If two or more external volumes use the same redundant cross-subsystem paths, these external volumes can be a group. This group is called a path group. In the redundant cross-subsystem paths, the cross-subsystem path that has the highest priority is called the primary path. The cross-subsystem paths other than the primary path are called alternate paths.

•Setting of the path groups

A path group is automatically set when you map the external volume or when you change a path group by using spreadsheets. You cannot set a new path group by itself. For more information about changing a path group by using spreadsheets, see “Changing Path Group” (page 115).

•Setting of cross-subsystem paths

Use fiber cables to establish multiple paths between the external storage system and the local storage system. At this time, connect to the external storage system from a different cluster port of the local storage system.

If multiple paths are established between the two storage systems, the starting points of the paths (that is, external ports of the local storage system) and the ending points (that is, WWNs showing the ports of the external storage system) will be displayed in a dialog box when you map an external volume. In this dialog box, you can set cross-subsystem paths by selecting the starting points and the ending points of the paths according to the actual cable connections. For details on how to set cross-subsystem paths, see “Configure Cross-subsystem Paths Dialog Box” (page 79).

•Setting of redundant cross-subsystem paths

You can set redundant cross-subsystem paths (add alternate paths) when you set the cross-subsystem paths. You can also add an alternate path or change the priority after completing the mapping of the external volume (see “Setting the Cross-subsystem Paths” (page 78)).

Path Mode

Path mode is either Single mode or Multi mode, depending on the connected external storage system.

•In the Single mode, only the path with the highest priority (primary path) is used to execute the I/O to the external volume. When an error occurs in the primary path, the path with the second highest priority is used.

•In the Multi mode, all of the set paths are used at the same time. The multiple paths are used to execute the I/Os to the external volume thus distributing the work load (round-robin processing).

For example, when a volume in the external storage system with the path mode of the Single mode is mapped as an internal volume using XP External Storage, the host I/O operations to the external volume are enabled using the primary path set in the mapping operation. The path is automatically switched to the alternate path when the primary path set in mapping operation cannot be used due to, for instance, maintenance operation in the storage system, or a failure in the channel processor. Because the path is switched to the alternate path, the I/O operation to the external volume continues even though an error occurred in the original path.

When the primary path cannot be used for three minutes continuously, the path is switched to the alternate path.

Examples of Alternate Paths

Figure 5 (page 20) illustrates an example of setting an alternate path. External storage system ports, WWN A and WWN B, are connected to CL1-A and CL2-A respectively, which are set to the external ports in the local storage system. You need to specify the port of a different cluster in

Configuring XP External Storage 19