines I would first recommend analysis of applications running in those environments and whether or not virtualization is actually necessary.