Cluster (file system)

Disc structure showing:
(A) a track (red),
(B) a geometric sector (blue),
(C) a disk sector of a track (magenta),
(D) a group of sectors or cluster (green).

A cluster (or allocation unit in Microsoft terminology) is a set of contiguous sectors that make up the smallest unit of storage on a disk. Files are stored in one or more clusters, depending on their allocation unit size. However, if the file size is less than the size of a cluster, it is completely occupied by the cluster.

Although a cluster may contain multiple hard drive sectors, it may be contained within them, since the smallest cluster in a FAT file system, for example, may be 8 bytes, while each Hard disk sector is 512 bytes, hard disk sectors mark divisions in it and are responsible for reading the needle and defragmentation in Microsoft Windows operating systems.

File systems