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Windows Server 2016 has a lot of new features and enhancements to existing features. One feature organizations have been waiting to see is Cluster Rolling Upgrade. The Cluster Rolling Upgrade feature allows you to upgrade cluster nodes running Windows Server 2012 R2 on Windows Server 2016 OSes without any downtime. In other words, there will be no downtime of Hyper-V and Scale-Out File Server clusters when Windows Server 2012 R2 cluster nodes are updated to Windows Server 2016. Cluster Rolling Upgrade not only eliminates downtime, but also reduces the cost associated with maintenance tasks.
In earlier versions of Windows OS, you had to create a new cluster in order to upgrade existing Windows Server cluster nodes to a new OS. If you needed to upgrade the node OS, you had to evict the node, install the new OS and then create a new cluster with the new node. The whole process required you to plan your maintenance window, which also includes downtime of Hyper-V and Scale-Out File Server clusters. The Cluster Rolling Upgrade feature allows you to upgrade your existing nodes to Windows Server 2016 OS in a cluster without evicting the cluster nodes. The process is seamless and can be automated using PowerShell cmdlets, reducing the time it takes to perform the complete operation.
There is also no need to obtain additional hardware. In other words, you perform an in-place upgrade of all the cluster nodes with a few simple steps.
In earlier versions of Windows OS, Microsoft didn't allow different Windows Server versions to coexist in the same cluster. However, this has been changed in Windows Server 2016. Microsoft permits different versions of Windows to coexist within the same cluster, which is known as Mixed-OS Mode. This is also sometimes known as Cluster Functional Level, introduced in Windows Server 2016. Note that when you perform a rolling upgrade of a cluster, the cluster remains in the Mixed-OS Mode, but there are new features of Windows Server 2016 cluster that won't be available until your cluster functional level is updated to Windows Server 2016.
The cluster nodes must be running Windows Server 2012 R2 for the Cluster Rolling Upgrade feature to work. Also, the Cluster Rolling Upgrade feature helps in zero-downtime only for Hyper-V and Scale-Out File Server clusters. For clusters such as SQL or Exchange, there will be a minimum downtime for the services hosted by the cluster.
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