text: 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. 