Fabric Manager Users Guide, Version 6.1, Revision A - QLogic

2–Advanced Fabric Manager Capabilities
Fabric Unicast Routing
NOTE:
Since a Mesh/Torus makes use of multiple SLs and LIDs, high performance
applications must query the SA for a PathRecord for every Source/Dest pair.
The SL will vary per PathRecord and is computed by the SM Based on the
present topology of the fabric. To allow interoperability with non-ideal
application implementations (such as verbs based MPIs which do not
interact with the SA), the QLogic FM will configure the 1st SL and the 1st LID
on each Host Channel Adapter for the Up/Down route. This permits such
applications to run, albeit with potentially increased latency and congestion.
Adaptive Routing
See the Mesh/Torus section below for more information about DOR-Up/Down
routing.
A serious limitation of static routing is that it must be done before traffic begins to
flow, hence static routes are balanced using "best guesses" by the FM of potential
application traffic patterns. However, once applications start to run, those routes
may be non-ideal. Adaptive routing is a powerful capability of all QLogic 12000
series switches, which allows the switches to scalably adjust their routes while the
applications are running to balance the routes based on actual traffic patterns.
The QLogic adaptive routing solution is highly scalable because it allows the
Fabric Manager to provide the topology awareness and program the switches with
the rules for adaptive routing. Then the FM gets out of the way and permits the
switches to dynamically and rapidly adjust the routes based on actual traffic
patterns.
This approach ensures a scalable solution since as switches are added, each new
switch will work in parallel with others to dynamically adapt to traffic. This
approach avoids the FM becoming a bottleneck and scales very well.
Adaptive routing provides a few important capabilities and options:
1. Adaptive routing can rapidly route around fabric disruptions and lost ISLs.
When adaptive routing is enabled, this capability will automatically occur and
limit the amount of lag time between an ISL going down and the traffic being
redirected to alternate routes.
2. Adaptive routing can automatically balance and rebalance the fabric routes
based on traffic patterns. It has the unique ability to handle changing traffic
patterns which may occur due to different computational phases or the
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