Q: What has changed in the quorum model in Windows Server 2012?
A: There were times when the quorum model in Windows Server 2012 could be difficult to understand and architect, such as knowing when to use a disk or file share witness and handling situations when nodes were taken offline, which then changed the need for an additional witness.
In Windows Server 2012 R2, this is now much simpler. The guidance in Windows Server 2012 R2 is to always configure a witness.
If there is shared storage, use a disk witness; if there's no shared storage, use a file share witness. The Windows Server 2012 R2 Failover Cluster will use the witness only if it needs it.
For example, if there are an even number of nodes, then the witness will be used and will be given a vote. If there are an odd number of nodes, then the witness isn't used and won't have a vote.
This is all done automatically. It really simplifies quorum configurations and really was needed for working with the dynamic quorum capability added in Windows Server 2012.