Quorum

A major concern in an automated failover system is the split-brain scenario. This occurs when two or more resource controllers believe they are in control of the only running instance of that resource. This is also known as a partitioned cluster. In the case of clustered Hyper-V Servers, such a situation would look like the following figure:

In the figure, the hosts cannot see each other, so each assumes that the other is down and starts the highly available virtual machine. Clients connect and begin making updates to two separate database copies, resulting in modifications that may be irreconcilable. Fortunately, this situation is practically impossible for most virtual machines. Even if multiple nodes try to bring the same virtual machines online, only one will be able to use its files, most importantly its VHDs, which exist on shared storage. For the virtual machines that use pass-through disks, the outcome isn't as clear. Some multisite clusters use replication technologies that...