Threshold buffers is not a valid option for Async Prefetch

  Explanation:  The THRESHOLD option is only valid with DETECTED
  ASYNC PREFETCH and not with ASYNC PREFETCH.

  User Action:  If you want the ASYNC PREFETCH feature, remove the
  THRESHOLD syntax.  If you want the DETECTED ASYNC PREFETCH
  feature, you must specify the DETECTED keyword and include the
  THRESHOLD syntax.