1. Continuous distribution (sequential structure)
a. Each file is assigned a set of adjacent extents.
b. The recording order in the logical file is the same as that of the disk block occupied by the file in the memory.
Advantages: easy to access in sequence and fast to read and write
Disadvantages:
*External debris will be generated. It can be made up by compact method, but it needs extra space and takes more time than compact memory.
*The size of the file should be given when creating the file; the utilization of storage space is not high, which is not conducive to the dynamic increase and modification of the file;
It is suitable for files that are accessed in a less sequential way, and it still retains the continuous file structure in popular UNIX systems. Such as exchange area
2. Link allocation (discontinuous) (linked structure)
Each file can be assigned a set of non adjacent disk blocks.
Set the link pointer to link multiple discrete disk blocks belonging to the same file into a linked list. The resulting file is called a linked file. There will be link costs.
(1) Advantages:
Discrete allocation to eliminate external fragmentation and improve utilization
At the same time, it is applicable to the dynamic growth of files; easy to modify
==>>Insufficient:
① Time cost of sequential retrieval: cannot support efficient disk block direct access. To access a file directly, you still need to look up many disk block numbers in fat in sequence.
② Space cost of link information: fat needs a large amount of memory. When the disk capacity is large, fat may take up more than a few MB of memory space. This is intolerable