Cloud-Buddy

As already addressed in the vSphere 4 u1 release notes, MSCS VMs are supported in a HA/DRS cluster, its amazing how many few have noticed the change. With all the functionalities that have been introduced over the years by VMware, its easy to miss a few things every now an then. Some consider MSCS a primitive form of clustering as opposed to HA/DRS clusters within ESX/i. However it must be noted that a HA/DRS cluster does not protect you from application failure or OS corruption. Neither does FT in vSphere. With a FT enabled VMs, it must be noted that when the primary VM blue screens, so does the secondary VM and you are left with two identical server both not functioning.

To sum it up, HA/DRS and even FT protects you from a hardware failure only. According to VMware, MSCS must be leveraged to maintain a 100% uptime for Windows guests. So what you can and cannot do with MSCS and VMware?

You can cluster two VMs on the same host, two VMs on seperates hosts and you can also cluster a physical and virtual machine. There are detailed guides published by VMware on how this can be achieved. (Click Here)

A 50K foot view of what you can and cannot do and this will also differ based on the version of ESX/I you are running:
Only two nodes in a MSCS cluster
MSCS cannot be an FT enabled VM
Though MSCS VMs can be in a HA/DRS cluster, both HA and DRS should be disabled for all the VMs that are a part of MSCS
Quorum and shared disk should not have the VMFS signature and should be presented to all the hosts in the cluster where the MSCS VMs reside (Think about it, it makes sense)
Don’t overcommit and try to create a reservation for your VM equal to the size of the memory assigned.
The VMware doc will have more details

Now the last part, DRS is disabled because under the hood, HA uses vMotion. Though vMotion is rapid and causes no outage for the users, MSCS heartbeat is very sensitive and may detect the few seconds of the stunning period as a node failure and consider that node to be down. This is certainly not what you want. Hence its best not to vMotion, which is why DRS is disabled as well.
Why is HA disabled? No one has been able to give a straight answer on that and it basically comes down to that its not supported.

As of now I really don’t know why you can’t have HA enabled for a VM that is part of a MSCS cluster.
The good news is, with 4 u1 and onwards, you can utilize the same hosts that are in a HA/DRS cluster to run your MSCS VMs, just don’t forget to disable these features for the VMs that are part of the MSCS cluster or else the VMware and MS support may stiff you in time of need.