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Storage Basics: Storage Area Networks

Many IT organizations today are scratching their heads debating whether the advantages of implementing a SAN solution justify the associated costs. Others are trying to get a handle on today's storage options and whether SAN is simply Network Attached Storage spelled backwards. In this article, I introduce the basic purpose and function of a SAN and examine its role in modern network environments. I also look at how SANs meet the network storage needs of today's organizations and answer the question, could a SAN be right for you.

Peel away the layers of even the most complex technologies and you are likely to find that they provide the most basic of functions. This is certainly true of storage area networks (SANs). Behind the acronyms and revolutionary headlines, lies a technology designed to provide a way of offering one of the oldest of network services, that of making access to data storage devices available to clients.

In very basic terms, a SAN can be anything from two servers on a network accessing a central pool of storage devices to several thousand servers accessing many millions of megabytes of storage. Conceptually, a SAN can be thought of as a separate network of storage devices physically removed from, but still connected to, the network. SANs evolved from the concept of taking storage devices, and therefore storage traffic, off the LAN and creating a separate back-end network designed specifically for data.

SANs represent the evolution of data storage technology to this point. Traditionally, on client server systems, data was stored on devices either inside or directly attached to the server. Next in the evolutionary scale came Network Attached Storage (NAS) which took the storage devices away from the server and connected them directly to the network. SANs take the principle one step further by allowing storage devices to exist on their own separate network and communicate directly with each other over very fast media. Users can gain access to these storage devices through server systems which are connected to both the LAN and the SAN.

This is in contrast to the use of a traditional LAN for providing a connection for server-storage, a strategy that limits overall network bandwidth. SANs address the bandwidth bottlenecks associated with LAN based server storage and the scalability limitations found with SCSI bus based implementations. SANs provide modular scalability, high-availability, increased fault tolerance and centralized storage management. These advantages have led to an increase in the popularity of SANs as they are quite simply better suited to address the data storage needs of today's data intensive network environments.

The advantages of SANs are numerous, but perhaps one of the best examples is that of the serverless backup (also commonly referred to as 3rd Party Copying). This system allows a disk storage device to copy data directly to a backup device across the high-speed links of the SAN without any intervention from a server. Data is kept on the SAN, which means the transfer does not pollute the LAN, and the server processing resources are still available to client systems.

SANs are most commonly implemented using a technology called Fibre channel (yes, that's fibre with an 're', not an 'er'). Fibre Channel is a set of communication standards developed by the American National Standards Institute (ANSI). These standards define a high-performance data communications technology that supports very fast data rates (over 2Gbps). Fibre channel can be used in a point-to-point configuration between two devices, in a 'ring' type model known as an arbitrated loop, and in a fabric model.

Devices on the SAN are normally connected together through a special kind of switch, called a Fibre Channel switch, which performs basically the same function as a switch on an Ethernet network, in that it acts as a connectivity point for the devices. Because Fibre channel is a switched technology, it is able to provide a dedicated path between the devices in the fabric so that they can utilize the entire bandwidth for the duration of the communication.

The storage devices are connected to Fibre Channel switch using either multimode or single mode fiber optic cable. Multimode for short distances (up to 2 kilometers), single mode for longer. In the storage devices themselves, special fiber channel interfaces provide the connectivity points. These interfaces can take the form of built in adapters, which are commonly found in storage subsystems designed for SANs, or can be interface cards much like a network card, which are installed into server systems.

So, the question that remains is this. Should you be moving away from your current storage strategy and towards a SAN? The answer is not a simple one. If you have the need to centralize or streamline your data storage then a SAN may be right for you. There is, of course, one barrier between you and storage heaven, and that's money. While SANs remain the domain of big business, the price tag's of SAN equipment is likely to remain at a level outside the reach of small or even medium sized businesses. As the prices fall, however, SANs will find their way into organizations of all sizes, including, if you want, yours.

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