Chapter 11. Advanced Copy Services 447
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.
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.