text: Generally, smaller grain sizes save space, but require more metadata access, which can 
  adversely affect performance. If you are not going to use the thin-provisioned volume as a 
  FlashCopy source or target volume, use 256 KiB to maximize performance. If you are going to 
  use the thin-provisioned volume as a FlashCopy source or target volume, specify the same 
  grain size for the volume and for the FlashCopy function. In this situation, ideally grain size is 
  equal to the typical I/O size from the host. 