A consistency group is a container for mappings. You can add many mappings to a consistency group.
The consistency group is specified when the mapping is created. You can also change the consistency group later. When you use a consistency group, you prepare and start that group instead of the individual mappings. This ensures that a consistent copy is made of all the source virtual disks (VDisks). Mappings to control at an individual level are known as stand-alone mappings. Do not place stand-alone mappings into a consistency group because they become controlled as part of that consistency group.
When you copy data from one VDisk to another, the data might not include all that you need to enable you to use the copy. Many applications have data that spans multiple VDisks and requires that data integrity is preserved across VDisks. For example, the logs for a particular database usually reside on a different VDisk than the VDisk that contains the data.
Consistency groups address the problem when applications have related data that spans multiple VDisks. In this situation, FlashCopy® operations must be performed in a way that preserves data integrity across the multiple VDisks. One requirement for preserving the integrity of data being written is to ensure that dependent writes are run in the intended sequence of the application.
You can set the autodelete attribute for FlashCopy consistency groups. If this attribute is set to on, the consistency group is automatically deleted when the last mapping in the group is deleted or moved out of the consistency group.
Consistency groups aggregate FlashCopy mappings, not the VDisks themselves. Therefore, a source VDisk with multiple FlashCopy mappings can be in different consistency groups. If a VDisk is the source VDisk for several FlashCopy mappings that are in the same consistency group, multiple identical copies of the source VDisk are created when the consistency group is started.
To create a FlashCopy mapping in a consistency group, the source VDisk cannot be the target of a mapping in the same consistency group. In addition, the target VDisk cannot be the source of another FlashCopy mapping in the same consistency group. You cannot move a FlashCopy mapping into a consistency group that contains similar FlashCopy mappings in the cascade.