file in the path table 3 for example if a file u201c b txt u201d is stored in the directory u201c bbb u201d the directory id corresponding to the file u201c b txt u201d is 4 refers to the table 1 an exemplary file table table 2 is shown as follows in the file table 4 each row stores a file name a length of the file name a directory id corresponding to the file and a file id the file id of each file in the file table 4 is different from the others the zero stands for a blank row a file path i e a path of a directory including the file can be obtained from the path table 3 according to a directory id corresponding to the file for example in the table 2 a directory id corresponding to a file u201c a jpg u201d is 4 i e the directory id of the directory u201c bbb u201d in the table 1 according to an equation of u201c a directory path equals a parent directory path plus the directory name u201d the file path of the file u201c a jpg u201d i e the directory path of u201c bbb u201d can be obtained from the table 1 according to the directory id 4 the directory path of u201c bbb u201d in the table 1 is u201c d pictures aaa bbb u201d so the file path of the file u201c a jpg u201d is u201c d pictures aaa bbb u201d the application 1 only stores the file id the application 1 sends a file id to the file management unit.