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Clusters and File Allocation
As I have been explaining in various sections in this discussion of hard disk structures and file systems, the purpose of the file system is to organize data. The most popular file system in the PC world is the FAT family of file systems: FAT12, FAT16, VFAT and FAT32. All of these file systems use a specific technique for dividing the storage on a disk volume into discrete chunks, to balance the needs of efficient disk use and performance. These chunks are called clusters. The process by which files are assigned to clusters is called allocation, so clusters are also sometimes called allocation units.
In this section I describe how FAT file system clusters and file allocation work. I start by describing what clusters are themselves. I then talk about the mechanism by which clusters are assigned to files. I explain how the FAT file system handles deleting files--and undeleting them as well. I then discuss how the cluster system can cause the file system to become fragmented. I conclude with a discussion of file system errors that can occur under the FAT file system.