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Increasingly, businesses rely on their information systems to be available to provide products, services, and decision support. The costs of downtime can be tremendous, especially for the new breed of Internet-based companies. In these situations, the hardware costs of having multiple servers and preventing downtime are small in comparison to the importance of server uptime. So far, we ve looked at several ways of protecting your information, including backup and recovery, replication, and the use of standby servers. Although these are good ways to ensure that your information is not lost, they fail to provide one feature the transparent fail-over of an unavailable server to another available one. In this situation, clients should not even notice that a major catastrophe has occurred in the server room or that critical resources were taken down for maintenance. What
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might seem like high levels of uptime on paper can translate into frustrating productivity or loss of business for users. Table 8-3 lists various uptime percentages and the actual amount of downtime that will be experienced if each level is met. Just 5 percent downtime can amount to more than a half month of server unavailability in a year. Notice that 99.999 percent uptime barely provides enough time for a full server reboot once per year! This is where the idea of clustering comes in. Simply put, clustering allows two or more physical servers to act as one logical server. Clients may see a whole array of servers as a single machine, and applications interact with it as such. The benefits are great when one server is unavailable, another can automatically take its place. This increases uptime and provides for high availability. Additionally, the servers can work together to balance the load between multiple clients. For example, if three managers decide to run large reports at the same time, each might be redirected to a separate machine for processing. In many cases, downtime might be required for server upgrades and/or maintenance (such as applying Windows NT Service Packs). Systems and network administrators should be able to take down a single server within a cluster with minimal impact to client machines. Again, all of these processes should occur behind the scenes and remain transparent to the user. Figure 8-22 is a logical depiction of how a cluster might operate. The ideas related to clustering probably seem quite logical, and you might think they d be easy to implement. In the real world, however, clustering on the Windows NT/2000 platform is much more complicated. There are limits to what can currently be done with hardware, software, and networking devices. In this section, we ll focus on the concepts, tools, and technologies available for evaluating and implementing clustering architecture. The intricate details of implementing a SQL Server clustering solution are beyond the scope of this book. However, we ll look at many of the issues related to planning for and evaluating clustering schemes for your business environment. Let s start by looking at some issues related to planning for clustering.
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Percentage of Uptime 95% 97% 99% 99.9% 99.99% 99.999% 99.9999%
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Downtime per Year (in seconds) 1,576,800 946,080 315,360 31,536 3,153.6 315.36 31.536
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Downtime per Year 18.25 days 10.95 days 3.65 days (87.6 hours) 8.76 hours 52.56 minutes 5.26 minutes 31.54 seconds
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Table 8-3.
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Figure 8-22.
A cluster: Multiple physical servers appear to clients as one.
Planning for Clustering
As with the decision to implement any type of technology, before you choose to implement clustering, you need to properly define the business need. We ll see in the next sections that implementing clustering can be time-consuming and costly. Before you undertake those expenses, you should ensure that clustering is necessary in your business environment. Here is a checklist of considerations that should help you make your decision: w Determine reasonable uptime requirements. Ask about network downtime in many companies, and the quick reaction will be No downtime is acceptable. Although it might seem to make sense that servers always be available, there are several tradeoffs involved in a 24 7 network environment: The hardware and software required to make this work can be very expensive. Much costly planning must go into the systems that provide for complete hardware redundancy. Applying upgrades and patches can be an overall benefit, but may require planned downtime. Based on these concerns, business leaders may find that having certain services such as a company intranet unavailable for small periods of time throughout the year is acceptable.
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