text: **Read I/O operations data flow**
  If the volume is mirrored (that is, two copies of the volume exist), one copy is known as the 
  *primary copy*. If the primary is available and synchronized, reads from the volume are 
  directed to that copy. The user selects which copy is primary when the volume is created, but 
  can change this setting at any time. In the management GUI, an asterisk indicates the 
  primary copy of the mirrored volume. Placing the primary copy on a high-performance 
  controller maximizes the read performance of the volume 
  
  For non-mirrored volumes, only one volume copy exists; therefore, no choice exists for read 
  source, and all reads are directed to the single volume copy. 
  
  **Write I/O operations data flow**
  For write I/O operations to a mirrored volume, the host sends the I/O request to the preferred 
  node, which is responsible for destaging the data from cache. Figure 7-1 on page 244 shows 
  the data flow for this scenario. 