Allocation unit

Allocation Unit is a term used in computer science and data management to refer to the smallest quantity of disk space that can be allocated to hold a file. All file systems used by Windows organize hard disks based on cluster size, which is determined by the number of sectors that the cluster contains. For example, on a disk that uses 512-byte sectors, a 512-byte cluster contains one sector, whereas a 4-kilobyte (KB) cluster contains eight sectors.

Overview

An allocation unit, or cluster, is the smallest logical amount of disk space that can be allocated to hold a file. The size of an allocation unit is chosen when the disk is formatted, and can be as small as a single sector, or as large as several kilobytes. The choice of allocation unit size is a trade-off. Smaller allocation units result in more efficient use of disk space, but larger allocation units can provide significant performance benefits.

Allocation Unit Size

The size of the allocation unit is determined by the file system that is used by the operating system. The most common file systems used by Windows are FAT, FAT32, and NTFS. Each of these file systems has a default allocation unit size, which is determined by the size of the disk volume.

• FAT: The default allocation unit size for FAT is 512 bytes. This is the smallest possible allocation unit size, and it results in the most efficient use of disk space. However, it also results in a large number of disk operations, which can slow down system performance.

• FAT32: The default allocation unit size for FAT32 is 4,096 bytes (4 KB). This is a compromise between disk space efficiency and system performance.

• NTFS: The default allocation unit size for NTFS is 4,096 bytes (4 KB). However, NTFS supports larger allocation unit sizes, up to 64 kilobytes (KB), for larger volumes.

See Also

• File system
• Disk storage
• Computer data storage