This section discusses the decisions that are necessary when planning to install a Dolphin Express Interconnect.
Depending on the application that will run on the cluster, the choice of Dolphin Express Interconnect equipped machines differs.
For best performance, all machines that run either NDB or MySQL server processes should be interconnected with the Dolphin Express interconnect. Although it is possible to only equip a subset of the machines with Dolphin Express, doing so will introduce new bottlenecks.
Machines that serve as application servers (clients sending queries to MySQL servers processes) typically have little benefit of being part of the interconnect. Please analyze the individual scenario for a definitive recommendation.
Machines that serve as MySQL frontends (like to run the MySQL Cluster management daemon ndb_mgmd) do not benefit from the Dolphin Express interconnect.
For small clusters of just two nodes, a number of possible approaches do exist:
Lowest cost: Connect the two nodes with one single-channel D351 adapter in each node.
Highest Performance: Connect the two nodes with one dual-channel D350 adapter in each node. This increases the bandwidth and adds redundancy at the same time.
Best Scalability: Use D352 adapters to connect the two nodes. The second dimension of this 2D adapter will not be used, but it is possible to expand this cluster in a fault-tolerant way.
When going to 4 nodes or more, the topology has to be the multi-dimensional torus, typically a 2D-torus built with D352 PCI-Express-to-SCI adapters. With D352, you can build clusters of any size up to a recommend maximum of 256 nodes. This is the typical topology for database clusters. For large clusters, it is possible to use a 3-D torus topology.
For 3- and 4-node cluster, special topologies without any fail-over delays can be built.
It is possible to install more than one interconnect fabric in a cluster by installing two adapters into each node. These interconnect fabrics work independently from each other and do very efficiently and transparently increase the bandwidth and throughput (a real factor of two for two fabrics) and add redundancy at the same time.
For some chipsets, PCI performance does not scale well, reducing the performance improvement of a second fabric. If this feature is important for you, contact Dolphin Support to make sure that you chose a chipset for which the full performance will be delivered.
Generally, nodes that are to be equipped with Dolphin Express interconnect adapters should be placed close to each other to keep cable lengths short. This reduces costs and allows for better routing of the cables.
For larger clusters, it makes sense to arrange the nodes in analogy to the the regular 2D-torus topology of the interconnect.
The maximum cable lenght is 7.5 meters.
The minimal cable bend radius is 25mm, which allows to place nodes 5cm apart. Connecting nodes or blades that are less than 5cm apart, like for 1U nodes in a 19" rack being a single rack unit apart, typically causes no problem as long as the effective bend radius is 25mm or more. Please contact Dolphin support for other cabling scenarios.
Once you have decided on the physical node placement, the cables should be ordered in the according lengths. The Dolphin sales representative will assist you in selecting the right cable lengths.