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 several non-contiguous physical blocks, each of which is connected by a pointer, and the previous physical block points to the next physical block.This method can improve disk utilization and is convenient for file deleting and inserting.

3.Indexed Allocation.Each file has a primary index in the FAT, and the index contains the entry for each partition assigned to the file. The index of the file is stored in a separate block, and the entry to the file in FAT points to this block.it is the best method to sace file