If both owner paths and non-owner paths exist, HDLM will select the path to be used next from the owner paths, and then from the non-owner paths. To prevent performance in the entire system from deteriorating, HDLM does not perform load balancing between owner paths and non-owner paths. Therefore, if some of the owner paths become unusable due to, for example, a failure, load balancing will be performed among the remaining usable owner paths. When all owner paths cannot be used, load balancing is performed among the non-owner paths.
For the example in Figure 1, suppose that the paths (A) and (B) are owner paths, and the paths (C) and (D) are non-owner paths. When the LU is accessed, the load is balanced between the two paths A and B, which are both owner paths. When one of the paths (A) cannot be used, then the LU is accessed from the only other owner path (B). When both of the owner paths (A and B) cannot be used, the load is then balanced between two other, non-owner paths (C and D).
The following describes a case in which non-owner paths exist.
When a global-active device is used and the non-preferred path option is set:
When a global-active device is used, the default settings of the storage system specify that all paths are owner paths. Load balancing is performed on all paths that access the primary and secondary volumes of global-active device pairs.
However, if the primary site and the secondary site are far apart, I/O performance might be low for I/O issued to a site other than the location of the host. In such a case, specify the non-preferred path option on the storage system at the site where the host is not located. A path for which the non-preferred path option is specified is a non-owner path and cannot be used until all the owner paths become unavailable.
If you specify the non-preferred path option on the storage system when the HDLM device is already configured, execute the refresh operation of the HDLM command, or restart the host.