1，首先，在要预留空间的分区上使用dd命令生成一个指定大小的文件：
创建一个10M的文件，名字是foobar：
In Unix-like operating systems， /dev/zero is a special file that provides as many null characters （ASCII NULL， 0x00； not ASCII character "digit zero"， "0"， 0x30） as are read from it. One of the typical uses is to provide a character stream for overwriting information. Another might be to generate a clean file of a certain size. Using mmap to map /dev/zero to RAM is the BSD way of implementing shared memory.
2，格式化这个文件：
3，将此文件mount，之后就可以向这个文件中写内容了，跟向一个目录中写内容是一样的：