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. When planning to use FlashCopy, observe the following guidelines: 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. 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. 3.15.1 FlashCopy guidelines  GLYPH<SM590000> Identify each application that must have a FlashCopy function implemented for its volume. GLYPH<SM590000> Identify storage pool or pools that will be used by FlashCopy volumes.  GLYPH<SM590000> Define which volumes need to use FlashCopy GLYPH<SM590000> For each volume, define which FlashCopy type best fits your requirements:  – Full copy – No copy – Thin-Provisioned – Incremental GLYPH<SM590000> Define how many copies you need and the lifetime of each copy. GLYPH<SM590000> Estimate the expected data change rate for FlashCopy types other than full copy. GLYPH<SM590000> Consider 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. GLYPH<SM590000> Define 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.  GLYPH<SM590000> Define 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. 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 Chapter 3. Planning  73