Out of the FC SAN inter-connectors such as Hub, Switch and Directors, FC Switch and FC Directors are majorly used devices in any Storage Area Network. These FC switches and FC directors may be connected in a number of ways to form different fabric topologies. Each topology provides certain benefits.
Fibre Channel (FC) SAN Topologies
FC SAN offers 3 types of FC Switch topologies. They are
Single-Switch Topology
In a single-switch topology, the fabric consists of only a single switch. Both the compute systems and the storage systems are connected to the same switch. A key advantage of a single-switch fabric is that it does not need to use any switch port for ISLs. Therefore, every switch port is usable for compute system or storage system connectivity. Further, this topology helps eliminate FC frames travelling over the ISLs and consequently eliminates the ISL delays.
A typical implementation of a single-switch fabric would involve the deployment of an FC director. FC directors are high-end switches with a high port count. When additional switch ports are needed over time, new ports can be added via add-on line cards (blades) in spare slots available on the director chassis. To some extent, a bladed solution alleviates the port count scalability problem inherent in a single-switch topology.
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Mesh Topology
A mesh topology may be one of the two types: full mesh or partial mesh. In a full mesh, every switch is connected to every other switch in the topology. A full mesh topology may be appropriate when the number of switches involved is small. A typical deployment would involve up to four switches or directors, with each of them servicing highly localised compute-to-storage traffic. In a full mesh topology, a maximum of one ISL or hop is required for compute-to-storage traffic. However, with the increase in the number of switches, the number of switch ports used for ISL also increases. This reduces the available switch ports for node connectivity.
In a partial mesh topology, not all the switches are connected to every other switch. In this topology, several hops or ISLs may be required for the traffic to reach its destination. Partial mesh offers more scalability than full mesh topology. However, without proper placement of compute and storage systems, traffic management in a partial mesh fabric might be complicated and ISLs could become overloaded due to excessive traffic aggregation.
Core-edge Topology
The core-edge topology has two types of switch tiers: edge and core.
The edge tier is usually composed of switches and offers an inexpensive approach to adding more compute systems in a fabric. The edge-tier switches are not connected to each other. Each switch at the edge tier is attached to a switch at the core tier through ISLs.
The core tier is usually composed of directors that ensure high fabric availability. In addition, typically all traffic must either traverse this tier or terminate at this tier. In this configuration, all storage systems are connected to the core tier, enabling compute-to-storage traffic to traverse only one ISL. Compute systems that require high performance may be connected directly to the core tier and consequently avoid ISL delays.
The core-edge topology increases connectivity within the FC SAN while conserving the overall port utilization. It eliminates the need to connect edge switches to other edge switches over ISLs. Reduction of ISLs can greatly increase the number of node ports that can be connected to the fabric. If fabric expansion is required, then administrators would need to connect additional edge switches to the core. The core of the fabric is also extended by adding more switches or directors at the core tier. Based on the number of core-tier switches, this topology has different variations, such as single-core topology and dual-core topology. To transform a single-core topology to dual-core, new ISLs are created to connect each edge switch to the new core switch in the fabric.
Link Aggregation
Link aggregation combines two or more parallel ISLs into a single logical ISL, called a port-channel, yielding higher throughput than a single ISL could provide. For example, the aggregation of 10 ISLs into a single port-channel provides up to 160 Gb/s throughput assuming the bandwidth of an ISL is 16 Gb/s.
Link aggregation optimizes fabric performance by distributing network traffic across the shared bandwidth of all the ISLs in a port-channel. This allows the network traffic for a pair of node ports to flow through all the available ISLs in the port-channel rather than restricting the traffic to a specific, potentially congested ISL. The number of ISLs in a port channel can be scaled depending on application’s performance requirement.
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