存储系统(卷)可指派给一个以上的存储组。
Storage system (volume) can be assigned to more than one storage group.
但存在继承性：sdo理解类型间的继承层次结构，给定属性要求给定类型，可指派子类型。
There is inheritance, though: SDO understands an inheritance hierarchy among types, and given a property expecting a given type, you can assign a subtype.
白盒子是可互相交换的，主应用程序可能把集群中的任何机器指派为控制者，而这个机器就把map和reduce任务分派给其它连接的白盒子。
White boxes are interchangeable, and the main application may assign any machine in the cluster as the controller, and this one in turn assigns map or reduce tasks to other connected white boxes.
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