The Raw Device Mapping (RDM) option is greyed out if:
- Your storage type does not support RDMs. For more information, see Comparing Types of Storage in the Configuration Guide for your version of VMware ESX.
- There are no available LUNs to be presented to an ESX/ESXi host. They may have already been committed to storing VMFS, or have already been mapped to other virtual machines as RDMs.
- vCenter Server has an older cache of known devices which can be used for RDMs, not including the intended LUN. Rescan your storage to pick up the changes. For more information, see Performing a rescan of the storage (1003988).
- You are using a SAS Direct Attached Storage (DAS) array that is being detected as Local Storage. For more information, see Creating Raw Device Mapping (RDM) is not supported for local storage (1017530).
- The option RdmFilter.HbaShared is selected in ESX/ESXi 4.1 or 5.x. To deselect this option, clickConfiguration > Software > Advanced Settings > RDM Filter, deselect the option, then click OK. For more information, see Configuring advanced options for ESX/ESXi (1038578).
- You are using an external shared SAS storage array like the Dell MD32xx. This requires that the checkboxRdmFilter.HbaShared is deselected in order to use a LUN as RDM. This is expected behavior because the RDM filter only applies to Fiber Channel, FCoE and iSCSI.