7.2 The Problems Clustering Does and Doesn't Solve

A common mistake made when employing cluster technology is to view clustering as the quintessential answer to all of the downtime problems an Exchange deployment faces. Proceeding with this notion is a fatal mistake, however. Clustering can only address issues within the domain of the cluster service technology. For example, if you have poor disaster recovery practices or software that is plagued with bugs, no magic clustering technology is going to save you. Clustering technology, especially that in Windows NT/ 2000, can only help solve issues it was designed to address. Stated simply, clusters most directly help you reduce single points of failure. A standalone server running Exchange has several points of failure. Hardware components like the system board, processors, power supplies with no redundancy, network cards, etc., may fail at some point in the life of a server. For example, industry-standard servers do not have any form of processor redundancy available, whereby a failed processor is lock-step fault tolerant and the system experiences zero downtime when a processor fails. This type of technology is only available in high-end systems such as Compaq's Himalayas. By deploying a cluster solution, many single points of failure can be eliminated. In some cases, software issues can even be thought of as single points of failure, and clusters may address some of these issues as well.

Another downtime problem that system operators often face is planned outages. Planned outages are often not "charged" against the availability...