content_image/971765.jpg
## 11.1.7 Interaction with cache 

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. 

<img src='content_image/971765.jpg'>

This copy-on-write process introduces significant latency into write operations. To isolate the active application from this additional 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 backend storage. Preparing FlashCopy benefits from the two-level cache because upper cache write data does not have to go directly to backend 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. 

