Also known as “allocation units”, clusters are essentially units of disk space as defined by a file system like FAT32 or NTFS during the partition formatting process. When files are saved to disk, they are stored in as many clusters as necessary to save the complete file. For example, if an NTFS partition is configured with a 4 K cluster size, a 32 K file would be saved using a total of 8 clusters. When a disk is defragmented, files are saved to contiguous disk clusters.
