Chapter 3. Planning 73
The system’s layer configuration cannot be changed if there are defined host objects with 
WWPNs. Therefore, ensure that the system layer is correctly planned and configured early 
during the deployment.
3.15.1  FlashCopy guidelines 
When planning to use FlashCopy, observe the following guidelines:
/SM590000Identify each application that must have a FlashCopy function implemented for its volume.
/SM590000Identify storage pool or pools that will be used by FlashCopy volumes. 
/SM590000Define which volumes need to use FlashCopy
/SM590000For each volume, define which FlashCopy type best fits your requirements: 
– No copy
– Full copy
– Thin-Provisioned
– Incremental
/SM590000Define how many copies you need and the lifetime of each copy.
/SM590000Estimate the expected data change rate for FlashCopy types other than full copy.
/SM590000Consider memory allocation for copy services. If you plan to define multiple FlashCopy 
relationships, you might need to modify the default memory setting. For more information, 
see 11.2.18, “Memory allocation for FlashCopy” on page 496.
/SM590000Define the grain size that you want to use. When data is copied between volumes, it is 
copied in units of address space known as grains. The grain size is 64 KB or 256 KB. The 
FlashCopy bitmap contains 1 bit for each grain. The bit records whether the associated 
grain was split by copying the grain from the source to the target. Larger grain sizes can 
cause a longer FlashCopy time and a higher space usage in the FlashCopy target volume. 
The data structure and the source data location can modify those effects. 
If the grain is larger than most host writes, this issue can lead to write amplification on the 
target system. This increase is because for every write IO to an unsplit grain, the whole 
grain must be read from the FlashCopy source and copied to the target. Such a situation 
might result in performance degradation.
If a thin-provisioned volume is used in a FlashCopy map, use the same grain size as the 
map grain size for best performance. Also, if a thin-provisioned volume is used directly 
with a host system, use a grain size that more closely matches the host IO size.
/SM590000Define which FlashCopy rate best fits your requirement in terms of the storage 
performance and the amount of time required to complete the FlashCopy. Table 3-4 lists 
the relationship of the background copy rate value to the number of grain split attempts per 
second.
For performance-sensitive configurations, test the performance observed for different 
settings of grain size and FlashCopy rate in your actual environment before committing a 
solution to production use. See Table 3-4 for some baseline data.
Table 3-4   Grain splits per second
User percentage Data copied per
second
256 KiB grain per
second
64 KiB grain per
second
1 - 10 128 KiB 0.5 2
11 - 20 256 KiB 1 4
21 - 30 512 KiB 2 8