使用一个固定大小的文件来模拟多级目录结构的磁盘存储结构。（固定大小就是生成一个空文件，然后往其中写入固定大小的内容）-Use a fixed-size file to simulate the structure of multi-level disk storage directory structure. (Fixed size is to generate an empty file, and then to write the contents of which fixed-size)