Here is an interesting article about Google and its data centers. As you can imagine these data centers are mission critical and processing billions of queries every day.
Jeff Dean say something which is absolutely spot on: "Our view is it's better to have twice as much hardware that's not as reliable than half as much that's more reliable," Dean said. "You have to provide reliability on a software level. If you're running 10,000 machines, something is going to die every day."