Microsoft Cluster Operating System (OS) Rolling Upgrade is a feature that keeps Hyper-V virtual machines (VMs) or Scale-Out File Server workloads running during an upgrade from a Windows Server 2012 R2 cluster to a Windows Server 2016 cluster.
The Cluster OS Rolling Upgrade allows administrators to update each individual node from Windows Server 2012 R2 to Windows Server 2016 to avoid workload downtime when the cluster is in mixed mode.
In earlier versions of Windows Server, an upgrade required taking the cluster offline, installing the new operating system on each node, bringing the upgraded cluster back online, and restarting workloads.
The Cluster OS Rolling Upgrade also gives administrators the ability to roll back the upgrade process up until the cluster functional level has been updated to Windows Server 2016 using a PowerShell cmdlet. Once the Update-ClusterFunctionalLevel cmdlet has been run, the process cannot be reversed.