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    Fault Tolerance

                    Virtual Machine and Data Protection is an important element to any corporation/enterprise that incorporates virtual infrastructure.  Of the items discussed or presented to us in our text, I feel that Fault Tolerance (FT) is one of the more important topics.  My reasoning behind this is that being able to recover from or withstand the impact of a casualty is important.  Time is money, and downtime can potentially result in losses for a company.  From my understanding, fault tolerance is a stricter form of HA (High Availability); FT aims for zero downtime while HA aims for minimal downtime.  On page 474 the text states this by saying FT is used to provide higher levels of virtual machine availability than what is possible with vSphere HA.  While HA uses backup or secondary servers/machines that startup when a primary failure occurs, FT uses a replicated machine (VM) that keeps a lockstep or mirrored state to the primary machine so in the case of a failure, the replicated machine can immediately jump in and take over.  Some of the uses for FT over HA could be an application where interruption in the session would cause errors or be unacceptable, any critical applications of processes (financial or medical application sessions or transactions), or any other applications that an interruption of system/resource availability is unacceptable.

                    FT is an amazing feature for those critical processes or applications, but it is not without its disadvantages.  Resources must be tightly monitored and allocated when FT is in use; the resources allocated to the primary machine must be identical to the backup machine, and they must also have access to all the same datastores and networks.  This can be pricey, especially if FT is in use for all machines, however when lost time equates to lost profits, it can pay for itself. Overall, HA will perform well enough, however FT provides that extra layer of security for the critical items.  FT is one of the most important areas, not only in Virtual Infrastructure, but in Networking, Security and outside IT as well (most people just refer to FT or HA as redundancy, and describe the level of redundancy as more redundant) as everything will experience a failure at some point, and FT or redundancy softens the blow and enables the business process to keep rolling.

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