text: **11.1.6 Grains and bitmap - I/O indirection**
  
  When a FlashCopy operation starts, a checkpoint is made of the source volume. No data is 
  copied at the time that a start operation occurs. Instead, the checkpoint creates a bitmap that 
  indicates that no part of the source volume was copied. Each bit in the bitmap represents one 
  region of the source volume. Each region is called a*grain*. 
  
  You can think of the bitmap as a simple table of ones or zeros. The table tracks the difference 
  between a source volume grains and a target volume grains. At the creation of the FlashCopy 
  mapping, the table is filled with zeros, which indicates that no grain is copied yet. When a 
  grain is copied from source to target, the region of the bitmap referring to that grain is updated 
  (for example, from “0” to “1”), as shown in Figure 11-2 on page 442. 