首先给出分布式系统的任务可用度与分布式互备份系统的定义。然后从任务可用度的角度出发 ,分别建立分布式互备份系统与分布式容错系统的任务可用度模型。最后 ,对分布式互备份系统和分布式容错系统的任务可用度进行比较和分析 ,得出许多有益的结论 ,它可以为分布式系统的可靠性设计与备份方案的选取提供理论依据。 First, the task availability of distributed system and the definition of distributed mutual backup system are given. Then from the point of view of task availability, task availability models of distributed mutual backup system and distributed fault-tolerant system are respectively established. Finally, the task availability of distributed mutual backup system and distributed fault-tolerant system are compared and analyzed, and many useful conclusions are drawn. It can provide theoretical basis for reliability design and backup scheme selection of distributed system.