HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Advanced HVAC Control, Information Technology and Open Systemsto switch to an alternative component that continues to function duringa failure. This redundancy is fundamental for a system that either recoversfrom or masks failures. Redundancy can be passive or active. Thegoal is to recover from failures in passive redundant systems and tomask failures so that they are transparent to the user in active redundantsystems.Passive RedundancyA passive redundant system provides access to alternative componentsthat are not associated with the current task and must be eitheractivated or modified in some way to pick up the failed component’sload. The transition is noticeable and may even interrupt service anddegrade system performance.Examples of passive redundant systems include standby serversand clustered systems. The mechanism for handling failures in passiveredundant systems is to switch over to an alternative server. The currentstate of the application may be lost, and the application may need to berestarted. The restart typically causes some interruption or delay in serviceto the users.Passive solutions are offered by NCR, Vinca, Microsoft and Novelland active redundant systems are offered by Stratus/Ascend and Tandem/Compaq.All require that the OS and the application have specificknowledge of the system architecture to take advantage of redundancy.This means that the OS and applications must be modified.Systems that recover from failures use a single system to run theapplication until a failure occurs. The detection of a failure may takeseveral seconds to several minutes before the recovery process begins.In the simplest type of recovery system, an operator moves thedisks from the failed system to another system and boots the secondsystem. In more sophisticated systems, the second system has knowledgeof the application and users, it reboots the applications and logs onthe users. In either case there is a pause in operation and a lose of data.Applications that have been modified to know the system architecturecan reboot automatically, providing a smoother recovery.Some automatic backup systems periodically copy certain filesonto another system that is then rebooted if the first system fails. Theconfigurations are known as clusters. These clusters include standby©2001 by The Fairmont Press, Inc. All rights reserved.