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Thursday, January 17, 2013

Storage 101 - Part 2

Storage 101 - Part 1

This article continues the series by discussing some of the hardware that is used in storage area networks. Specifically, this article will focus primarily on Fibre Channel host bus adapters.


In the first part of this article series, I explained that a SAN is different from NAS and from other types of networked storage. In this article, I want to discuss some of the hardware components that make up a typical SAN.

The Basic Architecture

SAN components generally fall into three categories – hosts, fabric, and storage. These three generic component types make up the SAN architecture. I will be discussing each of these component categories individually.


Hosts aren’t technically a SAN component since they are actually the servers that connect to the SAN rather than the SAN itself. Even so, the hosts do contain hardware that makes this connectivity possible.

The hardware that is present in a host varies depending upon how the host connects to the SAN. If the host connects to the SAN using the iSCSI protocol then the host will establish this connectivity through a standard network adapter. This might be a gigabit network adapter, a ten gigabit network adapter, or even a series of teamed (bonded) NICs.

Fibre Channel

Fibre Channel connectivity is a bit more interesting. Hosts connect to a Fibre Channel SAN by using a host bus adapter. A host bus adapter is similar to a network card in that it gets installed into a slot within the server and provides connectivity to the SAN. The primary difference between host bus adapters and Ethernet adapters is that host bus adapters are designed to work with fiber optic cables rather than Ethernet cables. Another difference is that unlike Ethernet, Fibre Channel does not use the TCP/IP protocol or carry Ethernet packets.

Another aspect of the host bus adapter that might seem peculiar to those who have only worked with Ethernet cards is that host bus adapters are dependent upon another component called the Gigabit Interface Converter. The Gigabit Interface Converter contains the lasers that are used for fiber optic communications and it also provides the physical connectivity to the fiber optic cable. Some host bus adapters have a gigabit interface converter built into the card, but other adapters require the gigabit interface converter to be plugged into a slot in the card.

As previously mentioned, Fibre Channel is based on the use of fiber optics. Fiber optics use lasers to send data in the form of pulses of light. These pulses of laser light travel across cables made of woven glass.

Every Fibre Channel device contains two ports – a send port and a receive port. A host bus adapter’s send port is connected to another device’s receive port. That other device is usually a switch, but I’m getting ahead of myself. Likewise, the host bus adapter’s receive port is connected to another device’s send port. What this means is that data is sent and received through two separate ports, which allows for full duplex communications. In other words, Fibre Channel host bus adapters are able to send and receive data at the same time.

So far I have given you a general overview of how host bus adapters work, but I haven’t really talked about why the gigabit interface converter is sometimes separate from the host bus adapter.
Host bus adapters with integrated gigabit interface connectors are beginning to become the norm, but there is a good reason why some adapters still require a separate gigabit interface connector. It’s because Fibre Channel is not a one size fits all medium.

To show you what I mean, think for a moment about USB. When first released, USB was designed to be universal – hence the name Universal Serial Bus. However, today there is no such thing as a standard USB cable. USB cables come in several different form factors of varying sizes and shapes. The need to incorporate USB connectivity into smaller and smaller electronic devices drove the development of small form factor USB interfaces such as micro USB.

This same basic principle also applies to host bus adapters. There are two main types of connectors that host bus adapters use. Having gigabit interface connectors that are separate from the host bus adapter itself makes for a modular design that allows the host bus adapter to be more versatile.
The first type of connector that you need to know about is called an SC connector. SC connectors are the older type of connector, but are still in use today. They are designed to be used with Fibre
Channel connections that operate at a speed of 1 gigabit per second or slower.

The other type of Fibre Channel connector is called an LC connector. This is the connector type that is most commonly used today. LC connectors are smaller than SC connectors and are used with Fibre Channel hardware that operates at much higher speeds than those devices that use SC connectors.

Devices that use LC connectors can operate at speeds of 2, 4, or 8 gigabits per second. In some cases a host bus adapter’s overall bandwidth is divided among multiple sets of ports. For example, some eight gigabit host bus adapters offer twin four gigabit ports. A port refers to a set of send and receive connectors, so a card with twin ports would contain two send connectors and two receive connectors.

You might occasionally see references to ten gigabit Fibre Channel, but at the present time 10 gigabit Fibre Channel is not based on the same technology as that of lower Fibre Channel speeds. 10 gigabit Fibre Channel is based on Fibre Channel over Ethernet (FCoE), which sends Fibre Channel traffic over 10 gigabit Ethernet. The fact that the Fibre Channel traffic has to be encapsulated into Ethernet packets means that 10 gigabit FCoE does not deliver the same level of performance that true 10 gigabit Fibre Channel would.

Just as Fibre Channel hardware can use different form factor connections and can operate at different speeds, there are also ratings for distance. Most of the host bus adapters on the market use short wave lasers. Short wave lasers operate in the 780 nm to 850 nm range and can reliably send and receive data at distances of up to 500 meters. This is typically more than adequate since SAN components are normally located in close physical proximity to one another.

Sometimes however, a SAN implementation might require data to be sent over a much longer distance. This is where long wave lasers come into play. Long wave lasers generally operate at a wavelength of 1300 nm. The distance that they can achieve varies depending on the device. Some long wave Fibre Channel hardware has a distance limitation of 10 KM (6.21 miles). However, some long wave Fibre Channel hardware can send and receive data at distances in excess of 100 KM (62.10 miles).


Now that I have discussed host hardware, I want to turn my attention in Part 3 to the fabric layer of storage area networks. The fabric layer consists of things like hubs, switches, and routers that have been specially designed for use with Fibre Channel.


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