Understanding the impact of using virtualized servers instead of real ones is perhaps one of the most complex issues when migrating from a traditional configuration to a cloud-based setup. Especially because virtualized servers are created equal … but only on paper.A Rackspace-funded “report” tries to find out the performance differences between Rackspace Cloud Servers and Amazon EC2. I guess the only conclusion we can get from their so-called report is that Cloud Server disk throughput is better than EC2′s. As the “CPU test” is a kernel compile which also stresses the disk, I don’t think we can reliably get any conclusion from these. An intrepid commenter ran a CPU-only test (Geekbench) and found out that EC2 performs slightly better than Rackspace in terms of raw processor performance. The same commenter, affiliated with Cloud Harmony, mentions that a simple hdparm test shows that Rackspace hdd has more than twice the throughput of EC2 hdd, at least in terms of buffered reads. Last but not least, don’t forget that for better disk performance Amazon recommends EBS instead of the VM disk. We cannot reliably make an informed cloud vendor choice just using VM benchmarks. Ideally, you should benchmark your own app on each cloud infrastructure and choose the one which gives you the best user-facing performance, because at the end of the day this is what matters most. Sadly, today this means experimenting with sometimes wildly different APIs and provisioning models.