Hard Drive Clusters

The smallest unit of space on the hard disk that any software program can access is a sector, which usually consists of 512 bytes. It is possible to have an allocation system for a disk whereupon each file is assigned as many individual sectors as is required. An an, a 1 Megabyte would require approximately 2,048 individual sectors to store its data, and the HPFS utilizes this type of allocation system.

The FAT file system, as is the case with most file systems, does not utilize individual sectors, and there are several performance reasons for this. By using individual sectors, the process of managing disks becomes overly cumbersome since files are being broken into 512-byte pieces.

In order for FAT to manage files with some form of efficiency is to group sectors into larger blocks referred to as clusters, or allocation units. Cluster size, however, is not a predetermined size, but rather is determined by the size of the disk volume itself, with small volumes (disk sizes) resulting in smaller clusters, and larger volumes (disk sizes) using larger cluster sizes. For the most part, a cluster ranges in size from 4 sectors or 2,048 bytes to 64 sectors or 32,768 bytes. You should be aware that you may, on some occasions, find 128-sector clusters in use at 65,536 bytes per cluster

Cluster size is determined at the time a disk volume is partitioned. Cluster size is an important consideration when setting up a hard disk so as to ensure that you maximizing the efficiency of the disk. Larger cluster sizes result in more wasted space because files are less likely to fill up an integer number of clusters e.g. if a volume uses clusters that contain 8,192 bytes, an 8,000 byte file will use one cluster, or 8,192 bytes on the disk. On the other hand, a 9,000 byte file will use two clusters, or 16,384 bytes on the disk.

References:

http://www.dewassoc.com/kbase/hard_drives/clusters.htm

http://www.ntfs.com/hard-disk-basics.htm

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