text: **Note:**Volume mirroring does not create a second volume before you split copies. Volume 
  mirroring adds a second copy of the data under the same volume. Therefore, you end up 
  having one volume presented to the host with two copies of data that are connected to this 
  volume. Only splitting copies creates another volume, and then both volumes have only 
  one copy of the data. 
  
  Starting with V7.3 and the introduction of the new cache architecture, mirrored volume 
  performance was significantly improved. Now, lower cache is beneath the volume mirroring 
  layer, which means that both copies have their own cache. 
  
  This approach helps when you have copies of different types, for example generic and 
  compressed, because now both copies use its independent cache and performs its own read 
  prefetch. Destaging of the cache can now be done independently for each copy, so one copy 
  does not affect performance of a second copy. 
  
  Also, because the IBM Storwize destage algorithm is MDisk aware, it can tune or adapt the 
  destaging process, depending on mdisk type and usage, for each copy independently. 