vsphere-esxi-vcenter-server-55-storage-guide

Chapter 17 Raw Device MappingIn physical mode, the VMkernel passes all SCSI commands to the device, with one exception: the REPORTLUNs command is virtualized so that the VMkernel can isolate the LUN to the owning virtual machine.Otherwise, all physical characteristics of the underlying hardware are exposed. Physical mode is useful torun SAN management agents or other SCSI target-based software in the virtual machine. Physical mode alsoallows virtual-to-physical clustering for cost-effective high availability.VMFS5 supports greater than 2TB disk size for RDMs in virtual and physical modes. You cannot relocatelarger than 2TB RDMs to datastores other than VMFS5.Dynamic Name ResolutionThe RDM file supports dynamic name resolution when a path to a raw device changes.VMFS uniquely identifies all mapped storage devices, and the identification is stored in its internal datastructures. Any change in the path to a raw device, such as a Fibre Channel switch failure or the addition ofa new HBA, can change the device name. Dynamic name resolution resolves these changes andautomatically associates the original device with its new name.Raw Device Mapping with Virtual Machine ClustersUse an RDM with virtual machine clusters that need to access the same raw LUN for failover scenarios. Thesetup is similar to that of a virtual machine cluster that accesses the same virtual disk file, but an RDMreplaces the virtual disk file.Figure 17‐3. Access from Clustered Virtual MachinesHost 3Host 4VM3VM4“shared” accessmapping fileVMFS volumeaddressresolutiionmappeddeviceComparing Available SCSI Device Access ModesThe ways of accessing a SCSI-based storage device include a virtual disk file on a VMFS datastore, virtualmode RDM, and physical mode RDM.To help you choose among the available access modes for SCSI devices, the following table provides a quickcomparison of features available with the different modes.Table 17‐1. Features Available with Virtual Disks and Raw Device MappingsESXi Features Virtual Disk File Virtual Mode RDM Physical Mode RDMSCSI Commands PassedThroughNo No YesREPORT LUNs is not passedthroughvCenter Server Support Yes Yes YesVMware, Inc. 159