New in vSphere 5.1–Support for five node Failover Clusters

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:

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:

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11 thoughts on “New in vSphere 5.1–Support for five node Failover Clusters

  1. This still applies to RDM, not VMDK right? Does 5.1 allows to have a MSCS using VMDK on multiple hosts or is it still limited to same host cluster?

    1. If you want to use VMDK files, you are still limited to having all VMs run on the same host (“cluster in a box” as VMware refers to it). Provided you’re running Windows 2008 SP2 or later you can now have 5 nodes, but they still all need to be on the same host if you want to use VMDKs.

      The other alternative to using RDMs at the vSphere level is to use iSCSI and configure in-guest initiators to connect to the shared storage volumes. That gets you around some of the limitations though is a bit more complex to manage overall.


      1. Cool, thanks Matt. I’m hopping you will be able to do it in the future. Maybe vSphere 6 or 7? That’d be a cool feature ;o)

      2. hey Matt,
        can we use iscsi san in vsphere for microsoft clustering ?
        in RDM or VMDK mode.. specially in RDM mode.. if yes then how many node we can add in it. ?

        1. Hey Sajid,

          You can use an iSCSI SAN for clustering virtual machines on vSphere if you use in-guest iSCSI only. iSCSI is not supported for use with Microsoft clusters if you’re using VMDKs (cluster in a box) or RDMs (cluster across boxes). That’s not to say it won’t work but just that it isn’t supported by VMware.

          This KB provides a lot of great info on what exactly is and isn’t supported when clustering virtual machines on vSphere:

          1. Hi Matt,
            you mean if i use iscsi initiator in side the vm and get the LUN then its supported but if i map RDM from esxi host to the vm then its not supported. ?
            kindly clear it for me ?

          2. You’re correct – if you want to use iSCSI to support virtual Failover Clusters, then your only option is using the iSCSI initiator inside the virtual machine. You cannot use iSCSI RDMs presented to the ESXi host or the solution won’t be supported.

            To be clear – iSCSI RDMs will technically work but it isn’t supported so it isn’t worth even discussing.

            Hope this helps clear it up.


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