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

2–Advanced Fabric Manager Capabilities
Link Speed Negotiation
To optimize the performance of the sweep, the SM can issue multiple concurrent
packets to a given device or the fabric. The number to issue at once must be
carefully balanced between the capabilities of the hardware and the goal of
making the sweep faster.
The SM allows configuration of sweep characteristics using a number of
parameters.
Optimized Fabric Programming
The InfiniBand standard allows the SM to route SMA packets using “Directed
Routed” or “LID Routed” mechanisms. Packets can also be routed with a mixture
of these two mechanisms. LID routed packets follow the normal routing used by
other traffic and are fully routed by hardware with low latency. Directed Routed
packets have the route explicitly specified in the packet, hop by hop, and allows
the SM to access components in the fabric prior to having the Switch Routing
tables fully programmed. Directed route packets are typically routed in switch
firmware and experience higher latency at each switch hop. Typically LID routed
SM packets incur much lower latency while traversing the fabric.
To optimize fabric programming in large fabrics, the QLogic Fabric Manager
supports “Early LID Routing”. This allows the SM to program routing tables in a
“cresting wave” approach such that the majority of fabric programming can use
LID routed packets or packets which are LID routed all the way to their final
hop.The net result of this advanced mechanism is fast programming of the fabric
and therefore more rapid fabric initialization and change handling. This feature
can be controlled using the SmaEnableLRDR and EhcaSmaEnableLRDR
parameters.
Scalable SMA Retries
The InfiniBand Architecture Specification defines that SMA packets are not flow
controlled. However to optimize fabric programming time, the QLogic Fabric
Manager can be configured to issue multiple SMA packets in parallel. This
approach can result in occasional packet loss which can have a negative effect on
the SMs performance. To allow for rapid recovery from such packet loss, while not
causing excessive retries to sluggish nodes, the QLogic Fabric Manager allows
for scalable retries with a increasing randomized backoff algorithm with each
attempt.
Link Speed Negotiation
When running with some DDR or QDR devices, links may only come up at SDR
speeds. To address this issue the QLogic Fabric Manager supports a Fabric
Manager based ability to negotiate link speeds and permit DDR or QDR operation
for such devices. This capability is enabled using the LinkSpeedOverride
option.
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