Continuous allocation

The continuous allocation method requires that each file occupies a set of contiguous blocks on the disk. The disk address defines a linear order for the disk. With this sorting, assuming that only one job is accessing the disk, accessing block b+1 after block b usually does not require moving the head. When the head needs to move, only one track needs to be moved. Therefore, the number of seeks required for accessing consecutively allocated files is the smallest, and the seek time required when a seek is indeed required is also the smallest.



Link assignment

Link allocation solves all the problems of continuous allocation. With link allocation, each file is a linked list of disk blocks; the disk blocks may be scattered anywhere on the disk. The directory contains pointers to the first and last blocks of the file.



Index allocation

Linked allocation solves the problem of external fragmentation and size declaration for continuous allocation. However, in the absence of FAT, link allocation cannot support efficient direct access, because the block pointers are scattered across the entire disk along with the blocks and must be read in order. Index allocation solves this problem by putting all the pointers together, the index block.