text: **11.6.13 Using Change Volumes with Global Mirror**
  
  Global Mirror is designed to achieve an RPO as low as possible so that data is as up-to-date 
  as possible. This design places several strict requirements on your infrastructure. In certain 
  situations with low network link quality, congested hosts, or overloaded hosts, you might be 
  affected by multiple 1920 congestion errors. 
  
  Congestion errors happen in the following primary situations: 
  
  (cid:2) At the source site through the host or network 
  (cid:2) In the network link or network path 
  (cid:2) At the target site through the host or network 
  
  Global Mirror has functionality that is designed to address the following conditions, which 
  might negatively affect certain Global Mirror implementations: 
  
  (cid:2) The estimation of the bandwidth requirements tends to be complex. 
  (cid:2) Ensuring that the latency and bandwidth requirements can be met is often difficult. 
  (cid:2) Congested hosts on the source or target site can cause disruption. 
  (cid:2) Congested network links can cause disruption with only intermittent peaks. 
  
  To address these issues,*Change Volumes*were added as an option for Global Mirror 
  relationships. Change volumes use the FlashCopy functionality, but they cannot be 
  manipulated as FlashCopy volumes because they are for a special purpose only. Change 
  volumes replicate point-in-time images on a cycling period. The default is 300 seconds. 