In many environments, there will be requirements or special cases that warrant the use of Raw Device Mappings (RDM). The use of RDMs allows the guest operating system running in a VM to create its native filesystem on a LUN device.
Present a LUN to the ESXi hosts. You might need to contact your storage administrator for the same. Once the LUN is presented, rescan the HBAs to make sure that the LUN is visible to the ESXi hosts. Keep in mind that only a LUN with no VMFS on it can be presented as an RDM. The benefits of using raw device mappings have been outlined in the vSphere 5.5 Storage guide on page 156 at http://bit.ly/vsphere55_Storage_Guide.