Tristan Louis has an interesting blog entry about how many servers are at Google, based on the company’s S-1 filing. Overall, Tristan thinks that there are between 30,000 and 80,000. I think that he’s off by a factor of two. Tristan thinks that Google is buying dual-processor 2-GHz machines, but we know that Google has traditionally purchased the cheapest machines that it could buy, so he should have done the math with single-processor 1-GHz machines.
Google uses cheap machines beacuse it’s primarily concerned about hard disk bandwidth, rather than computes.