Storage Area Network (SAN) is a high performance network that connects storage devices and the back-end of servers. The deployment of a SAN allows the servers on a LAN or a WAN to access any number of storage devices.

SANs have led to the development of several new methods for attaching servers to storage devices such as optical jukeboxes, tape libraries, and disk arrays.

The I/O bandwidth of the networks that were earlier used to connect the data storage devices and the processors was not commensurate with the capacities of the disk arrays and computers that utilized the data stored in them. The access to data is further complicated by the different database software run on different platforms. Managing different file systems and data formats requires trained manpower. The traditionally distributed storage has been a huge drain on management resources and inefficient as well in terms of capacity utilization of hardware resources. Scalability is also an issue when disk capacity is tied down to a single server or client. Sharing of data often requires creating duplicate copies, moving these copies slows down the LAN/WAN and often co-ordination between applications such as BI, CRM, and ERP that are spread over the entire organization becomes very difficult. 

SAN topologies are predominantly developed using fiber channels. Fiber channel is an open technical standard developed for networking and is especially useful for handling storage communications as it offers flexible connectivity and fast access to data. Optical fibers are used for long-distance networking and copper cable links are preferred for shorter distances due to their lower cost. Fiber channels can support different protocols and a large number of devices, a quality very desirable in any networking solution.  

The American National Standards Institute (ANSI) has laid down the standards on which fiber channel networks are based. These standards define the manner in which data is to be moved across networks. The standards are exhaustive and cover physical interfaces, data encoding practices and protocols, data delivery methodologies, and common services. Fiber channels offer the advantage of a high level of hardware processing to ensure high performance. The serial data transport scheme used in a fiber channel can be implemented using simple cables and connectors. The information can be routed easily through switched networks. Since fiber channel transport layers are protocol independent, they enable the transmission of multiple protocols. Apart from being extremely flexible in its application, fiber channel delivers data at the rate at which the receiving application is able to handle; besides there is no loss of data. 

Tape systems make use of tapes arranged serially; parallel arrangements are not possible. Tape systems consist of drives, autoloaders, and libraries. Tape drives connect the tapes to the devices and enable the reading/writing from and to the tapes. Tape autoloaders are tape drives that perform the function of auto backup; they are used for devices that generate a lot of data constantly. Tape libraries are autonomous sets of tape drives and autoloaders. They are used in situations where the storage capacity required is very high. Tape systems are used for offline storage because of their cost efficiency.

Most storage networks use the SCSI protocol for communication between servers and disk drive devices. They do not use SCSI low-level physical interface (e.g. cables), however, as its bus topology is unsuitable for networking. A mapping layer to other low-level protocols is used to form a network:

Storage sharing

Historically, data centers first created “islands” of SCSI disk arrays. Each island was dedicated to an application and visible as a number of “virtual hard drives” (i.e. LUNs). Essentially, a SAN connects storage islands together using a high-speed network, thus allowing all applications to access all disks.

Operating systems still view a SAN as a collection of LUNs, and usually maintain their own file systems on them. These local file systems, which cannot be shared among multiple operating systems / hosts, are the most reliable and most widely used. If two independent local file systems resided on a shared LUN, they would be unaware of this fact, would have no means of cache synchronization and eventually would corrupt each other. Thus, sharing data between computers through a SAN requires advanced solutions, such as SAN file systems or clustered computing. Despite such issues, SANs help to increase storage capacity utilization, since multiple servers share the storage space on the disk arrays. The common application of a SAN is for the use of transactionally accessed data that require high-speed block-level access to the hard drives such as email servers, databases, and high usage file servers.

In contrast, NAS allows many computers to access the same file system over the network and synchronizes their accesses. Lately, the introduction of NAS heads allowed easy conversion of SAN storage to NAS.

Perhaps the biggest benefit of a SAN is that it complements expensive business applications that demand instant and real-time information. ERP and CRM systems can fulfill their business promise only if the right type of data is made available at the right time to the right person. To this end, a SAN is most useful and appropriate.

One of the early problems with Fibre Channel SANs was that the switches and other hardware from different manufacturers were not entirely compatible. Although the basic storage protocols FCP were always quite standard, some of the higher-level functions did not interoperate well. Similarly, many host operating systems would react badly to other operating systems sharing the same fabric. Many solutions were pushed to the market before standards were finalized and vendors innovated around the standards. Note: fibre channel and SAN are not synonymous.

SANs are primarily used in large scale, high performance enterprise storage operations. It would be unusual to find a single disk drive connected directly to a SAN. Instead, SANs are normally networks of large disk arrays. SAN equipment is relatively expensive and as such, fibre channel host bus adapters are rare in desktop computers. The iSCSI SAN technology is expected to eventually produce cheap SANs, but it is unlikely that this technology will be used outside the enterprise data center environment. Desktop clients are expected to continue using NAS protocols such as SMB and NFS. The exception to this may be remote storage replication.

Video editing workgroups require very high data transfer rates. Outside of the enterprise market, this is one area that greatly benefits from SANs.

Per-node bandwidth usage control, sometimes referred to as quality-of-service (QoS), is especially important in video workgroups as it ensures fair and prioritized bandwidth usage across the network if there is insufficient open bandwidth available. Avid Unity, Apple’s Xsan and Tiger Technology MetaSAN are specifically designed for video networks and offer this functionality.

Storage virtualization refers to the process of completely abstracting logical storage from physical storage. The physical storage resources are aggregated into storage pools, from which the logical storage is created. It presents to the user a logical space for data storage and transparently handles the process of mapping it to the actual physical location. This is currently implemented inside each modern disk array, using vendor’s proprietary solution. However, the goal is to virtualize multiple disk arrays, made by different vendors, scattered over the network, into a single monolithic storage device, which can be managed uniformly.

RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops allowed in a path from the source to a destination. This hop limit, however, limits the size of networks that RIP can support.

The WAIS protocol and servers were primarily promoted by Thinking Machines Corporation of Cambridge, Massachusetts. Thinking Machines produced WAIS servers which ran on their massively parallel CM-2 (connection machine) and SPARC-based CM-5 MP supercomputers. WAIS clients were developed for various operating systems including Windows, Macintosh, NeXT and UNIX. TMC, however, released a free open source version of WAIS to run on UNIX in 1991.

With the advent of Z39.50:1992, the termination of support for the free WAIS from Thinking Machines and the establishment of WAIS Inc as a commercial venture, the U.S. National Science Foundation funded CNIDR to create a clearinghouse of information related to Internet search and discovery systems and to promote open source and standards. CNIDR created a new freely available open-source WAIS.

Public WAIS was often used as a full text search engine for individual Internet Gopher servers, supplementing the popular Veronica system which only searched the menu titles of Gopher sites.