content_image/972102.jpg
After the initial synchronization is complete, you can change the copy direction (see Figure 6-12). The ability to change roles is used to facilitate disaster recovery. 

<img src='content_image/972102.jpg'>

Attention: When the direction of the relationship is changed, the roles of the volumes are altered. A consequence is that the read/write properties are also changed, meaning that the master volume takes on a secondary role and becomes read-only. 

## Consistency Groups 

A Consistency Group (CG) is a collection of relationships that can be treated as one entity. This technique is used to preserve write order consistency across a group of volumes that pertain to one application, for example, a database volume and a database log file volume. 

After a remote copy relationship is added into a Consistency Group, you cannot manage the relationship in isolation from the Consistency Group. So, for example, issuing a stoprcrelationship  command on the stand-alone volume would fail because the system knows that the relationship is part of a Consistency Group. 

Note the following points regarding Consistency Groups: 

GLYPH<SM590000> Each volume relationship can belong to only one Consistency Group. 

GLYPH<SM590000> Volume relationships can also be stand-alone, that is, not in any Consistency Group. 

GLYPH<SM590000> Consistency Groups can also be created and left empty, or can contain one or many relationships. 

GLYPH<SM590000> You can create up to 256 Consistency Groups on a system. 

GLYPH<SM590000> All volume relationships in a Consistency Group must have matching primary and secondary systems, but they do not need to share I/O groups. 

