Chapter 11. Advanced Copy Services 443
Figure 11-3   Copy on Write steps
With IBM FlashCopy, the target volume is immediately accessible for both Read and Write 
operations. Therefore, a target volume can be modified, even if it is part of a FlashCopy 
mapping. As shown in Figure 11-4, when a Write operation is performed on the target 
volume, the grain that contains the blocks to be changed is first copied from the source (Copy 
on Demand). It is then modified with the new value. The bitmap is modified so the grain from 
the source is not copied again, even if it is changed or if a background copy is enabled.
Figure 11-4   Copy on Demand steps
FlashCopy indirection layer
The FlashCopy indirection layer governs the I/O to the source and target volumes when a 
FlashCopy mapping is started, which is done by using the FlashCopy bitmap. The purpose of 
the FlashCopy indirection layer is to enable the source and target volumes for read and write 
I/O immediately after the FlashCopy is started.
The indirection Layer intercepts any I/O coming from a host (read or write operation) and 
addressed to a FlashCopy volume (source or target). It determines whether the addressed 
volume is a source or a target, its direction (read or write), and the state of the bitmap table for 
the FlashCopy mapping that the addressed volume is in. It then decides what operation to 
perform. Hereunder are the different I/O indirections.
Note: If all the blocks of the grain to be modified are changed, there is no need to copy the 
source grain first. There is no copy on demand and it is directly modified.