说明：  通过merge-sort算法的实现，掌握外存算法所基于的I/O模型与内存算法基于的RAM模型的区别；理解不同的磁盘访问优化方法是如何提高数据访问性能的。
(Merge-sort algorithm, to grasp the core algorithm based I/O model memory algorithm is based on the distinction RAM model understand how different disk access optimization method to improve data access performance.)