Data cluster

A cluster is a logical grouping of sectors of a disk.

" Cluster " or " allocation unit " is a term used in Windows parlance, with other operating systems, the term " block " in use. Both terms can be used interchangeably. When using the term " block ", however, is between the data blocks on the disk and the blocks of the file system ( = cluster) to distinguish clearly.

The file system can generally address only complete cluster, it is without the help of special programs not possible to address individual sectors or even individual bytes within a cluster. Therefore, files always occupy on disk, an integer amount of clusters. The larger the cluster, the less administrative effort must be expended for large files, and the lower the external fragmentation a file. However, since each file ( at the end ) in the middle one-half cluster unused ( wasted) is (internal fragmentation), larger clusters in view of the efficient utilization of the storage space of a disk, are disadvantageous, particularly in the storage of a large number of small files.

The maximum number of clusters on a disk varies depending on the file system. FAT16 only allows 65 524 ( 216-12 ) clusters, FAT32, however, supports up to 228 ( 268,435,456 ), NTFS even 264 clusters.

From the maximum number of clusters and the selected cluster size - typically 512-32768 bytes - determines the maximum size of a file system. In a cluster size of only 512 bytes a FAT32 file system, for example, up to 128 gibibyte ( about 137 gigabytes) include (228 x 512 bytes), larger clusters are used, this results in a correspondingly larger limit the file system.