Cluster (file system)

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Not to be confused with clustered file system.

Disk structure:
(A) track
(B) geometrical sector
(C) track sector
(D) cluster

In computer file systems, a cluster is the unit of disk space allocation for files and directories. In order to reduce the overhead of managing on-disk data structures, the filesystem does not allocate individual disk sectors, but contiguous groups of sectors, called clusters.

On a disk that uses 512-byte sectors, a 512-byte cluster contains one sector, whereas a 4-kilobyte (kB) cluster contains eight sectors.

A cluster is the smallest logical amount of disk space that can be allocated to hold a file. Storing small files on a filesystem with large clusters will therefore waste disk space; such wasted disk space is called slack space. For cluster sizes which are small versus the average file size, the wasted space per file will be statistically about half of the cluster size; for large cluster sizes, the wasted space will become greater. However, the read and write speed of a disk increases as cluster size increases. Typical cluster sizes range from 1 sector (512 B) to 128 sectors (64 KiB).

More recently, there are clusters that may not be physically contiguous at the disk: if it's rotation is too fast compared to the read-writing speed of the head, than its sectors may be conveniently distributed at the disk, e.g., a distance apart at the same disk track. This should not be confused with fragmentation, as in this example they are still logically continuous.