Chapter 19. Host multipathing 275When multiple paths to a LUN are available, a consistent method of the use of those pathsmust be determined. This method is called the load balance policy. The following fivestandard policies in Windows Server 2008 apply to multiconnection sessions and MPIO.Other operating systems can implement different load balancing policies: Failover only: Only one path is active at a time, and alternative paths are reserved for pathfailure. Round robin: I/O operations are sent down each path in turn. Round robin with subset: Some paths are used as in round robin, while the remainingpaths act as failover only. Least queue depth: I/O is sent down the path with the fewest outstanding I/Os. Weighted paths: Each path is given a weight that identifies its priority, with the lowestnumber having the highest priority.19.2 Multipathing software optionsThe multipathing solution can be provided by the following resources: Third-party vendors:– Storage vendors provide support for their own storage arrays, such as the IBM DataONTAP DSM for Windows. These solutions often are specific to the particular vendor’sequipment.– Independent third-party vendors offer heterogeneous host and storage support, suchas Symantec and Veritas DMP. Operating system vendors as part of the operating system, for example, WindowsMSDSM, Solaris MPxIO, AIX MPIO, Linux Device-Mapper Multipath, HP-UX PVLinks,VMware ESX Server NMP19.2.1 Third-party multipathing solutionThird-party multipathing solutions are provided by storage vendors or independent softwarevendors, such as Symantec. The advantage of the use of multipathing solutions that areprovided by storage vendors is that it provides a unified management interface for alloperating systems. This unified interface makes administering a heterogeneous hostenvironment easier. In addition, storage vendors know their array the best, and so themultipathing solution provided by the storage array vendor can provide optimal performance.Conversely, the multipathing solutions that are provided by storage vendors have the followingdisadvantages: Most of these solutions come with fee-based software licenses, and often require ongoinglicense maintenance costs. The solutions that are provided by storage vendors lock the customer into a single storageplatform. Some of these solutions do have support for other storage arrays, but theremight be long qualification or support delays. These solutions usually do not interoperate well with multipathing solutions from otherstorage vendors that must be installed on the same server.The multipathing solutions that are provided by Symantec also require fee-based softwarelicenses. However, these solutions provide support for heterogeneous storage andheterogeneous host OS.
