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In computing, a storage area network (SAN) is a network designed to attach computer storage devices such as disk array controllers and tape libraries to servers. As of 2006, SANs are most commonly found in enterprise storage. more...

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A SAN allows a machine to connect to remote targets such as disks and tape drives on a network for block level I/O. From the point of view of the class drivers and application software, the devices appear as locally attached devices.

There are two variations of SANs:

A network whose essential purpose is the transfer of data between computer systems and storage elements. A SAN consists of a communication infrastructure, which provides physical connections, and a management layer, which organizes the connections, storage elements, and computer systems so that data transfer is secure and robust. The term SAN is usually (but not necessarily) identified with block I/O services rather than file access services.; A storage system consisting of storage elements, storage devices, computer systems, and/or appliances, plus all control software, communicating over a network.;

Defining SAN

Storage networks are distinguished from other forms of network storage by the low-level access method that they use. Data traffic on these networks is very similar to those used for internal disk drives, like ATA and SCSI.

In a storage network, a server issues a request for specific blocks, or data segments, from specific disk drives. This method is known as block storage. The device acts in a similar fashion to an internal drive, accessing the specified block, and sending the response across the network.

In more traditional file storage access methods, like SMB/CIFS or NFS, a server issues a request for an abstract file as a component of a larger file system, managed by an intermediary computer. The intermediary then determines the physical location of the abstract resource, accesses it on one of its internal drives, and sends the complete file across the network.

Most storage networks use the SCSI protocol for communication between servers and disk drive devices, though they do not use its low-level physical interface. Typical SAN physical interfaces include 1Gbit Fibre Channel, 2Gbit Fibre Channel, 4Gbit Fibre Channel, and (in limited cases) 1Gbit iSCSI. The SCSI protocol information will be carried over the lower level protocol via a mapping layer. For example, most SANs in production today use some form of SCSI over Fibre Channel system, as defined by the FCP mapping standard. iSCSI is a similar mapping method designed to carry SCSI information over IP.

Benefits

Sharing storage usually simplifies storage administration and adds flexibility since cables and storage devices do not have to be physically moved to move storage from one server to another. Note, though, that with the exception of SAN file systems and clustered computing, SAN storage is still a one-to-one relationship. That is, each device (or Logical Unit Number (LUN)) on the SAN is "owned" by a single computer (or initiator). In contrast, Network Attached Storage (NAS) allows many computers to access the same set of files over a network. It is now possible to combine the SAN and NAS using a NAS head.

Read more at Wikipedia.org