Certain limitations exist when you use RDMs.

Not available for block devices or certain RAID devices – RDM uses a SCSI serial number to identify the mapped device. Because block devices and some direct-attach RAID devices do not export serial numbers, they cannot be used with RDMs.

Available with VMFS-2 and VMFS-3 volumes only – RDM requires the VMFS-2 or VMFS-3 format. In ESX, the VMFS-2 file system is read only. Upgrade it to VMFS-3 to use the files that VMFS-2 stores.

No snapshots in physical compatibility mode – If you are using an RDM in physical compatibility mode, you cannot use a snapshot with the disk. Physical compatibility mode allows the virtual machine to manage its own snapshot or mirroring operations.

Snapshots are available in virtual mode.

No partition mapping – RDM requires the mapped device to be a whole LUN. Mapping to a partition is not supported.