There's a maxim in the data center business that you can't manage what you can't measure, and eBay has come up with the mother of all measurement systems for calculating data center efficiency. The online auction giant has devised a methodology that looks at the cost of its IT operations in dollars, kilowatt hours and carbon emissions, and ties those costs back to a single performance metric -- in eBay's case, the number of buy and sell transactions its customers make at eBay.com.