This article continues the discussion of storage area networks by talking about the storage fabric and about the three most commonly used fabric topologies.
Introduction
In the second part of this article series, I talked all about hosts and host hardware. In this article, I want to turn my attention to the storage fabric.As previously explained, SANs consist of three main layers – the host layer (which I talked about in the previous article), the fabric layer, and the storage layer. The fabric layer consists of networking hardware that establishes connectivity between the host and the storage target. The fabric layer can consist of things like SAN hubs, SAN switches, fiber optic cable, and more.
Fabric Topologies
Before I get too far into my discussion of the fabric layer, I need to explain that SANs are really nothing more than networks that are dedicated to the sole purpose of facilitating communications between hosts and storage targets. That being the case, it should come as no surprise that there are a number of different topologies that you can implement. In some ways SAN fabric topologies mimic the topologies that can be used on regular, non-SAN networks. There are three main fabric topologies that you need to know about. These include point to point, arbitrated loop, and switched fabric.
Point to Point
Point to point is the simplest and least expensive SAN fabric topology. However, it is also the least practical. A point to point topology is essentially a direct connection between a host and a storage target. The simplicity and cost savings come into play in the fact that no additional SAN hardware is needed (such as switches and routers). Of course the price for this simplicity is that the fabric can only include two devices – the host and the storage target. The fabric cannot be expanded without switching to a different topology. Because of this, some would argue that point to point isn’t even a true SAN topology.Arbitrated Loop
The simplest and least expensive “true SAN” topology is an arbitrated loop. An arbitrated loop makes use of a Fibre Channel hub. Hubs are kind of like switches in that they contain ports and devices can communicate with each other through these ports. The similarities end there however.
Fibre channel hubs lack the intelligence of a switch, and they do not segment communications like a switch does. This leads to a couple of important limitations. For starters, all of the devices that are attached to a hub exist within a common collision domain. What this means is that only one device can transmit data at a time. Otherwise, if two devices attempted simultaneous communications the transmissions would collide with each other and be destroyed.
Because of the way that Fibre Channel hubs work, each hub provides for a certain amount of bandwidth and that bandwidth must be shared by all of the devices that are connected to the hub. This means that the more devices you connect to a Fibre Channel hub, the more each device must compete with other devices for bandwidth.
Because of bandwidth limitations and device capacity, the arbitrated loop topology is suitable only for small or medium sized businesses. The limit to the number of devices that can be connected to an arbitrated loop is 127. Even though this probably sounds like a lot of devices, it is important to remember that this is a theoretical limit, not a practical limit.
In the real world, Fibre Channel hubs are becoming more difficult to find, but you can still buy them. Most of the Fibre Channel hubs that I have seen recently only offer eight ports. That isn’t to say that you can only build a hub with eight devices however. You can use a technique called hub cascading to join multiple hubs together into a single arbitration loop.
As the arbitration loop grows in size, there are a few things to keep in mind. First, the 127 device limit that I mentioned previously is the limit for the entire arbitration loop, not just for a single hub. You can’t exceed the device limit just by connecting an excessive number of hubs together.
Another thing to consider is that the hub itself counts as a device. Therefore, an eight port hub with a device plugged into each port would actually count as nine devices.
Probably the most important thing to remember with regard to hub cascading is that hardware manufacturers have their own rules about hub cascading. For example, many of the Fibre Channel hubs on the market can be cascaded twice, which means that the maximum number of hubs that you could use in your arbitration loop would be three. If you assume that each hub contains eight ports then the entire arbitration loop would max out at 24 devices (although the actual device count would be 27 because each of the three hubs counts as a device).
Keep in mind that this represents a best case scenario (assuming that the manufacturer does impose a three hub limit). The reason why I say this is because in some cases you might have to use a hub port to connect the next hub in the cascade. Some hubs offer dedicated cascade ports separate from the device ports, but others do not.
Earlier I mentioned that using an arbitration loop was the cheapest and easiest way to build a SAN. The reason for this is that Fibre Channel hubs do not typically require any configuration. Devices can simply be plugged in, and the hub does the rest. Keep in mind however, that arbitration loops tend to be slow (compared to switched fabrics) and that they lack the flexibility for which SANs have come to be known.
Switched Fabric
The third topology is known as a switched fabric. Switched fabric is probably the most widely used Fibre Channel topology today. It is by far the most flexible of the three topologies, but it is also the most expensive to implement. When it comes to SANs however, you usually get what you pay for.
As the name implies, the switched fabric topology makes use of a Fibre Channel switch. Fibre Channel switches are not subject to the same limitations as hubs. Whereas an arbitration loop has a theoretical limit of 127 devices, a switched fabric can theoretically scale to accommodate millions of devices. Furthermore, because of the way that a switched fabric works, any device within the fabric is able to communicate with any other device.
As you can see, Fibre Channel switches are very powerful, but they also have the potential to become a single point of failure. A switch failure can bring down the entire SAN. As such, switched fabrics are usually designed in a way that uses redundant switches. This allows the SAN to continue to function in the event of a switch failure. I will discuss switched fabrics in much more detail in the next article in this series.
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