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In computer file systems, a cluster (sometimes also called allocation unit or block) is a unit of disk space allocation for files and directories.To reduce the overhead of managing on-disk data structures, the filesystem does not allocate individual disk sectors by default, but contiguous groups of sectors, called clusters.
File Allocation Table (FAT) is a file system developed for personal computers and was the default filesystem for the MS-DOS and Windows 9x operating systems. [citation needed] Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices.
Because file size references are stored in eight instead of four bytes, the file size limit has increased to 16 exabytes (EB) (2 64 − 1 bytes, or about 10 19 bytes, which is otherwise limited by a maximum volume size of 128 PB, [nb 2] or 2 57 − 1 bytes), raised from 4 GB (2 32 − 1 bytes) in a standard FAT32 file system. [1]
While storage devices usually have their size expressed in powers of 10 (for instance a 1 TB Solid State Drive will contain at least 1,000,000,000,000 (10 12, 1000 4) bytes), filesystem limits are invariably powers of 2, so usually expressed with IEC prefixes.
For a 512-byte allocation, the average unused space is 256 bytes. For 64 KB clusters, the average unused space is 32 KB. Generally, the allocation unit size is set when the storage is configured. Choosing a relatively small size compared to the files stored, results in excessive access overhead.
The FAT file system is a file system used on MS-DOS and Windows 9x family of operating systems. [3] It continues to be used on mobile devices and embedded systems, and thus is a well-suited file system for data exchange between computers and devices of almost any type and age from 1981 through to the present.
Using the default cluster size of 4 KB, the maximum NTFS volume size is 16 TB minus 4 KB. Both of these are vastly higher than the 128 GB [ a ] limit in Windows XP SP1 . The size of a partition in the Master Boot Record (MBR) is limited to 2 TiB with a hard drive with 512-byte physical sectors, [ 25 ] [ 26 ] although for a 4 KiB physical sector ...
The file system block size represents the minimum allocation unit. XFS allows file systems to be created with block sizes ranging between 512 bytes and 64 KB, allowing the file system to be tuned for the expected degree of usage.