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Network redundancy can be achieved using multiple connections to the same network, multiple networks in an asymmetric configuration, or multiple live networks. Balance complexity and cost against benefits when choosing a network redundancy scheme, because you re affecting every client and server in the end-to-end system. Manual failover is a realistic and practical approach to shuffling multiple hosts between networks. Network services such as naming need to be among the most reliable you have, if you use them, because many other higher-level services and management utilities depend on them.
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CHAPTER
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Data Centers and the Local Environment
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Occurrences in [the environment] are beyond the reach of exact prediction because of the variety of factors in operation, not because of any lack of order in nature. Albert Einstein
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In this chapter, we move to Level 5 of the Availability Index, the Local Environment, as shown in Figure 10.1. Environmental issues tend to be less technical than some of the other Index levels. Nevertheless, the environment in which your critical systems operate can have a serious impact on system availability. If the processes and the environment around your systems are well run, they will probably slip into obscurity and never be noticed by your users unless they are the cause of a significant outage.
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10
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A V A I L A B I L I T Y
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Local Environment Networking Disk and Volume Management Reliable Backups Good System Administration Practices INVESTMENT
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Figure 10.1 Level 5 of the Availability Index.
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Administered properly, data centers can be wonderful places from which to operate critical systems. But just like anything else, if they are mismanaged, they can be bad news. Data centers are, in many ways, throwbacks to the days of large mainframe computing. Mainframes required carefully controlled temperature and humidity, and they needed to be placed where certain people could get regular access to them. Since they had specific power requirements, data centers represented a single place where 220-volt lines could be run, battery backup units could be installed, and large amounts of air-conditioning could be installed. Raised floors made it easy to hide the spider webs of cables that often emanated from these machines to terminals, printers, and tape units. Fire protection could be centralized there; old Halon1 fire suppression systems could be deployed in a limited area. Although most modern computers are not as picky about temperature and humidity as the older mainframes were, today s data centers have many of the same features. One reason for the similarity, of course, is that many modern data centers still have mainframes happily chugging along. In fact, some highend servers from vendors like Sun and HP are looking more and more like
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Until 1994, fire suppression systems based on Halon, a liquefied, compressed gas that stops the spread of fire by chemically disrupting combustion, were commonly found in data centers and other facilities where water could not be used. In 1994, the Clean Air Act ended production of new Halon, but many facilities still use recycled Halon. Other chemicals have been introduced to replace Halon.
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mainframes. As a result, power conditioning, fire protection, and environmental controls are still valuable and practical with today s systems. Modern fire protection systems are safer and cleaner than Halon, but in the data center they can still be centralized into a relatively small space. Power requirements run the gamut, and 220-volt lines are still common requirements for larger systems. Data centers offer significant advantages and disadvantages as compared to leaving critical systems in regular work areas. The following are some advantages: Centralized resources. A data center is the perfect place to coordinate all the specialized requirements of large, critical machines: conditioned 220-volt power, fire protection, security, battery backups, raised floors, atmospheric control, and so on. If systems were deployed at random throughout the facility, those resources would need to be available everywhere. By limiting their deployment to the data center, a tremendous cost saving can be achieved. Physical security. Data centers work well to prevent inappropriate physical access to systems. But since they are centralized and they contain lots of large, impressive-looking equipment, there is a tendency to show them off through data center tours (see Data Center Tours, later in this chapter), which limit security. But there are disadvantages too: Limited possibilities for expansion. If your systems need to expand beyond the capacity of your rack, it may not be possible to get another rack adjacent to the first one. This can lead to complicated and messy layouts of systems, as they spread themselves in a seemingly random fashion all over the data center, like fast-food toys in a family minivan. In that event, there may be issues with cable lengths, especially when SCSI is involved, that make such expansion impossible. Putting all your eggs in one basket. Centralizing your systems can be a double-edged sword. Countering all the advantages of centralization, that we discussed previously, is the prospect that a single calamity could strike the data center and wipe out most or all of the company s computing resources at the same time. Of course, this can be addressed by maintaining multiple data centers at separate facilities, a topic we cover at length in 20, The Disaster Recovery Plan. Some people prefer to keep their clustered systems at opposite ends of their data center, to limit the impact of a localized external calamity such as a ceiling or floor collapse, or an overhead pipe leak. Too much physical security. Some security is critical, but too much security will do little more than impede the ability of system administrators to get their jobs done. In our experience, good security doesn t keep the
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