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Wednesday, 3 February 2016

What is VMware Raw Device Mapping (RDM)

Raw Device Mapping:

Raw device mapping (RDM) is an option in the VMware virtualization that enables a logical unit number (LUN) to be directly connected to a virtual machine (VM) from the storage area network (SAN). RDM uses SAN commands (SCSI) to improve performance in I/O-intensive applications.

RDM can be configured to operate in virtual compatibility mode or physical compatibility mode. Virtual mode provides benefits that are available in VMFS, such as snapshots. In Virtual mode RDM, only READ/WRITE SCSI Command is passed directly to storage and rest are virtualized by VMKernel. In virtual mode RDM, file extension used is rdm.vmdk for pointer file.

Physical mode provides access to most hardware functions and physical device characteristics of the storage system that is mapped. In physical mode RDM, only SCSI command that is virtualized by VMKernel is ReportLUN. Rest all commands are passed directly to storage. In physical mode RDM, file extension used is rdmp.vmdk for pointer file.

You can use physical mode RDM for below use cases.
  • VM running with applications that are SAN aware
  • Cluster configurations
  • Performing P2V, you can skip copying data of existing mapped LUN on physical server if data size is too large, instead re-map the LUN to VM after P2V is done.
Please feel free to comment if have any feedback or discussion point.

1 comment:

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