Architecture Limitations with Replication of VM connected with Physical Mode RDM

[RanjnaAggarwal]

what are Architecture Limitations with Replication of VM connected with Physical Mode RDM in vsphere replication. Why VMware is not supporting this?

[vbrowncoat]

vSphere Replication does not work with physical mode RDMs. This is because of the way that vSphere Replication and physical mode RDMs work.

As outlined in this: VMware KB: Difference between Physical compatibility RDMs and Virtual compatibility RDMs

It isn't currently possible for VR to do what it needs to do with physical mode RDMs.

Does this answer your question?

[jhunter1 · Accepted solution]

I will add some color to GS's response. Of particular interest when it comes to the way physical RDM (pRDM) works, these two bullet points from the KB article sum it up:

• 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.

There is a vSphere Replication (VR) agent built into vSphere that performs a few functions. In particular, it tracks the writes to the virtual machine. When a replication cycle occurs, the changed data is replicated to the target location. The VR agent must be able to "see" these writes to track them. Since physical mode passes all SCSI commands directly to the device, the VR agent cannot track these changes for replication.