FAT, NTFS, ext2/ext3/ext4 file systems store files in data blocks (also called data clusters under Windows). The cluster or block size remains at a constant number of sectors after being initialized during the formatting of the filesystem. In general, most operating systems try to store the data in a contiguous way so as to minimize data fragmentation. The seek time of mechanical drives is significant for writing and reading data to/from a hard disk, so that is why it is important to keep the fragmentation to a minimum level.