11.1.7 Interaction with cache 11.1.8 Background Copy Rate IBM Spectrum Virtualize-based systems have their cache divided into upper and lower cache.  Upper cache serves mostly as write cache and hides the write latency from the hosts and  application. Lower cache is a read/write cache and optimizes I/O to and from disks.  Figure 11-10 shows the IBM Spectrum Virtualize 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. 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. Chapter 11. Advanced Copy Services  447 Figure 11-10 New cache architecture