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This chapter is from the book

This chapter is from the book

Improvements in Clustering and Storage Area Network Support

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 2008 now not only works, but also provides a series of significant improvements that actually make clustering work a whole lot better.

As IT administrators are tasked with the responsibility of keeping the network operational 24 hours a day, 7 days a week, it becomes even more important that clustering works. Fortunately, the cost of hardware that supports clustering has gotten significantly less expensive; in fact, any server that meets the required specifications to run Windows Server 2008, Enterprise Edition can typically support Windows clustering. The basic standard for a server that is used for enterprise networking has the technologies built in to the system for high availability. Windows Server 2008, Enterprise Edition or Datacenter Edition is required to run Windows 2008 clustering services.

Clustering is covered in detail in Chapter 29, "System-Level Fault Tolerance (Clustering/Network Load Balancing)."

No Single Point of Failure in Clustering

Clustering by definition should provide redundancy and high availability of server systems; however, in previous 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 failover from one system to another. Windows 2008 clustering removed this requirement of a static quorum drive. Two major technologies facilitate this elimination of a single or central point of failure, which include majority-based cluster membership verification and witness-based quorum validation.

The majority-based cluster membership allows 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 the cluster needs to failover to another node. Rather than assuming 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 2008. Effectively, Windows 2008 server clusters have better information to determine whether it is appropriate to failover 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 both organizations with distributed data centers and also provides more resiliency in the cluster operations components.

Stretched Clusters

Windows 2008 also introduced 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, technologically not the right thing to do 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.

Improved Support for Storage Area Networks

Windows 2008 also has improved its support for storage area networks (SANs) by providing enhanced mechanisms for connecting to SANs as well as switching between SAN nodes. In the past, a connection to a SAN was a static connection, meaning that a server was connected to a SAN just as if the server was physically connected to a direct attached storage system. However, the concept of a SAN is that if a SAN fails, the server should reconnect to a SAN device that is now online. This could not be easily done with Windows 2003 or prior. SCSI bus resets were required to disconnect a server from one SAN device to another.

With Windows 2008, a server can be associated with a SAN with a persistent reservation to access a specific shared disk; however, in the event that the SAN fails, the server session can be logically connected to another SAN target system without having to script device resets that have been complicated and disruptive in disaster recovery scenarios.

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