Redundant Link Versus MultiPath

Resolving The Problem

Given the following example:



There are two MultiPath paths, because there are two fibre adapter cards on the system that connect to the same LUN. For each of the two fibre cards on the System i, there are two Redundant Links to the same LUN. For each MultiPath, only one Redundant Link will be active. So for fibre card 1, the system will choose and activate one of the following two Redundant Links:

Fibre Card 1 (MultiPath 1): Redundant Link A: Switch 1, Kona 1 Redundant Link B: Switch 1, Kona 2

It is the same for fibre card 2:

Fibre Card 2 (MultiPath 2): Redundant Link C: Switch 2, Kona 1 Redundant Link D: Switch 2, Kona 2

If the system activates Link A for Fibre Card 1, and if Link C is activated for Fibre Card 2, then both MultiPaths would be using the same Kona. In this case, if Kona 1 stops working, then the connection to the LUN is lost for both paths (Fibre Card 1 and 2). After a link completely fails (loss of light), the system will select another Link to try and recover. However, by this time some jobs, applications, network servers, and so on, may time out and fail before another redundant link is activated.

If, instead, Link D is activated for Fibre Card 2, then Multipath 1 would use Fibre Card 1, Switch 1, and Kona 1, while Multipath 2 would use Fibre Card 2, Switch 2, and Kona 2. In this case, if Switch 1 or Kona 1 stopped working, the system would continue to access the LUN through Multipath 2 with no interruption.

Since the user has no control over which redundant link the system chooses to use, the mix of multipath and redundant links could end up creating a single point of failure and a potential outage instead of providing the high availability solution they were looking for. It is usually best to use a redundant link-only configuration, or a multipath-only configuration.

Q&A:

Q1 - What happens if MultiPath is using the same Kona for both paths because of the Redundant Links chosen by the system?
A1 - If there is a full failure of the links (loss of light), both paths will fail but will recover as soon as the system is able to activate redundant links through the other Kona.

Q2 - Does MultiPath have more priority than Redundant Links?
A2 - MultiPath and Redundant Links are independent functions (however, they can be used together). MultiPath uses all active paths to the target LUN to improve performance, throughput, and redundancy. Redundant Links only supply backup redundancy in case the current active link fails. When an i5/OS IOA sees two or more active ports on the same DS6K, it picks just one of those ports and uses it until the actual connection fails (for example, loss of light). All other ports remain unused by that IOA until the failover is required. This is the definition of Redundant Links.

An example of Redundant Links with no MultiPath follows:



An example of MultiPath without Redundant Links follows:



Q3 - Are Redundant Links automatic or can they be turned off?
A3 - They are automatic, based on the SAN configuration (as shown in the examples above). They can only be turned off by removing the redundant connections to the DS6000 (in other words, removing cables, disabling ports, and so on)

Q4 - Do the slowest connections determine data transfer performance?
A4 - In Redundant Link configurations, the algorithms for choosing used ports are common across all IOAs While not guaranteed, all IOAs are likely to choose the same common port; this by itself can cause unwanted performance issues, all IOAs using the same DS6000 port. All paths in a Multipath configuration to a LUN are utilized equally. While this is generally a performance gain, the overall performance can be held back due to a single slow path.