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Definition: cluster

(1) Two or more systems working together. See clustering.

(2) Also called an "allocation unit" or "file allocation unit," it is a number of storage sectors treated as a unit. A cluster is the smallest unit of storage the operating system's file system can manage. That means if a 1KB file is stored in a system with a 4KB cluster (see table below), the 1KB file takes up 4KB of disk space.

Following are the cluster sizes for the various Windows file systems starting with Windows XP. When FAT32 came out, the reduction of cluster sizes from 16KB and 32KB down to 4KB saved a lot of space for small files. See file system, FAT, FAT32 and lost cluster.

                  Cluster Size

    Drive       FAT16       FAT32
    Size        (1984)      (1996)

     8-32MB     512 bytes   --
    32-64MB     1KB         512 bytes
    64-128MB    2KB         1KB
   128-256MB    4KB         2KB
   256-512MB    8KB         --
 512MB-1GB     16KB         --
     1-2GB     32KB         --
     2-4GB     64KB         --
 256MB-8GB      --          4KB
     8-16GB     --          8KB
    16-32GB     --         16KB


     7-512MB    4KB
 512MB-1GB      4KB
   1GB-2GB      4KB
   2GB-2TB      4KB
   2TB-16TB     4KB
  16TB-32TB     8KB
  32TB-64TB    16KB
  64TB-128TB   32KB
 128TB-256TB   64KB