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vSphere 5.5, RDMs and Microsoft Clustering

I was having a conversation with one of our tech support guys (Greg Williams) recently about the relaxation on the requirement to allow Raw Device Mappings (RDMs) to be presented to different hosts using different SCSI identifiers and still do vMotion operations in vSphere 5.5. You can read that post here where I described how the restriction has been relaxed. Greg mentioned that he was handling a case where customers wished to share a physical mode/passthru  RDM between VMs on different ESXi hosts with a view to running Microsoft Clustering Services (MSCS) on top. We call this CAB or Clustering Across Boxes.

MSCS is probably the only real use case for passthru RDMs these days. We need to use passthru RDMs in MSCS as these allow all SCSI commands to be passed all the way down to the disks. In the case of MSCS, it uses SCSI reservations on disks to gain quorum/ownership/control of services in the event of a failure/fail-over. If we didn’t use these passthru RDMs and used VMDKs or virtual mode/non-passthru RDMs instead, the SCSI reservation would get translated to a file lock. This would be okay if we were running MSCS using VMs on the same ESXi host (we call this a CIB configuration, short for Clustering in a Box) but it wouldn’t allow MSCS to work correctly across ESXi hosts.

We wondered if this lifting of restrictions on the LUN ID for vMotion would also allow us to present the RDM to different ESXi hosts with different IDs, and still allow virtual machines on those hosts to share the RDM and configure MSCS. Well, to cut to the chase, that restriction is still in place. If the RDM is mapped using different identifiers to different hosts, you will not be able to share that RDM to the VMs that you want to have running MSCS.

It would appear that vCenter continues to check for an identical VML (VMware Legacy) identifier when sharing a RDM to multiple VMs on different ESXi hosts. If the VML ID is different (which it will be if a different SCSI ID is used to present the RDM to different ESXi hosts), then you will not be able to configure MSCS.

Bottom line: if you wish to use Microsoft Clustering Services with vSphere 5.5, you must continue to follow the practice of presenting the passthru RDM/LUN with the same SCSI ID to all ESXi hosts that will have virtual machines sharing the RDM. This is true even in vSphere 5.5, where we have relaxed some RDM restrictions to enable vMotion of VMs across ESXi hosts where the RDM is presented with a different ID.

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