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Figure 17.4 Active-active configuration, before a failover.
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Mirrored Public Network Figure 17.5 Active-active configuration, after a failover.
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So, the question remains: Which configuration style is better As with so many of the questions that we examine throughout the book, the complete answer is not entirely clear-cut. Active-passive is unquestionably better for pure availability. But because of the added implementation costs, it is just as unquestionably more difficult to sell it to management. From a cost perspective, active-passive is superior. It requires fewer systems, and as a result, less space, less electricity, less cooling, and less administration. In the end, like nearly everything else we have discussed, it depends on your needs. If your applications are so critical that you need to squeeze every possible iota of availability out of them, then active-passive is the way to go. If rare outages are tolerable, then active-active clustering should be fine. If you believe that active-passive is truly the way to go, but your management is not convinced, we recommend selling it to them the same way you might sell life insurance. Clustering software and configurations are just like life insurance, after all. Although nobody likes to admit it, system outages are inevitable. When they occur, they cost money, and depending on when they occur and the importance of the system that has failed, they can cost a lot of money. The parallels to life insurance are obvious.
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Every few months I send my life insurance company a check. In return for that timely payment, I receive . . . nothing! (Not even a lousy calendar.) And personally, I hope that relationship continues for a very long time. If I were to stop paying those bills, the insurance would go away, and when my family finally needs it, they won t get any of the benefits. (Personal note to my wife: Relax, I have no intention of stopping my life insurance payments.) If your company chooses not to invest in a standby server, then when it needs it, it won t get the benefits that a reliable standby server can provide. Just as I (at least in theory) eat right and exercise so that I will live a long, healthy life, and therefore get to keep making those insurance payments and keep getting nothing in return, we build our computers with spare NICs, mirrored disks, mature applications, and all the other stuff we have discussed. Someday, despite my best efforts (and although I really don t like to think about it), that life insurance policy will finally pay out. And despite your best efforts, someday that critical server is going to fail. When it does, you ll want the most reliable server backing it up. Otherwise, downtime will be the inevitable result. The most reliable takeover server is one that isn t doing anything, except waiting to step in. Evan
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However, more than four out of every five two-node clustered configurations that we have been involved with over the years have been active-active configurations. Realistically, they are more economical, and the risks that they bring along are acceptable for most environments. Once again, your mileage may vary. Choose the configuration that makes the most sense for you.
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Back in 15, we defined a service group as a set containing one or more IP addresses, one or more disks or volumes, and one or more critical processes. A service group is the unit that fails from one server to another within a cluster. In the early days of clustering software, especially in active-passive configurations, each cluster ran a single service group, and when that group was on a system in the cluster, that system was active; otherwise, it was not. There was no concept of multiple service groups. Later, when FMS grew more sophisticated and active-active clusters began to appear, two systems shared two service groups. When the cluster and its components were operating normally, there was still just one service group allocated to each server. As servers grew larger and able to handle more capacity, it became clear that each server could easily manage more than one service group without suffering any significant performance impact. If a cluster member could manage more than one service group, it was reasonable to require that each service group must be able to failover separately from any others. Otherwise, the multiple service groups would, in reality, be a single service group, and no advantage would be gained. The introduction of multiple service groups to clustering added value because they gave FMS the ability to failover intelligently. Service groups could be split up between multiple nodes in the cluster after some members of the cluster had disappeared. Service groups could failover to less heavily loaded systems, or be reapportioned between nodes based on just about any rule set. For service groups to maintain their relevance and value, they must be totally independent of each other. If because of external requirements, two service groups must failover together, then they are, in reality, a single group. Service group failover, as shown in Figure 17.6, is, therefore, the capability for multiple service groups that ran together on one server to failover to separate machines when that first server fails. In this figure, we see two nodes: fish and chips. Fish has two active service groups, A and B, and chips has two other service groups, C and D. Each group can also run on its respective partner server; A can run on chips as A', and so on. Even though the service groups can move separately from fish to chips and back again, the advantage of this scheme, from an availability perspective, is negligible.