ESX server does not typically perform I/O load balancing across paths for a given storage device.At any given time, only single path is used, which is called as Active Path. The ESX server host automatically sets multipathing policy according to the make and model of the array it detects. If the detected array is not supported, it is treated as active/active.

Manually changing MRU to Fixed is not recommended. If you are using Fixed Policy, you can see which path is the preferred path with an asterisk mark.

It is recommended to use fixed policy when SP are active/active and for MRU should be used when SP are active/passive mode

RDM is a special mapping file in VMFS volume that manages metadata for its mapped device. Mapping file has a .vmdk extension, but the file contains only disk information describing the mapping to the LUN on the ESX server system

Benefits of Raw Device Mapping (RDM)

• User-friendly persistent name
• Dynamic name resolution
• Distributed file locking: distributed locking on a RDM makes it safe to use a shared RAW SCSI devices without losing date when two VM are accessing the same LUN.
• File permissions
• File system operations
• Snapshots
• Vmotion

In RDM there are two modes physical modes and virtual modes

In Physical mode, VMKernel passess are SCSI commands to the device except REPORT LUN command is virtualized so the Vmkernel can isolate the Lun from the owning VM.

All mapped LUNs are uniquely identified by VMFS, RDM lets you give a permanent name to a device name, which is relative to the first visible LUN. so that any change in HBA,FC failure can change Vmhba because name includes initiator,HBA,SP,LUN. Dynamic resolution can compensate this.

Vmkfstools can used for managing RDM from SVC console, typical operations are querying mapping information, create mapping file and to import or export a virtual disk.