* Contiguous Allocation
* Linked Allocation
* Indexed Allocation.
Each method has its advantages and disadvantages. Accordingly, some systems support all three (e.g. Data General's RDOS). More commonly, a system will use one particular method for all files.
Contiguous File Allocation
- Each file occupies a set of contiguous block on the disk
- Allocation using first fit/best fit.
- A need for compaction
- Only starting block and length of file in blocks are needed to work with the file
- Allows random access
- Problems with files that grow.
Linked File Allocation
- Each file is a linked list of blocks
- NO external fragmentations
- Effective for sequential access.
- Problematic for direct access
File Allocation Table (FAT)
- Variation of the link list(MS/DOS and OS/2)
- A section of the disk at the beginning of each partition (Volume) is set aside to contain a FAT
- FAT has one entry for each disk block, pointing to the next block in the file.
- The link list is implemented in that section
- Actual file blocks contain no links
Indexed File Allocation
- Indexed allocation is bringing all the pointers together
- Much more effective for direct access
- Inefficient for small files