包括局部化保持，拓展块保留，内存映射和高速缓冲。
They include locality preservation, wilderness preservation, memory mapping, and caching.
该块保留了下一个有效标识的值，并且会向下一个PoId提供next方法。
The block maintains the value of the next valid identifier, and will provide the next PoId with the next method.
keep - with - previous和keep - with - next特性指定块应和前一块还是下一块保留在一起。
The keep-with-previous and keep-with-next properties specify whether a block should remain with the previous or next block.
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