What is File Allocation?

The structure that offers the FAT file system its name is the file allocation table. In order to know what this important table does, you should first understand how space on the hard disk is allocated under operating systems that use FAT family file systems (including DOS and most versions of Windows.)

Data is stored in solo 512-byte sectors on the hard disk. In theory it is possible for every file to be allocated to a number of individual sectors and this is actually done for some file systems (such as HPFS) though for performance reasons individual sectors are not allocated to files in the FAT system. The reason is that it would consume a lot of overhead (time and space) to keep track of pieces of files that were this small a 10 GB disk partition has (20,000,000 sectors) the hard disk is instead broken into larger pieces called clusters or alternatively allocation units. Every cluster contains a number of sectors. Usually, clusters vary in size (from 2,048 bytes to 32,768 bytes) which corresponds to 4 to 64 sectors the entire. The file allocation table FAT is where information about clusters is stored. Every cluster has an entry in the FAT that describes how it used. This is what tells the OS (operating system) which parts of the disk are currently used by files and which are free for use. The FAT entries are utilized by the operating system to chain together clusters to form files.

The file allocation tables FAT are stored in the area of the disk immediately following the volume boot sector. Every volume actually contains two identical copies of the FAT ostensibly; the second one is intended to be a backup of sorts in case of any damage to the first copy. Damage to the FAT is able to of course result in data loss since this is where the record is kept of which parts of the disk contain which files. The initiative behind the backup copy is that it could be used in the event that the primary becomes damaged.

In the conventional FAT system though, the backup FAT system does not work too well. The trouble is that the two copies are kept right next to each other on the disk, in order that if, for instance, bad sectors appear on the disk where the first copy of the FAT is stored, chances are pretty high that the second copy will be affected as well. Another trouble is that disk utilities often duplicate the primary FAT to the backup FAT location. This signifies that any corruption that arises in the primary FAT may be duplicated to the backup copy before it is noticed.

Under FAT32 some enhancement were made to the FAT backup scheme. First- either copy of the FAT is able to be designated the "primary" and either the "backup". Second Method- the process by which the FAT is copied from the primary to the backup location is able to be disabled and the combination of these features permits the second FAT to be used and protected in the event of problems with the first.