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Windows Server 2012 Technology Primer : Improvements for Continuous Availability (part 1) - No Single Point of Failure in Clustering,Stretched Clusters, 64-Node Clusters

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4/14/2014 3:07:26 AM
Although clustering of servers has been around for a long time in Windows (dating back to Windows NT 4.0, when it was available, but really didn’t work), clustering in Windows Server 2012 now not only works, but also provides a series of significant improvements. In Windows Server 2012, you can set up clustering without a lot of the complexity that was previously necessary to get high availability working in Windows; even the previous requirement of having a storage-area network (SAN) with cluster share volumes (CSV) is no longer. All these improvements in Windows Server 2012 make continuous availability a viable solution for enterprise-class high availability.

No Single Point of Failure in Clustering

Clustering, by definition, should provide redundancy and high availability of server systems. However, in earlier versions of Windows clustering, a “quorum drive” was required for the cluster systems to connect to as the point of validation for cluster operations. If at any point the quorum drive failed, the cluster would not be able to fail over from one system to another. Windows Server 2008 / 2008 R2 and Windows Server 2012 clustering removed this requirement of a static quorum drive. Two major technologies facilitate this elimination of a single or central point of failure: majority-based cluster membership verification and witness-based quorum validation.

The majority-based cluster membership enables the IT administrator to define what devices in the cluster get a vote to determine whether a cluster node is in a failed state and so needs to fail over to another node. Instead of assuming that the disk will always be available, as in the previous quorum disk model, now nodes of the cluster and shared storage devices participate in the new enhanced quorum model in Windows Server 2012. Effectively, Windows Server 2012 server clusters have better information to determine whether it is appropriate to fail over a cluster in the event of a system or device failure.

The witness-based quorum eliminates the single quorum disk from the cluster operation validation model. Instead, a completely separate node or file share can be set as the file share witness. In the case of a GeoCluster, where cluster nodes are in completely different locations, the ability to place the file share in a third site and even enable that file share to serve as the witness for multiple clusters becomes a benefit for organizations with distributed datacenters and also provides more resiliency in the cluster operation’s components.

Stretched Clusters

Windows Server 2012 also supports the concept of stretched clusters to provide better server and site server redundancy. Effectively, Microsoft has eliminated the need to have cluster servers remain on the same subnet, as has been the case in Windows clustering in the past. Although organizations have used virtual local-area networks (VLANs) to stretch a subnet across multiple locations, this was not always easy to do, and, in many cases, was not the right thing to do technologically in IP networking design.

By allowing cluster nodes to reside on different subnets, plus with the addition of a configurable heartbeat timeout, clusters can now be set up in ways that match an organization’s disaster failover and recovery strategy.

64-Node Clusters

Clustering provides redundancy of nodes in the cluster, but now with Windows Server 2012, an organization can have 64 nodes in the cluster, a significant improvement over the 16-node limit in earlier versions of Windows. With 64 nodes in a cluster, an organization can set up cluster nodes for high availability, for redundancy, and to serve as standby servers in the event of a server failure in the cluster. With potentially 64 nodes in the cluster, the organization can greatly improve high availability and continuous operations of the network systems.

 
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