Cluster (file system)

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In certain filesystem types like the File Allocation Table (FAT) filesystem of MS-DOS or the NTFS filesystem of Windows NT, 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.

For , on a disk that uses 512-byte sectors, a 512-byte cluster contains one sector, whereas a 4-kibibyte (KiB) 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. Typical cluster sizes range from 1 sector (512 B) to 64 sectors (32 KiB).