Storage area network

Categories: Computer networks | Computer storage | Computer buses

"SAN" redirects here. For other uses, see SAN (disambiguation).

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 2005, SANs are common in enterprise storage.

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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 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 devices, though they do not use its low-level physical interface.

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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.

SANs tend to increase storage capacity utilization, since multiple servers can share the same growth reserve.

Other benefits include the ability to allow servers to boot from the SAN itself. This allows for a quick and easy replacement of faulty servers since the SAN can be reconfigured so that a replacement server can use the LUN of the faulty server. This process can take as little as half an hour and is a relatively new idea being pioneered in newer data centres.

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SAN types

SANs are normally built on an infrastructure specially designed to handle storage communications. Thus, they tend to provide faster and more reliable access than higher level protocols such as NAS.

The most common SAN technology is Fibre Channel networking with the SCSI command set. A typical Fibre Channel SAN is made up of a number of Fibre Channel switches which are connected together to form a fabric or network.

An alternative, and more recent (2003), SAN protocol is iSCSI which uses the same SCSI command set over TCP/IP (and, typically, Ethernet). In this case, the switches would be Ethernet switches and the network would not be referred to as a fabric.

Another newcomer is the ATA-over-Ethernet (AOE) protocol which embeds the ATA protocol inside of raw Ethernet frames. While a raw Ethernet protocol like AOE cannot be routed over a network like iSCSI, it does provide a simple discovery model with fairly low overhead.

Connected to the SAN will be one or more servers(hosts) and one or more disk arrays, tape libraries, or other storage devices. In the case of a Fibre Channel SAN, the servers would use special Fibre Channel host bus adapters (HBAs) and optical fiber. iSCSI SANs would normally use Ethernet network interface cards, or special TOE cards.

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Compatibility

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 (such as 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 OSes sharing the same fabric. Many systems were pushed to the market before standards were finalised and vendors innovated around the standards.

The combined efforts of the members of the Storage Networking Industry Association (SNIA) improved the situation during 2002 and 2003. Today most vendor devices, from HBAs to switches and arrays, interoperate nicely, though there are still many high-level functions that do not work between different manufacturers' hardware.

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SANs at work

SANs are primarily used in large scale, high performance enterprise storage operations. It would be unusual to find a Fibre Channel disk drive connected directly to a SAN. Instead, SANs are normally networks of large disk arrays. Fibre Channel host bus adapters are also rare in desktop computers. Accordingly, SAN equipment is relatively expensive. The nascent (as of 2004) iSCSI SAN technology is expected to produce cheaper SANs, but it is unlikely that this technology will be used outside the enterprise data center environment.

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SANs in a SOHO

With the increasing rise of digital media in all phases of life and its effect on storage needs, its natural that SANs will enter into the SOHO market. Currently this is dominated by NAS type of systems.

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See also

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External links

es:Storage Area Network fr:Storage Area Network it:Storage Area Network nl:Storage Area Network ja:ストレージエリアネットワーク pl:Storage Area Network pt:Storage area network