text: **11.6.6 Synchronous remote copy**
  
  Metro Mirror is a fully synchronous remote copy technique that ensures that writes are 
  committed at both the master and auxiliary volumes before write completion is acknowledged 
  to the host, but only if writes to the auxiliary volumes are possible. 
  
  Events, such as a loss of connectivity between systems, can cause mirrored writes from the 
  master volume and the auxiliary volume to fail. In that case, Metro Mirror suspends writes to 
  the auxiliary volume and enables I/O to the master volume to continue to avoid affecting the 
  operation of the master volumes. 
  
  Figure 11-88 on page 522 shows how a write to the master volume is mirrored to the cache of 
  the auxiliary volume before an acknowledgment of the write is sent back to the host that 
  issued the write. This process ensures that the auxiliary is synchronized in real time if it is 
  needed in a failover situation. 