ARCHIVED: What does cluster size mean when referring to hard drives?

This content has been archived, and is no longer maintained by Indiana University. Resources linked from this page may no longer be available or reliable.

When installing a hard drive, a preparatory process consisting of partitioning and formatting allocates the space into logical units. The operating system determines how these units are sized and arranged. Although this will differ between different operating systems and their file systems, all systems call these logical units clusters.

Clusters are used to store files and data on the hard drive. The size of the cluster is important because space can be wasted on systems that are inefficient in sizing the clusters.

For example, if you have a 2GB hard drive partitioned with FAT16, a common file system used by DOS and some versions of Windows 95, your cluster size is 32KB. This means that any file you store will have a minimum file size of 32KB, or one cluster. If you write a Word document that is 6KB long, the system will tell you the file is 6KB in size, but it will really take up 32KB on the hard drive; 26KB of space is wasted. There is no way to break the clusters up into smaller pieces without changing the file system.

Using a more efficient file system, such as FAT32 or NTFS, will save hard drive space because the file system uses a smaller cluster size. FAT32 is the file system shipped with Windows 95 OSR2 and Windows 98, while NTFS is the file system in Windows NT, 2000, and XP.

A 2GB hard drive partitioned with FAT32 has a cluster size of 4KB. The same 6KB file stored on this drive will take up two 4KB clusters. Only 2KB of space are wasted under this system.

Cluster size is set automatically when you partition the drive depending on the file system you use. UITS recommends that you leave these default settings unchanged, since changing them can damage data or interfere with the drive operations.

This is document ahim in the Knowledge Base.
Last modified on 2018-01-18 12:43:43.

Contact us

For help or to comment, email the UITS Support Center.