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In article , Alisha's
Addict wrote: .. Yep - the spec allows for x-redundancy. Although you'd want to keep a lid on the amount of redundancy : More redundancy = more resilience More redundancy = more bits = more cost. After a while, the law of diminishing returns comes in, making it daft to have too many redundant legs. Dual would make a big improvement in resilience, Triple would be a partial improvement over dual but after triple, you're probably getting into gold-plating levels. It also depends on how much space there is. If you do your dual redundance with the cables in the same runs, you're only effectively getting single redundancy cos a cut in one place will cut both cables. Increasing redundancy to high levels can have subtle benefits and subtle problems. Lots of telephone switches and utterly mission-critical computers have triple-redundant elements, so you can take one element out of service for maintenance while still having a hot standby failover element. As redundancy increases, you may start imposing significant overhead keeping all the elements synchronized. Even worse is what is called the Byzantine Corruption Problem, where some of your elements are giving you incorrect information. Voting logic is one of the ways to approach Byzantine Corruption. Depending on your system policy, you can let the minority or majority control. For example, in a medical radiation treatment system, if any of the three processors indicates an unsafe condition, it immediately closes the shutter on the radioactive element. Better to stop the treatment and have a human check, than nuke the patient. In other systems, the majority rules, on the assumption that corruption is the exception case. Even more complex systems may have redundant monitoring devices that independently agree that the working elements are operating correctly, using modeling and the like. |
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