Data Protection for VMware and Application Data in Mission Critical Environments

Figure 7: VMFS
Raw Device Mapping (RDM)
A Raw Device Mapping (RDM) is a special file in a VMFS volume that acts as a proxy for a raw
device. RDM LUN supports direct access
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between the virtual machine and physical storage
subsystem, and is useful with SAN snapshots and other layered applications running in a virtual
machine. There are two types of RDM devices—physical compatibility RDM, also called pass-through
RDM, and virtual compatibility RDM, also known as non-pass-through RDM. Pass-through RDM
volumes allow SCSI commands to pass through directly from the guest operating system to the SAN.
Thus, they enable scalable backups better using native SAN features. Non-pass-through RDM volumes
preserve some features of VMware disk such as snapshots, however, a larger set of SCSI commands
are filtered when using non-pass-through RDM.
Figure 8: Raw Device Mapping (RDM)
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Fibre Channel or iSCSI only
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