text: Figure 11-10 New cache architecture 
  
  This copy-on-write process introduces significant latency into write operations. To isolate the 
  active application from this extra latency, the FlashCopy indirection layer is placed logically 
  between upper and lower cache. Therefore, the extra latency that is introduced by the 
  copy-on-write process is encountered only by the internal cache operations and not by the 
  application. 
  
  The two-level cache provides more performance improvements to the FlashCopy mechanism. 
  Because the FlashCopy layer is above lower cache in the IBM Spectrum Virtualize software 
  stack, it can benefit from read prefetching and coalescing writes to back-end storage. 
  Preparing FlashCopy benefits from the two-level cache because upper cache write data does 
  not have to go directly to back-end storage, but to lower cache layer instead. 
  
  **11.1.8 Background Copy Rate**
  
  The Background Copy Rate is a property of a FlashCopy mapping. A grain copy from the 
  source to the target can occur when triggered by a write operation on the source or target 
  volume, or when background copy is enabled. With background copy enabled, the target 
  volume eventually becomes a clone of the source volume at the time the mapping was started 
  (T0). When the copy is completed, the mapping can be removed between the two volumes 
  and you can end up with two independent volumes. 