file slack space
DOS allocates disk space in multiple-sector chunks called "clusters".
Clusters typically hold 2K, 4K, or 8K of data.

When DOS creates a file, it must allocate at least one cluster for
that file.  Since file sizes vary, some portion of the last cluster is
wasted (it contains no file data).

  лK  лK  лK  лK  лK  лK  лK  лK  In this figure, a 2-byte file
 hi                               (just the word "hi") is stored in
                                  a 4096-byte cluster, so 4094 bytes
                                  of storage are "wasted" -- that's
4K cluster              the "file slack".
  DoubleSpace's allocation scheme that minimizes slack space; a 1-byte
file occupies only 512 bytes of physical storage.