contiguous allocation
contiguous allocation requires that each file occupy a set of contiguous blocks on the disk. Disk addresses define a linear ordering on the disk. With this ordering, assuming that only one job is accessing the disk, accessing block b+1 after block b normally requires no head movement. When head movement is needed(from the last sector of one cylinder to the first sector of the next cylinder), the head need only move from one track to the next.

linked allocation
linked a