Three ways to allocate disk space

at present , Common external memory allocation methods for disks are Continuous distribution 、 Link assignments and Index allocation Three . When using different distribution methods , Different file physical structures will be formed .

1 Continuous distribution

The continuous allocation method requires each file to occupy a continuous set of blocks on the disk , As shown in the figure . The file structure formed in this way is called Sequential file structure , The physical file at this time is called Sequence file . This allocation method ensures that the order of recording in the logical file and the order of disk blocks occupied by the files in the memory are coincident .

2 Link assignments
Link allocation adopts discrete allocation , Eliminate the external debris , Therefore, the utilization rate of disk space is significantly improved ; And because it is based on the current needs of the document , Assign it the necessary disk blocks , When files grow dynamically , It can be reallocated dynamically , Therefore, there is no need to know the size of the file in advance . Besides , The addition of documents 、 Delete 、 It is also very convenient to change .

Link assignment can be divided into implicit link and explicit link .

2.1 Implicit linking

file , Each directory entry in the directory includes pointers to the first disk block and the last disk block of the linked file . Disk blocks are distributed anywhere on the disk , Except for the last disc , Each disk block has a pointer to the next disk block , These pointers are transparent to the user .

2.2 Explicit links

Explicit linking is used to link the pointer of each physical block of the file , Explicitly stored in a linked table in memory . This table only has one set on the whole disk .

3 Index allocation
When opening a file , Just call the number of the disk block number occupied by the file into the memory , There is no need to put the whole FAT Call in memory . So , Put the disk block numbers corresponding to each file together , Index allocation method is an allocation method based on this idea .

3.1 Single level index

It assigns an index table to each file , Record all disk block numbers allocated to the file in the index block , Therefore, the index block is an array containing many disk block numbers . When creating a file , Just fill in the directory entry created for it with a pointer to the index block .

3.2 Multi level index

When the file is too large , Too many index blocks , Single level indexing is inefficient . here , Build another level of index for these index blocks , It is called the first level index , You can also index , It is called the second level index and so on . It is called multi-level index allocation .

3.3 Mixed index allocation

take A combination of multiple index allocation methods And form a distribution mode , Such as direct address , Primary indirect address , Multiple indirect addresses .