Raw Device Mapping (RDM): Virtual mode and Physical mode

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
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
    • Microsoft failover cluster quorum disk 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.

One thought on “Raw Device Mapping (RDM): Virtual mode and Physical mode

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.