The conventional approach for storage in organizations is to have a set of application servers with storage interconnected through a LAN. In this approach, data is shared between application servers using ad-hoc methods which vary according to application’s requirements. Additionally, this LAN which is used to transfer data is also used to exchange application messages with clients. Some of the problems of the conventional approach are:
- Unconnected islands of information
- Duplication of data
- Inconsistency between copies
- Extra operational complexity to replicate and manage replicated data
In order to address these problems a novel approach for data storage is proposed: Storage Area Networks (SANs). The key point of SANs is to decouple applications servers from the storage. The idea is to provide universal interconnection between computers and storage devices, thus enabling sharing and a better management of data. This is typically achieved with the use of a dedicated fiber-based network infrastructure.
SAN is a generic term and can refer to many technologies, protocols, hardware and software. A formal definition of SAN (extracted from ) is:
“A storage area network (SAN) is a network of interconnected computers and data storage devices. The term SAN specifically includes both the interconnection infrastructure and the computers and storage that it links together.”
Benefits of SAN
- Cost savings
- No extra temporary space to stage data
- Reduce costs with tape and robotic media handlers, since now they can be transparently shared among many applications/services
- Reduce operational costs with copying data from place to place, etc
- Operational benefits
- There’s no need to devise mid schedule data transfers between pairs of servers.
- There’s no need to worry about whether copies of data being used by two computers running different applications are synchronized (i.e., have exactly the same contents), because the two computers are working from the same copy of data.
- Possibility to distribute data in different zones, since storage is decoupled from application servers
- Removes I/O traffic from the LAN, increasing performance of existing applications
- Simplifies back-up, since it can be done in the SAN-side, instead of in the applications
Desired properties of a SAN
- A SAN must be highly available, because highly critical data will be stored.
- A San must be scalable – it’s I/O performance must grow as the number of interconnected devices grow.
SAN – Software vs Hardware
“Hardware makes SANs possible; software makes SANs happen” 
SANs are typically associated with hardware/networking technologies, however, it’s only possible to take full advantages of highly-available and high-performance SANs through software. Some of the capabilities that can be leveraged through software are:
- Sharing tape drives and online storage devices
- Application failover
- Sharing data between different clients and servers
- Direct Data Movement between Devices
SAN vs Network-Attached-Storage (NAS)
SANs are typically mistaken with NAS, since both involve sharing data across many storage devices over a network. However, Network-Attached Storages normally operates at the file-system level, while SAN goes beyond and operates at the block level. This means the file systems reside entirely in the client/application machines, that access blocks through the network. Another difference is that current NAS solutions are based on Ethernet and TCP/IP, while NAS are based on dedicated fiber channels. As a rule of thumb:
- Choose NAS for simplicity of data sharing, particularly among computers and operating systems of different types.
- Choose SAN for the highest raw I/O performance between data client and data server. Be prepared to do some additional design and operational management to make servers cooperate (or at least not interfere) with each other.
Even though SAN is not a new concept, its potential has not been completely explored. There still a lot of research and development going on and some companies such as EMC, Nexenta, NetApp, among others are exploring this potential. Some of the coming challenges/solutions in this area involve:
- Interoperability between existing solutions and adoption/improvement/creation of standards
- Smarter Storage and Appliances
- Support for Heterogeneous Computer Systems
- Data Storage as a Service / Cloud Storage
This text is based on  – Learn more on: http://snia.org/education/storage_networking_primer/san
 What Storage Networking Is and What It Can Mean to You
from Storage Area Network Essentials, by Richard Barker & Paul Massiglia.
Copyright© 2002, John Wiley & Sons, Inc. New York. Used by Permission.