442 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
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.