text: Figure 11-2 A simplified representation of grains and bitmap 
  
  The grain size can be 64 KB or 256 KB. The default is 256 KB. The grain size cannot be 
  selected by the user when creating a FlashCopy mapping from the GUI. The FlashCopy 
  bitmap contains 1 bit for each grain. The bit records whether the associated grain is split by 
  copying the grain from the source to the target. 
  
  After a FlashCopy mapping is created, the grain size for that FlashCopy mapping cannot be 
  changed. When a FlashCopy mapping is created, if the grain size parameter is not specified 
  and one of the volumes is part of a FlashCopy mapping, the grain size of that mapping is 
  used. 
  
  If neither volume in the new mapping is part of another FlashCopy mapping, and at least one 
  of the volumes in the mapping is a compressed volume, the default grain size is 64 KB for 
  performance considerations. But other than in this situation, the default grain size is 256 KB. 
  
  **Copy on Write and Copy on Demand**
  IBM Spectrum Virtualize FlashCopy uses Copy on Write (CoW) mechanism to copy data from 
  a source volume to a target volume. 
  
  As shown in Figure 11-3 on page 443, when data is written on a source volume, the grain 
  where the to-be-changed blocks are stored is first copied to the target volume and then 
  modified on the source volume. The bitmap is updated to track the copy. 