Virtualization Pro

May 8 2008   8:45PM GMT

VMotion and RDMs

Kutz Profile: Akutz

Yesterday I was a speaker at “Virtualization: Getting from Pilot to Production.” During my second session I claimed that you could VMotion a virtual machine (VM) that uses a Raw Device Mapping (RDM) to access a raw logical unit number (LUN). Two audience members challenged this claim, saying that they had previously run into a scenario where it was not possible to VMotion a VM that makes use of RDMs. I was sure I was right, and they were positive they were correct. It turns out we were *all* spot on. You can VMotion a VM that uses RDM as long as the RDM is configured in virtual compatibility mode. When you map a SAN LUN using a RDM, you choose between two modes of operation: physical and virtual. Per VMware documentation:

Virtual mode for an RDM specifies full virtualization of the mapped device. It appears to the guest operating system exactly the same as a virtual disk file in a VMFS volume. The real hardware characteristics are hidden. Virtual mode allows customers using raw disks to realize the benefits of VMFS such as advanced file locking for data protection and snapshots for streamlining development processes. Virtual mode is also more portable across storage hardware than physical mode, presenting the same behavior as a virtual disk file.

Physical mode for the RDM specifies minimal SCSI virtualization of the mapped device, allowing the greatest flexibility for SAN management software. In physical mode, the 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 for the owning virtual machine. Otherwise, all physical characteristics of the underlying hardware are exposed. Physical mode is useful to run SAN management agents or other SCSI target based software in the virtual machine. Physical mode also allows virtual-to-physical clustering for cost-effective high availability.

Additionally, you can also VMotion a VM with an RDM that uses network port ID virtualization (NPIV), as long as you use virtual compatability mode.

So there you have it. The audience members were right. My memory is not as shot as I thought it was, and everyone is happy.

2  Comments on this Post

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  • Jean-Yves Chevallier
    Thank you Andrew. I often do presentations about N-Port ID Virtualization and I often get this question as well, from users who believe RDM is incompatible with VMotion and don't know about physical and virtual modes. The company I work for (Emulex) demonstrates VMotion with NPIV (and RDM) as often as we have a chance. It works... Do you see many customers who use RDM in physical mode?
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  • San95
    I have a rdm in a vsphere 4.0 esx server cluster that consists of 4 esx servers. I've set up the rdm in virtual compatibility mode and I can vmotion it back and forth between the 4 servers however as soon as I reboot one of those esx server, I can no longer do live vmotion to that esx server unless I turn the vm with rdm off. I've called vmware and they are claiming it's a bug and were no help.
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