text: **Copy services topology**
  One or more clusters can participate in a copy services relationship. One typical and simple 
  use case is disaster recovery, where one site is active and another performs only a disaster 
  recovery function. In such a case, the solution topology is simple, with one cluster per site and 
  uniform replication direction for all volumes. However, multiple other topologies are possible 
  that allow you to design a solution that optimally fits your set of requirements. For more 
  information about examples of valid relationships between systems, search for “Metro Mirror 
  and Global Mirror partnerships” at this IBM Knowledge Center web page. 
  
  **Global Mirror versus Metro Mirror**
  Decide which type of copy service you are going use. This decision should be requirements 
  driven. Metro Mirror allows you to prevent any data loss during a system failure, but has more 
  stringent requirements, especially regarding intercluster link bandwidth and latency, and 
  remote site storage performance. Also, it incurs performance penalty because writes are not 
  confirmed to host until data reception confirmation is received from the remote site. 
  
  Because of finite data transfer speeds, this remote write penalty grows with the distance 
  between the sites. A point-to-point dark fiber-based link typically incurs a round-trip latency of 
  1 ms per 100 km (62.13 miles). Other technologies provide longer round-trip latencies. 
  Inter-site link latency defines the maximum distance for any performance level. 