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

2–Advanced Fabric Manager Capabilities
Mesh/Torus Topology Support
Figure 2-7. 2D Mesh with 1 Toroidal Dimension Fabric Example
Disruption Handling
A very important part of any routing algorithm is its ability to handle fabric
disruptions. In High Performance Computing (HPC), fabrics design changes are
rare, however fabric disruptions are quite common. Disruptions can be the result
of simple maintenance operations, such as replacing a cable or rebooting a
switch. Disruptions can also be the result of more wide-spread events, such as
loss-of-power for a significant part of the fabric.
Unlike Fat Tree topologies, Mesh fabrics have the potential for numerous credit
loops. It is important that the routing algorithm ensure the fabric routing is
deadlock free while still providing optimal latency. Also the routing algorithm must
be able to properly route the fabric within the constraints of the hardware (number
of VLs/SLs) while handling a potentially large number of fabric disruptions.
The QLogic Fabric Manager’s support for dor-updown provides an optimized
solution to this complex challenge. It can optimally route the fabric deadlock free,
and it can handle a large number of disruptions, all while staying within the
constraints of existing hardware. As such it is an ideal solution and provides better
operation of the fabric than some of the alternatives (For example, Lash, Lash-Tor,
Segment Based Routing, and so on), all of which either compromise performance
or are unable to handle more than a few disruptions within the capabilities of the
hardware.
The updown spanning tree and the multicast spanning tree match so there will not
be conflicts between the updown unicast traffic and the multicast traffic. Matching
the two spanning trees also prevents credit loops and prevents failover speedups.
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