VMware published a whitepaper about Performance Characteristics of VMFS and RDM. VMware ESX Server offers three choices for managing disk access in a virtual machine—VMware Virtual Machine File System (VMFS), virtual raw device mapping (RDM), and physical raw device mapping. It is very important to understand the I/O characteristics of these disk access management systems in order to choose the right access type for a particular application. Choosing the right disk access management method can be a key factor in achieving high system performance for enterprise�class applications. This study provides performance characterization of the various disk access management methods supported by VMware ESX Server. The goal is to provide data on performance and system resource utilization at various load levels for different types of work loads. This information offers you an idea of relative throughput, I/O rate, and CPU efficiency for each of the options so you can select the appropriate disk access method for your application.
