Cloud computing is a significant shift in the way companies build and run IT resources. It promises pay-‐as-‐you-‐go economics and elastic capacity. Every major change in IT forces IT professionals to “rebalance” their application strategy—just look at client-‐server computing, the web, or mobile devices. Today, cloud computing is prompting a similar reconsidering of IT strategy. But it’s still early days for clouds. Many enterprises are skeptical of on-‐ demand computing, because it forces them to relinquish control over the underlying networks and architectures on which their applications run. In late 2009, performance monitoring firm Webmetrics approached us to write a study on cloud performance. We decided to assess several cloud platforms, across several dimensions, using Webmetrics’ testing services to collect performance data. Over the course of several months, we created test agents for five leading cloud providers that would measure network, CPU, and I/O constraints. We also analyzed five companies’ sites running on each of the five clouds. As you might imagine, this resulted in a considerable amount of data, which we then processed and browsed for informative patterns that would help us understand the performance and capacity of these platforms. This report is the result of that effort. Testing across several platforms is by its very nature imprecise. Different clouds require different programming techniques, so no two test agents were alike. Some clouds use large-‐scale data storage that’s optimized for quick retrieval; others rely on traditional databases. As a result, the data in this report should serve only as a guideline for further testing: your mileage will vary greatly.