9.1.1 System Structure

The following is a description of basic system structure required to use the Storage Cluster function. The example of system structure is shown in "Figure 9.1 Storage Cluster Function: Example of System Structure". Data is transferred between the Primary Storage and the Secondary Storage in the synchronization mode using the REC path.

Figure 9.1 Storage Cluster Function: Example of System Structure

For the Primary Storage and Secondary Storage that configure Storage Cluster, the REC path is used to confirm that the other storage device exists. If the REC path is disconnected, because the existence of other storage device cannot be confirmed, a failover may be performed due to a misjudgment even though both the Primary Storage and Secondary Storage are operating. To prevent this misjudgment, configure a server to monitor the Primary Storage and Secondary Storage. In the Storage Cluster function, this server is called "Storage Cluster Controller".

Point

To perform an automatic failover operation to the Secondary Storage when an abnormality is detected in the Primary Storage, install the Storage Cluster Controller as shown in "Figure 9.1 Storage Cluster Function: Example of System Structure" and then connect the Storage Cluster Controller to the Primary Storage and the Secondary Storage with the management LAN.
Configure the Storage Cluster function on the Management Server. Connect the Management Server to the Primary Storage and the Secondary Storage with the management LAN. If a Storage Cluster Controller is installed, connect both the Management Server and the Storage Cluster Controller with the management LAN.

See

Refer to "9.1.9 Confirming the Existence Using the Storage Cluster Controller and the Failover Execution Trigger" for a role of the Storage Cluster Controller.

REC Path

REC path is the path in which the Storage Cluster function transfers data between the Primary Storage volumes and the Secondary Storage volumes.

An ETERNUS Disk storage system manages copy sessions used in the Storage Cluster function and Advanced Copy sessions separately. Since the ETERNUS Disk storage system automatically controls copy sessions used in the Storage Cluster function, it is not required to configure copy sessions and copy groups in this product.

Point

When an REC route temporary fault (communication break) occurs, a differential copy is executed after the REC route recovers, and the data is automatically recovered in the equivalent state. As failover does not occur during the period until the REC route is recovered, we recommend a redundant REC route configuration.

Split Mode

With the Storage Cluster function, the operation mode can be specified for the Primary Storage volume when the REC path is disconnected. This operation mode is called "split mode".
One of the following can be specified for the split mode.

Split mode	Explanation
Read/Write (default)	This mode gives priority to business continuity and continues writing data to volumes in the Primary Storage. In this case, data is written only on the volumes in the Primary Storage, causing the data to be nonequivalent to data in the Secondary Storage.
Read	This mode gives priority to maintenance of data equivalent state and inhibits writing data to the volumes in the Primary Storage.