Commonly used data structures such as binary search tree (Binary Search Tree, BST) each node can only hold one data, resulting in a high tree height, the data of logically adjacent nodes may be far away, if in memory This is not a big problem for data manipulation in the middle, but because the speed of reading and writing to the disk is much slower than that of the memory, the unit of reading and writing to the disk each time is much larger than the smallest unit of reading and writing the memory.