Change volumes

Change volumes are used in Global Mirror relationships where cycling mode is set to Multiple. Change volumes can also be used on other types of remote-copy relationships to automatically maintain a consistent image of a secondary volume when a relationship is being resynchronized. Change volumes create periodic point-in-time-copies of the source volumes and replicate them to the secondary site. Using change volumes lowers bandwidth requirements by only addressing the average throughput and not the peak.

If you are using Global Mirror with cycling mode set to Multiple, the copying process is similar to Metro Mirror and standard Global Mirror. Change volumes must be configured for both the primary and secondary volumes in each relationship. A copy is taken of the primary volume in the relationship using the change volume that is specified when the Global Mirror relationship with change volumes is created. The background copy process reads data from the stable and consistent change volume, copying the data to the secondary volume in the relationship. Copy-on-write technology is used to maintain the consistent image of the primary volume for the background copy process to read. The changes that took place while the background copy process was active are also tracked. The change volume for the secondary volume can also be used to maintain a consistent image of the secondary volume while the background copy process is active.

When the background copy process completes, the change volume at the primary merges the record of changes that were received by the Global Mirror relationship. If the process completes abnormally, such as when the relationship is stopped, then the Global Mirror relationship becomes Inconsistent Stopped or Consistent Stopped.

Change volumes are used with a volume with copies on two sites using HyperSwap topology. Similar to change copies that are used in Global Mirror relationships, change volumes for HyperSwap are used to create point-in-time replicas of volume copies on separate sites. Change volumes maintain a consistent and stable record of changes and make resynchronization of data quicker.