As has been the case with the last few vSphere releases, VMware has crammed vSphere 5.1 full of new features and functionality. I won’t try to go into all of the details since there are many blog posts out there already.
Since virtualizing business critical applications is near and dear to my heart, one of the changes that immediately jumped out to me is a big change to the support for virtualized Windows Failover Clusters. Since the first version of the Setup for Failover Clustering guide was released (probably back in the VI3 days or maybe even earlier), there has been one restriction: Virtualized clusters are limited to just two nodes. That was true whether both were virtual, or one was physical and one was virtual.
The limitation was technically an on-paper limitation – nothing would stop you from adding more than 2 nodes to a cluster. You could run into issues with SCSI locking with more than 2 node clusters so VMware didn’t support it.
With vSphere 5.1, the limitation has been raised to allow support for up to five node clusters provided you are running at least Windows 2008 SP2 or higher. If you are running an older version of Windows, you’re still limited to just 2 nodes.
Here’s a link to the above referenced Setup for Failover Clustering and and Microsoft Cluster Service document updated for vSphere 5.1: http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-setup-mscs.pdf
One thing to note – the well known and very handy “Microsoft Clustering on VMware vSphere: Guidelines for Supported Configurations” KB still lists the maximum node limit at 2. I would expect VMware to update that KB soon (and I will be reaching out to them about it shortly).
This also only applies to what are known as “shared disk clusters” or clusters that share the same disk resource among active/passive nodes. For solutions that leverage non-shared disk clusters, such as Exchange 2010 DAGs or SQL 2012 AAGs, there is no such limit and the only limit is whatever is supported by the application.
This is good news to those that still need to support virtualized clusters, or for migration/long term coexistence between physical and virtualized clusters.
Update 9/19/12 - Cormac Hogan has posted some more technical details on the change in support for virtualized clusters at his blog. Read about it here: http://cormachogan.com/2012/09/19/vsphere-5-1-storage-enhancements-part-10-5-node-mscs-support/