With the release of VMware vSphere 4.1 come a boat load of new features. There are plenty of sites out there where you can read about them so I’m not going to go on and on, but there is one that stood out that I thought I’d try and clarify. For more info on 4.1 check out this page on VMware’s web site:
Prior to vSphere 4.1 virtual machines participating in a MSCS/WFC could not be combined with VMware HA. Prior to 4.0 it was recommended that VMware clusters that housed these VMs have VMware HA disabled. After the release of 4.0 we updated guidance to have VMware HA enabled, but have the auto-restart feature of VMware HA disabled for the individual VMs…effectively disabling VMware HA for the VMs but not for the cluster.
The reason that this isn’t supported in versions prior to vSphere 4.1 is due to the fact that VMware HA does not obey affinity rules. This becomes a serious issue when you have a shared storage cluster, i.e. Exchange 2007 single-copy cluster, SQL cluster, etc. If you have two members of a shared storage cluster that are mapping to the same raw-device mapping (RDM) and they are powered up on the same host there is a chance for data corruption. The way around this in previous versions is to disable auto-restart of the VMs - hence no support for MSCS nodes and VMware ha.
In 4.1 we introduce the concept of DRS groups. This lets you define a group for VMs and a group for hosts and set VM-Host affinity rules between those groups. Take for instance VM1 and VM2 are a MSCS cluster and Hosts 1-4 are a VMware HA cluster. You can create two VM groups each containing one VM and you can create two host groups each containing two separate hosts (group 1 = hosts 1-2, group 2 = hosts 3-4). You can then set a rule so that VM1 is only powered up on hosts 1-2 and VM2 is only powered up on hosts 3-4. This now guarantees that we will not have a corruption issue due to VMs being powered up on the same host.
Of course, this doesn’t those out there that will want to use VMware HA with Exchange CCR and DAG since there is no shared storage involved. Combining the two technologies is still not supported by MSFT for whatever reason, but just be aware that it can be done successfully and safely with vSphere 4.1.