Functions of a Consistency Group
In medium- and large-sized database applications, data, logs, and change records are stored on associated LUNs of storage systems. The data correlation between those LUNs is ensured by upper-layer host services at the primary site. When data is replicated to the secondary site, the data correlation must be maintained. Otherwise, the data at the secondary site cannot be used to recover services. To maintain the data correlation, you can add the remote replication pairs of those LUNs to the same consistency group. This section compares storage systems running a consistency group with storage systems not running a consistency group to show you how a consistency group ensures service continuity.
Users can perform synchronization, splitting, and primary/secondary switchovers for a single remote replication pair or perform these operations for multiple remote replication pairs using a consistency group. Note the following when using a consistency group:
- Remote replication pairs can be added to a consistency group only on the primary storage system. In addition, secondary LUNs in all remote replication pairs must reside in the same remote storage system.
- LUNs in different remote replication pairs in a consistency group can belong to different working controllers.
- Remote replication pairs in one consistency group must work in the same remote replication mode.
The following compares remote replication without using a consistency group with remote replication using a consistency group to show how a consistency group is used to prevent data from becoming unusable.
Remote Replication Without Using a Consistency Group
Associated LUNs that are not added to a consistency group are at a risk for becoming unavailable, as shown in Figure 1-6.
Remote Replication Using a Consistency Group
If associated LUNs are added to a consistency group, data unavailability is prevented, as shown in Figure 1-7.
