#VCP #vSphere #Storage A mechanism in ESXi for a VM to have direct access to a LUN on a SAN. It is basically a symbolic link. The LUN which is used for RDM is not formatted for VMFS but anything chosen on the VM when initializing the RDM out of the OS. there are 2 different modes a RDM can be mapped - Virtual compatibility Mode, RDM acts like a virtual disk file, Snapshots can be used. - Physical compatibility mode, Direct access to the SCSI device ![[_media/vSphere - Raw device mapping(RDM|vSphere - Raw device mapping(RDM|[RDM)2024-07-15.png]]2024-07-15.png]]2024-07-15.png) For RDMs vSphere uses N-Port ID virtualization. A WWPN and WWNN are paired in a VM .vmx file and the VMkernel instantiates a virtual port on the Host HBA used for LUN access. ## Use cases - Hardware Specific SCSI, when an application needs to send hardware specific SCSI commands for it to work. - Clustered VMs, MSCS (Microsoft Cluster configuration) requires RDM - SAN Management Software, Which needs direct access to a LUN for Storage resource management etc. - Migrations for physical to virtual for example