For UNIX and VMS administrators, clustering is old hat. But for someone coming from a PC LAN environment, clustering might be a new concept. Clustering lets multiple servers work together as a unified computing resource to service a group of tasks, provide fault tolerance and continuous availability, or offer dynamic scalability. For networked users, clustering provides nonstop database, email, file, and other system services. In theory, you should be able to blast a hole through one of your clustered servers, and your users never notice the difference.
In "Clusters for Everyone," Mark Smith gives you an overview of the current Windows NT clustering market. He discusses who the players are, how their clustering solutions fit in the market, and how you can use these clustering solutions in an NT environment. In the product reviews that follow this introduction, the Windows NT Magazine Lab examines seven NT clustering solutions.
The Lab picked clustering solutions that represent major clustering technologies: disk mirroring, file replication, and fault tolerance. Some vendors implement their clustering solution with the focus on software. For example, Octopus' Octopus SASO and NSI's Double-Take are primarily software-based solutions. Several vendors combine hardware and software products to create a solution. Cubix's RemoteServ/IS combines Cubix technology with Citrix WinFrame. For NT Cluster-in-a-Box, Data General combines ALR servers, Data General's CLARiiON disk subsystem, and VERITAS' FirstWatch clustering software. IBM resells its Intel-based systems and disk array with either Vinca's StandbyServer for NT or Microsoft's Wolfpack. Amdahl uses its Intel-based servers and LVS disk array with FirstWatch, Wolfpack, or NCR's LifeKeeper software.
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All the vendors of the products the Lab reviewed, and most of the vendors we list in "Buyer's Guide for Clustering Solutions," will support the Wolfpack APIs. For a quick overview of the other basic features and capabilities of the products we tested, see "Clustering Solutions Feature Summary." In "Clustering Terms and Technologies," you'll find explanations of common clustering terms used throughout the product reviews.
Getting Technical
Clusters work in many different ways, depending on the technology you choose. Through specific hardware and software, you can set up two-node clusters that eliminate just about every common single point of failure: power supplies, disks, processors, and network connections. If enough components fail on one of the nodes, the other node takes over. All the solutions available for NT offer two-node, shared-nothing clusters: The server nodes in the cluster are self-contained, independent computers (no shared memory bus, no shared disk). Unfortunately, shared-nothing clustering presents difficult problems to overcome, such as how to offer fully fault-tolerant, available systems and how to perform dynamic scaling. Microsoft and clustering product vendors have not yet addressed these problems in either NT or the add-on products. The technology is coming, but you might have to wait a year or two before you can get it. NT 5.0, Wolfpack 2.0, SQL Server 7.0, Exchange Server 6.0, Internet Information Server (IIS) 4.0, Transaction Server 1.0, and so on will each bring us a step closer to a self-contained, scalable BackOffice solution--Oracle Parallel Server on NT may provide a solution sooner.
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