Multipathing Software Options; Third-Party Multipathing Solution - IBM N Series Hardware Manual

When multiple paths to a LUN are available, a consistent method of using those paths needs
to be determined. This method is called the load balance policy. There are five standard
policies in Windows Server 2008 that 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 alternate paths are reserved for path
failure.
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 remaining
paths 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 lowest
number having the highest priority.

19.2 Multipathing software options

The multipathing solution can be provided by:
Third-party vendors:
– Storage vendors provide support for their own storage arrays such as the IBM Data
ONTAP DSM for Windows. These solutions are generally specific to the particular
vendor's equipment.
– Independent third-party vendors offer heterogeneous host and storage support such
as Symantec and Veritas DMP.
Operating system vendors as part of the operating system:
– For example, Windows MSDSM, Solaris MPxIO, AIX MPIO, Linux Device-Mapper
Multipath, HP-UX PVLinks, VMware ESX Server NMP

19.2.1 Third-party multipathing solution

As mentioned previously, third-party multipathing solutions are provided either by storage
vendors or by independent software vendors such as Symantec. The advantage of using
multipathing solutions provided by storage vendors is that it provides a unified management
interface for all operating systems. This unified interface makes administering a
heterogeneous host environment easier. In addition, storage vendors know their array the
best, and so the multipathing solution provided by the storage array vendor can provide
optimal performance. Conversely, the multipathing solutions provided by storage vendors
have the following disadvantages:
Most of these solutions come with fee-based software licenses, and typically require
ongoing license maintenance costs.
The solutions provided by storage vendors lock the customer into a single storage
platform. Some of these solutions do have support for other storage arrays, but there
might be long qualification/support delays.
These solutions usually do not interoperate well with multipathing solutions from other
storage vendors that must be installed on the same server.
The multipathing solutions provided by Symantec also require fee-based software licenses.
However, these solutions provide support for heterogeneous storage and heterogeneous
host OS.
Chapter 19. Host multipathing
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