Raw device mapping (RDM) is an option in the VMware server virtualization environment that enables a storage logical unit number (LUN) to be directly connected to a virtual machine (VM) from the storage area network (SAN).
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RDM is one of two methods for enabling disk access in a virtual machine. The other method is Virtual Machine File System (VMFS). While VMFS is recommended by VMware for most data center applications (including databases, customer relationship management (CRM)applications and enterprise resource planning (ERP) applications, RDM can be used for configurations involving clustering between virtual machines, between physical and virtual machines or where SAN-aware applications are running inside a virtual machine.
According to a 2008 VMware Performance Study comparing VMFS and RDM on ESX Server 3.5: “For random workloads, VMFS and RDM produce similar input/output (I/O) throughput. For sequential workloads with small I/O block sizes, RDM provides a small increase in throughput compared to VMFS. However, the performance gap decreases as the I/O block size increases. For all workloads, RDM has slightly better CPU cost.”
RDM, which permits the use of existing SAN commands, is generally used to improve performance in I/O-intensive applications. RDM can be configured in either virtual compatibility mode or physical compatibility mode. Virtual mode provides benefits found in VMFS, such as advanced file locking and snapshots. Physical mode provides access to most hardware functions of the storage system that is mapped.