A few weeks ago I got an assignment to review a colleague’s infrastructure design based on VMware. The design was fine (because they used my design document as template 🙂 ) but one thing triggered me.

There was a special port group on one of the virtual switches names ‘MSCLUSTER’. No problem so far but I heard the client recently migrated to iSCSI storage.

The combination Microsoft Clustering Services (MSCS) combined with iSCSI triggered  my alarm bells. As I recalled Microsoft Clustering on iSCSI is not a problem but Microsoft Clustering on VMware with iSCSI is not supported (by VMware).

So, I rejected the design based on this support issue.

This message was not well received, as it turned out they had just migrated their clustered Exchange servers and had been running this way for a while now. Proposing to the client to migrate again was no option. So they tried to  question my findings. They had been running this setup without any problems and Exchange CCR clustering wasn’t real Microsoft Clustering because no disks were clustered, only the Exchange services.

A quick search told me that Exchange CCR clustering is based on Microsoft Cluster Service-Majority Node Set clusters, which do not require shared storage. The last sentence, ‘which do not require shared storage‘, threw me off. Because I’m no Microsoft Clustering expert and certainly no Exchange guru I had to turn to my VMware resources.

My question was twofold. One, Is Exchange 2007 CCR on VMware with iSCSI supported? And two, If VMware doesn’t support this, is this going to change in the future?


So, is Exchange 2007 CCR on VMware with iSCSI supported? The VMware response is simple and straight forward.

  • CCR is supported on iSCSI by Microsoft, but not by VMware.

  • All clustered servers (including CCR/DAG) must use RDM disks, this is a VMware requirement, Microsoft doesn’t care.

  • VMware only supports Fiber Channel for clustered servers even though CCR doesn’t use a shared disk model.

  • For standalone servers (non-clustered), FC and iSCSI are supported by both VMware and Microsoft.


So VMware does not support Exchange CCR clustering on iSCSI but is this going to change in the future?

The response is promising. The Exchange team within VMware requested the vSphere product team to support iSCSI for CCR and DAG clusters.  They also would like to remove the RDM requirement for CCR and DAG. Response from the product team is that they are testing and will update the support stance in a future release.

Bottom line: Maybe in a future release VMware is probably going to support Exchange CCR and DAG clustering on VMware with iSCSI. At the moment it’s not clear in which future release this support will be added.


Personally I see very little advantages to use Exchange CCR over VMware HA or even FT for your Exchange servers.

OK, with Exchange CCR clustering you can back-up the offline node, no manual actions are needed for site recovery and data is available off-site. This can all be achieved with alternate solutions like snapshotting, Site Recovery Manager and data replication.