  Predicting remaining tape media capacity? 
 The Question is:
 
How can you find out how much storage capacity is left on a tape?
 
 
 The Answer is:
 
  As this is more of a prediction than a calculation, there is no
  entirely reliable means of determining the remaining storage.  Well,
  you could keep track of the amount of data required to fill the tape
  media to capacity -- that's one way certain to produce a result.
 
  You could estimate the amount of information written to the tape and
  then subtract that from the reported media capacity, but the calculations
  will not reflect the implications of any potential media errors (disk
  devices are generally kept logically perfect via bad block revectoring),
  nor will the calculations reflect the relative effectiveness (or the
  potential ineffectiveness) of any data compression that might be in
  use.  While data compression typically compresses the data (and often
  by some unknown and data-specific factor), various degenerate data cases
  can cause the compression algorithm to actually increase the amount of
  storage required over that of uncompressed data.
 
  Most tape operations simply overflow onto additional media as required,
  potentially using extra media and empirical experience with the amount
  of storage required, or using automatic tape loaders, or using operator
  (manual) assistance (see REPLY and similar commands).
 
