text: **11.6.9 Practical use of Metro Mirror**
  
  The master volume is the production volume, and updates to this copy are mirrored in real 
  time to the auxiliary volume. The contents of the auxiliary volume that existed when the 
  relationship was created are deleted. 
  
  **Switching copy direction:**The copy direction for a Metro Mirror relationship can be 
  switched so that the auxiliary volume becomes the master, and the master volume 
  becomes the auxiliary, which is similar to the FlashCopy restore option. However, although 
  the FlashCopy target volume can operate in read/write mode, the target volume of the 
  started remote copy is always in read-only mode. 
  
  While the Metro Mirror relationship is active, the auxiliary volume is not accessible for host 
  application write I/O at any time. The IBM Storwize V7000 allows read-only access to the 
  auxiliary volume when it contains a consistent image. IBM Storwize allows boot time 
  operating system discovery to complete without an error, so that any hosts at the secondary 
  site can be ready to start the applications with minimum delay, if required. 
  
  For example, many operating systems must read LBA zero to configure a logical unit. 
  Although read access is allowed at the auxiliary in practice, the data on the auxiliary volumes 
  cannot be read by a host because most operating systems write a “dirty bit” to the file system 
  when it is mounted. Because this write operation is not allowed on the auxiliary volume, the 
  volume cannot be mounted. 