text: Background copy I/O is scheduled to avoid bursts of activity that could have an adverse effect 
  on system behavior. An entire grain of tracks on one volume is processed at around the same 
  time, but not as a single I/O. Double buffering is used to try to use sequential performance 
  within a grain. However, the next grain within the volume might not be scheduled for some 
  time. Multiple grains might be copied simultaneously, and might be enough to satisfy the 
  requested rate, unless the available resources cannot sustain the requested rate. 
  
  Global Mirror paces the rate at which background copy is performed by the appropriate 
  relationships. Background copy occurs on relationships that are in the InconsistentCopying 
  state with a status of Online. 