performance testing tool with a capacity to monitor only 2 servers

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http://serverfault.com – we are looking at a performance testing tool that can monitor only 2 servers. We will be be testing our environment that has more than 2 servers that include 3or 4 web servers, 2 app servers and a db server not to mention a couple of load balancers. My concern is that can we even do a meaningful end to end performance test with a tool that can only monitor 2 servers and be able to co relate all the results to see where the bottleneck is? Any insight will be great....thanks. (HowTos)