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In a post to his personal blog, Liu said that he used a combination of publicly available data and DNS queries within EC2 to arrive at the total number of racks in use by the cloud service, then multiplied that by the number of individual servers in each.
However, the researcher said, there are a couple of obvious caveats to his investigation. The total number of servers in each rack is an estimation on his part, and Amazon may well configure its systems differently than he imagines. What's more, any rack without an active instance running on it would be impossible to count, throwing off Liu's accuracy.
Nevertheless, there has been no shortage of media reaction to his post, thanks in part to the fact that it represents one of the best estimates yet of the Amazon cloud's size. The company does not divulge much information on EC2, making such educated guesses necessary.