Consistency groups

Consistency groups simplify management of related remote-copy relationships because copy actions are applied simultaneously to all relationship in the group. Consistency groups can be used to maintain consistent copies when data has dependencies across host applications or volumes.

When data for a single application is spread across more than one volume, a consistency group can manage the independent relationships for all volumes that are related to that application, thus ensuring consistent, synchronized data. When multiple applications run on different host systems, each application has data on different volumes, and these applications exchange data with each other. In these situations, specific rules exist as to how the relationships can be updated. These rules ensure that the set of secondary volumes contain usable data. The key property is that these relationships are consistent. Consistency groups help if consistent copies are created for all these uses.

Different types of relationships can belong to one consistency group; however, they do not have to belong to a consistency group. Relationships that are not part of a consistency group are called independent relationships. A consistency group can contain zero or more relationships. All relationships in a consistency group must have matching primary (master) and secondary (auxiliary) systems or sites. All relationships in a consistency group must also have the same copy direction and state. A copy type is automatically assigned to a consistency group when the first relationship is added to the consistency group. After the consistency group is assigned a copy type, only relationships of that copy type can be added to the consistency group. Global Mirror relationships with different cycling modes cannot belong to the same consistency group.

Remote-copy consistency group states

Table 1 describes the remote-copy consistency group states.

Table 1. Remote-copy consistency group states

Management GUI icon	State	Description
	Consistent synchronized	For consistency groups that have either Metro Mirror and Global Mirror (without change volumes) relationships, replication between the primary system and target system has started. The target volume contains consistent data. It is possible to complete data recovery operations and switch the direction of the remote copy. The two copies differ only in the data that is in transit between the two systems.
	Consistent copying	Indicates that the data is being copied between the primary volume and the secondary volume. The two copies differ only in the data that is in transit between the two systems. The secondary volume contains consistent data. It is possible to complete data recovery operations and switch the direction of the remote copy.
	Consistent stopped	Indicates that the copy between the primary and secondary volumes is stopped, but secondary volume contains consistent data. Data recovery is possible but switching the copy direction is not possible without potentially losing data on the primary system.
	Consistent disconnected	Indicates that the connection between the primary volumes and the secondary volumes is unavailable, but the data on the secondary volumes is consistent. The volumes in this half of the consistency group are all operating in the secondary role and cannot accept read or write I/O operations. It is possible to recover data from the secondary volume in this state, but you cannot switch the direction of the copy without data loss on the primary volume.
	Inconsistent copying	Indicates that data is being copied between the primary volumes and the secondary volumes and data is not consistent on secondary volumes. The primary volumes are accessible for read and write I/O operations, but the secondary volumes are not accessible for either operation. Usually this state occurs after the consistency group is started from the Inconsistent stopped state. It also occurs when the force option is used on a start operation when the group is in the Idling or Consistent stopped state. In this state recovering data or switching the copy direction is not possible.
	Inconsistent stopped	Indicates that the data between the primary volumes and the secondary volumes is not consistent. The primary volumes are accessible for read and write I/O operations, but the secondary volumes are not accessible for either operation. A copy process must be started to make the secondary volumes consistent. In this state, you cannot recover data or switch the copy direction.
	Inconsistent disconnected	Indicates that the auxiliary system cannot contact the primary system. The data between the secondary volume and the primary volume is inconsistent. It is not possible to complete data recovery operations or switch the direction of the remote copy until data is consistent. This status can occur if the primary system has experienced an unplanned outage.
	Idling	Indicates that the consistency group is stopped, but both systems are currently reading and writing data to the volumes in the remote copy. If the remote copy is restarted, data loss can occur.
	Idling disconnected	Indicates that the systems in the consistency group cannot connect to one another. The data between the secondary volume and the primary volume is inconsistent. It is not possible to complete data recovery operations or switch the direction of the remote copy until data is consistent. This state can occur if the primary system has experienced an unplanned outage.
	Empty	The consistency group does not contain any relationships.