HP XP External Storage Software - Manual (Page 23)

Cross-subsystem Paths
A cross-subsystem path is a route from a local storage system port to an external storage system port. To
prepare for possible failures of the cable, the switch, or the channel processor, it is recommended that
you create redundant cross-subsystem paths. This redundancy allows you to continue performing the I/O
operations to the external volumes when you maintain the cable. You can set up to eight paths.
A group of redundant cross-subsystem paths is called a path group. In a path group, the cross-subsystem
path that has the highest priority is called the primary path. The cross-subsystem paths other than priority
path are called alternate paths.
• Setting of the path groups
A path group is automatically set when you map the external volume. You cannot set a new
path group by itself.
• Setting of cross-subsystem paths
Use fibre 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 the 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 which
show 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 ” on page 78.
• 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 ” on 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 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.
Note: 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 on page 24 illustrates an example of setting an alternate path. In Figure 5 on page 24, 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 the local storage system for the alternate path. Therefore, CL1 port and CL2 port are specified as
shown in Figure 5 on page 24.
XP24000 External Storage Software User’s Guide 23