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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.
The limit on partition size was dictated by the 8-bit signed count of sectors per cluster, which originally had a maximum power-of-two value of 64. With the standard hard disk sector size of 512 bytes, this gives a maximum of 32 KB cluster size, thereby fixing the "definitive" limit for the FAT16 partition size at 2 GB for sector size 512.
File size is a measure of how much data a computer file contains or how much storage space it is allocated. Typically, file size is expressed in units based on byte . A large value is often expressed with a metric prefix (as in megabyte and gigabyte ) or a binary prefix (as in mebibyte and gibibyte ).
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 compression algorithm is designed to support cluster sizes of up to 4 KB; when the cluster size is greater than 4 KB on an NTFS volume, NTFS compression is not available. [71] Data is compressed in 16-cluster chunks (up to 64 KB in size); if the compression reduces 64 KB of data to 60 KB or less, NTFS treats the unneeded 4 KB pages like ...
In computer file systems, a block allocation map is a data structure used to track disk blocks that are considered "in use". Blocks may also be referred to as allocation units or clusters. [1] CP/M used a block allocation map in its directory. Each directory entry could list 8 or 16 blocks (depending on disk format) that were allocated to a file.
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]
The Atari Portfolio supports a sector size of 512 for volumes larger than 64 KB, 256 bytes for volumes larger 32 KB and 128 bytes for smaller volumes. Magneto-optical drives used sector sizes of 512, 1024 and 2048 bytes. In 2005 some Seagate custom hard disks used sector sizes of 1024 bytes instead of the default 512 bytes. [18]