244 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
Figure 7-1   Data flow for write I/O processing in a mirrored volume 
As shown in Figure 7-1, the writes are sent by the host to the preferred node for the volume 
(1). Then, the data is mirrored to the cache of the partner node in the I/O Group (2), and 
acknowledgment of the write operation is sent to the host (3). The preferred node then 
destages the written data to all volume copies (4). The example that is shown in Figure 7-1 
shows a case with a destage to a mirrored volume; that is, one with two physical data copies.
With V7.3, the cache architecture changed from an upper-cache design to a two-layer cache 
design. With this change, the data is only written once, and is then directly destaged from the 
controller to the disk system. 
Figure 7-2 shows the data flow in a stretched environment.
Figure 7-2   Design of an Enhanced Stretched Cluster
UCA UCA
Site1
Preferred Node IO group Node Pair
Write Data with location
Site2
Non-Preferred Node
Mirror
Copy 1
Preferred
Storage at Site 1 Storage at Site 2
Copy 2
Non preferred
LCA1
LCA 2
Destage
Reply with location
LCA1
LCA 2
Token write data message with location
Copy 1 Non preferred
Copy 2 
Preferred
Destage
Data is replicated
once across ISL.