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).

┌─½K┬─½K┬─½K┬─½K┬─½K┬─½K┬─½K┬─½K┐ 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.