VMware ESX Server is a software platform that efficiently multiplexes the hardware resources of a server among virtual machines. This paper studies the performance of a key component of the ESX Server architecture: its storage subsystem. We characterize the performance of native systems and virtual machines using a series of disk microbenchmarks on several different storage systems. We show that the virtual machines perform well compared to native, and that the I/O behavior of virtual machines closely matches that of the native server. We then discuss how the microbenchmarks can be used to estimate virtual machine performance for disk-intensive applications by studying two workloads: a simple file server and a commercial mail server.