Consistency groups

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For Global Mirror, Metro Global Mirror, and the multi-target session types, Copy Services Manager manages the consistency of dependent writes by creating a consistent point-in-time copy across multiple volumes or storage systems. A consistency group is a set of target volumes in a session that have been updated to preserve write order and are therefore recoverable.

Copy Services Manager automatically assigns an ID to each consistency group on the storage systems. However, you can choose to assign a custom name to a storage system consistency group. A custom-defined name allows you to associate multiple sessions with a specific storage system consistency group.

Data exposure is the period when data is written to the storage at the primary site until data is replicated to storage at the secondary site. Data exposure is influenced by factors such as:
  • Requested consistency-group interval time
  • Type of storage systems
  • Physical distance between the storage systems
  • Available bandwidth of the data link
  • Input/output (I/O) load on the storage systems

To manage data exposure, you can change the consistency group interval time. The consistency group time interval specifies how often a Global Mirror and Metro Global Mirror session attempts to form a consistency group. When you reduce this value, it might be possible to reduce the data exposure of the session. A smaller interval value causes the session to attempt to create consistency groups more frequently, which might also increase the processing load and message-traffic load on the storage systems.