Connection failures can occur if an adapter, cable, or switch in the path between the server and the storage device fails for any reason. For example, if you have two adapters in a server pointing to the same device and the adapter in the primary path fails, the connection between the server and the device fails. The multipath connection feature automatically fails over to the secondary path, using the second adapter and its related path to continue communications. The failover is transparent to users and applications.
To achieve the desired availability, your storage solution can implement one or more host bus adapters in the server and multiple adapters in a SAN. These adapters can interconnect with cables from the server to the device, with cables from each set of adapters to the same switch, or with cables from each set of adapters homed to different switches in your SAN. NSS multipathing is interoperable with third-party storage products such as SANs and hardware RAIDs.
IMPORTANT:The NSS Media Manager supports fault tolerance with the NSS multipath solution; it does not support load balancing across the multiple paths. Some third-party multipathing systems (such as EMC PowerPath) support load balancing across the multiple connection paths. When you use third-party multipath solutions, make sure you do not turn on NSS MPIO.
NSS automatically identifies multiple connection paths and randomly selects one path to serve as the primary, active interconnect. You can specify path priorities to control which path serves as the primary path and to set up the failover sequence of the alternate available paths. NSS multipath support enables you to dynamically manage multiple paths according to the priorities you set.
In the event of a failure in the active interconnect, NSS automatically and dynamically recovers to another available failover path. The path with the highest priority of the paths that are available (up) becomes the primary path. The paths not selected remain as failover device paths for the new primary path.
Set a path’s priority to determine its position in the failover sequence of the alternate available paths. The default priority is zero (0), which is no priority. Assign a value between 1 (highest priority) to 4 million (lowest priority).
The NLM™ programs needed to take advantage of multipath support (mm.nlm and nwpa.nlm) are installed and loaded when you install NetWare® 6 and later. You can manage multipathing using the browser-based iManager Storage plug-in, NSS Management Utility (NSSMU), or server console commands.