So what is a Storage Area Network?
A Storage Area Network, in its purest sense, is a separate computer network, typically based on a 'fabric' of fibre channel, switches and hubs that connects storage devices to a heterogeneous set of servers on a many-to-many basis. A SAN can also enable direct storage-to-storage interconnectivity, and lends itself exploiting new breeds of clustering technology and to get the best out of Network Attached Storage devices that can intelligently provide disk and tape capabilities to one or more servers.

The technologies that constitute a SAN are as follows:

• The critical enabler is fiber technology. This is the ability to send data across fibre optic channels at very high speeds and bandwidths to sites as far away as 10 kilometers.
• Fiber enabled servers, disk arrays and other intelligent storage and other devices are then connected to the 'fabric' by fiber through sophisticated switches and hubs. These may be configured in highly scalable manner to provide access from hundreds of servers in many locations to hundreds of terabytes of shared storage resources attached directly to the Storage Area Network.
• Zoning and other security facilities are emerging to enable the same levels of security that we have come to demand on the mainframe. What these technologies do is enable the removal of storage devices from their servers and make the storage generally available across the network on a 'many to many' basis.

In its simplest form SAN simply provides server to storage access across fiber - a 'SCSI on steroids' capability. In a more sophisticated form SAN enables a disk or tape array to be accessed by two or more servers at high speed across the fibre channel.

The benefits of this include Improved Performance,  Greater Connectivity and the elimination of redundant data. A Storage Area Network also expedites the use of advanced clustering solutions thereby enabling new levels of availability and business continuity.  A SAN also enables us to relocate backup, restore, file migration and replication of data from the servers and local/wide-area networks and have direct data movement from disks/tapes to other disks/tapes across the SAN fiber.

The benefits here include:

• backups done quietly behind the scenes server power is freed up for the business applications network capacity is released for the users
• remote disaster recovery sites up to date - to the minute!

All this without having to change existing applications, database management systems or the way the user connects to their applications via their current local area network - we are simply moving the storage away from the servers onto its own network. And the applications will be delivering 'local' speed of access to data on the SAN where the data may in fact be many kilometers away.In other words, a SAN represents the removal of storage from their servers and makes the storage generally available across the network. 

An alternative and interesting way to visualize a Storage Area Network is shown below.   The Storage Area Network is shown within the center.  This is where the corporate data resides - a pool of shared on-line, off-line and intelligent storage devices made available via the fiber fabric.

Users connect via a Local Area Network (LAN) and/or Wide Area Network (WAN) through the application servers into the data.  In this way we can see that the application server is simply acting as an access path into the data.  This is a new way of looking at our business-computing environment.   Rather than connecting to, for example, the email server we are accessing our email data through the server.   Further, if we have a suitable architecture, then should the application server fail, compatible spare server capacity can take over the user processing needs easily. This is a crucial point to note - this new storage topology facilitates a new architecture for enhancing availability, and leads us to the interesting notion that application servers in a SAN-aware cluster can be considered as 'Application Server Peripherals' around the users' data. It also shows that the typical 'failover' high availability solution of today that requires 100% redundant hardware can be replaced by one where, say, a 32-machine cluster can enable each server to provide 'application takeover' facilities for its peers. This provides equal or better availability with far less hardware.   This new architecture provides a number of significant benefits:

All of these points provide quantifiable business benefits relating to increased "Return on Investment" and enable the IT function to deliver on, or indeed to exceed, required Service Level Agreements.  We can now view a distributed set of storage devices as though they were one single homogeneous mass.  Hence, if a given application requires additional disk capacity, then we can re-assign disk capacity from any other application on the SAN - even though these other resources may be located many miles away from the primary application.  This gives us an enormous improvement in "Return on Investment" because we can re-use storage devices that previously were hidden behind a given server.

So what does this do for my business?
Why should the president or managing director of a company find storage area networks even vaguely interesting? How will it improve a company's financial or other performance?   Storage Area Networks may enable some companies to operate in a business sense in a radically new way, or in a way they have aspired to in recent years but the technology had really not been up to the challenge.Such companies and government organizations are typically geographically dispersed and depend upon timely and accurate information as a primary corporate asset. In these organizations data is the life-blood of the business.  Today the data required is stored in multiple, isolated locations.

• Unmanaged data duplication means inaccurate information on which to base business decisions.
• Isolated data means that the 'total picture' cannot easily be ascertained
• Timely information is often impossible to assemble when required.
  

The information required by these companies may be contained in huge data warehouses, but is more likely to be in the form of a combination of widely dispersed sources. These might include data feeds from outside the company, Intranet web sites, multiple business applications and other databases, email servers; but the vast amount of information is still held as normal files on the file systems. To enable businesses to operate with a wide-open shared data concept requires systems to have high performance access to data for all authorized users across the business.A Storage Area Network can enable isolated data sources within say a 30 km area to be interconnected and made generally available.  Within a Storage Area Network or across one or more interconnected SANs, a most useful capability is automatic controlled replication of critical corporate data to where it is needed in business terms. This may be for a disaster recovery site or to provide local copies for performance reasons. Such replication needs to faithfully reflect the master copy, albeit a few seconds or minutes later. An example might be a corporate Intranet web site with an employer address book that everyone can use in replacement of out-of-date photocopies. And every time you connect to your network your private copy is synchronized on to your computer. Imagine that for corporate data across subsidiaries within a global company. Another business requirement is that the information is always available. 'Always' does not mean 99.999% system availability - it means 100% availability by users and their applications to the data. A Storage Area Network provides inherently very high availability by the use of automatic data redundancy, automatic backups and the maintenance of nearby disaster recovery copies. Clustered servers with shared access to the data that can dynamically switch users and applications between peers will dramatically improve user and business application availability. On the fly change of all components is also a promise of SAN - whether that be a replacement disk that is about to fail, a server that needs maintenance, a software upgrade or a part of the network that is to be enhanced.  The SAN architecture should also assist in providing a smoother more consistent service by adding load balancing to the high availability and performance already mentioned.

The inevitable conclusion
Many companies will evolve towards SAN technology without really realizing it, as new disk arrays, offline storage devices, servers and software are all made fiber and SAN connectable. Others will adopt Storage Area Networks aggressively to gain the potential benefits of far better utilization of resources, new levels of availability, performance and the ability to better share corporate data across their company. Earlier adopters of Storage Area Networks must select proven suppliers with a pedigree for producing reliable hardware or software. They must advance aggressively to gain benefits of a SAN, but with due care and planning. Management tools and suitable 'SAN aware' utilities should be deployed early to help control this new 'unknown'.

Storage Area Networks will take several years to mature during which we will see these early adopters work through an evolution from 'SCSI on steroids' to an environment with outstanding availability and performance for global applications and globally shared data. The key to success is the establishment of a 'storage centric' environment supported by a high performance low latency fiber fabric that provides users with highly available access to clusters of application servers with many-to-many connectivity to shared online and offline storage.