物理和虚拟兼容性RDM的区别

An RDM is a special mapping file in a VMFS volume that manages metadata for its mapped device. The mapping file is presented to the management software as an ordinary disk file, available for the usual file-system operations. To the virtual machine, the storage virtualization layer presents the mapped device as a virtual SCSI device.

 

RDM has two compatibility modes:

• Physical mode specifies minimal SCSI virtualization of the mapped device, allowing the greatest flexibility for SAN management software.
• VMkernel passes all SCSI commands to the device, with one exception - The REPORT LUNs 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 while running SAN management agents or other SCSI target-based software in the virtual machine.
• Physical mode also allows virtual-to-physical clustering for cost-effective high availability.
• Virtual Machine Snapshots are not available when the RDM is used in physical compatibility mode.
• You can use this mode for Physical-to-virtual clustering and cluster-across-boxes.
• VMFS5 supports greater than 2TB disk size for RDMs in physical compatibility mode only. The following restrictions apply:
• You cannot relocate larger than 2TB RDMs to datastores other than VMFS5.
• You cannot convert larger than 2TB RDMs to virtual disks, or perform other operations that involve RDM to virtual disk conversion. Such operations include cloning.
• To expand the size of the RDM refer  Expanding the size of a Raw Device Mapping (RDM) (1007021)

• Virtual mode specifies full virtualization of the mapped device.
• VMkernel sends only READ and WRITE to the mapped device. The mapped device appears to the guest operating system exactly the same as a virtual disk file in a VMFS volume.
• The real hardware characteristics are hidden.
• If you are using a raw disk in virtual mode, you can realize the benefits of VMFS, such as advanced file locking for data protection and snapshots for streamlining development processes.
• Virtual mode is more portable across storage hardware than physical mode, presenting the same behavior as a virtual disk file.
• You can use this mode for both Cluster-in-a-box and cluster-across-boxes.
• To expand the size of the RDM refer  Expanding the size of a Raw Device Mapping (RDM) (1007021)

Note: RDM is not available for direct-attached block devices or certain RAID devices. You cannot map a disk partition as RDM. RDMs require the mapped device to be a whole LUN.

 

For limitations and characteristics of RDMs during all types of migration, see: Migrating virtual machines with Raw Device Mappings (RDMs)