the following introduce three file allocation methods

1.Contiguous Allocation: It requires that each file occupy a set of contiguous blocks on the disk. Disk address define a linear address ordering on the disk. For instance , the address will ordered one by one such as a,a+1,a+2，a+3...., but when a part of the block is deleted it will be one fragment , it is difficult to find one large and contiguous blocks to save files.

2.Linked Allocation: The information of a file is stored in sever