How do I tell where a bottleneck is on a server? [closed]

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http://serverfault.com – I have a LAMP server running with a large database serving a large number of requests. If things start slowing down, how do I tell if the bottleneck is the database, the processor, disk I/O, or bandwidth, or something else? How do I do this on a live server, ie, I don't want to take things down for benchmarking. (HowTos)