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Storage Mastery: Fibre Channel

Editor’s note: This is the third post in a blog series by CBT Nuggets trainer Anthony Sequeira that will cover IT storage technology.

mastering_storage_series_EMAIL2The traditional Storage Area Network (SAN) tends to use Fibre Channel as the network connectivity technology. This is certainly not the only option, but is still one of the most popular today. This post examines this technology (Fibre Channel) in detail — and serves as a primer to understanding many subsequent technologies such as converged I/O (Fibre Channel over Ethernet).

Fibre Channel (FC) is a high-speed network technology that runs over optical fiber cables and serial copper cables. In the SAN, optical fiber cables are preferred for front-end connectivity, while copper cables are preferred for backend disk connectivity. FC technology is extremely scalable. The theoretical maximum for nodes is approximately 15 million.

The architecture of Fibre Channel actually supports three basic interconnectivity options:

  • Point-to-Point (P2P)
  • Arbitrated Loop (FC-AL)
  • Fabric Connect

As you might guess, the P2P is the simplest FC configuration. It features two devices connected directly to each other. For example, a server connected directly to a storage array. As you also might guess, this interconnectivity option does not scale.

The FC-AL configuration features devices that are connected to a shared loop. This technology has the logical characteristics of a token ring topology, and the physical characteristics of a star topology. In this environment, devices on the loop must “arbitrate” in order to communicate on the loop. Interestingly, FC-AL can be implemented without any interconnecting device, but in actual practice, hubs were often used. Like the P2P design, scalability was the major limitation with the FC-AL approach. Specifically, the issues were:

  • Shared bandwidth
  • 8-bit addressing supporting up to 127 devices on the loop
  • Adding or removing devices results in loop re-initialization and a pause in loop traffic

While FC-AL in the SAN became legacy, we still see FC-AL in place in storage. For example, this is popular in storage arrays still for the internal FC connectivity. A great example is the NetApp Fabric Attached Storage (FAS) devices.

The Fabric Connect option is the most modern and implemented technology today. The term fabric refers to a logical space in which all nodes communicate with one another in the fabric. Each port in the fabric has a unique 24-bit Fibre Channel address for communication. The topology is often described by the number of tiers it possesses. These tiers are defined by the number of FC switches that span two nodes that are farthest from each other in the SAN.

Ports are the basic building blocks of the Fibre Channel network infrastructure. Ports within the Fabric Connect can be of the following types, and this is just to name a few:

  • N_port – end point in the fabric – also known as a node port; in actual practice this port is typically a host port (HBA) or a storage array port
  • NL_port – a node port in the older FC-AL interconnectivity design; this port is also known as a good loop port
  • E_port – a port that connects two FC switches; this port is known as an expansion port
  • F_port – a port in the FC switch that connects to an N-port; this port is also known as a fabric port
  • FL_port – a fabric port that participates in the FC-AL; it connects to the NL_port
  • G_port – a generic port that can operate as an E_port or an F_port; this port typically determines its role automatically during switch initialization

Until next time, I hope you found this informative, and I’d like to thank you for reading.

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