Rhoderick Milne posted a great article about some issues and features that should be validated when deploying DAG on top of VMWare.
When resolving issues with on-premises Exchange sometimes the issue may be directly within Exchange, other times the root cause may lie outside Exchange. Depending upon the exact nature of the case we may have to investigate network switches, load balancers or storage. When Exchange is virtualized then the hypervisor and it’s configuration also may require attention.
This was the case with a recent customer engagement. Initially the scope was upon Exchange with symptoms including Exchange servers dropping out of the DAG, databases failing over and poor performance for users. As with most cases that get escalated to me, there is rarely only a single issue in play and multiple items have to be addressed. The customer was using ESX 5 update 1 as a hypervisor solution, and Exchange 2010 SP3. Exchange was deployed in a standard enterprise configuration with a DAG, CASArray and a third party load balancer.