Storage area network

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SAN redirects here; for other meanings, 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 2004, SANs are common in enterprise storage.

Contents

Defining SAN

A SAN is distinguished from other ways of networking storage by the low-level block storage access method used. Put simply, data traffic on a SAN is very similar to those (like ATA and SCSI) used for internal disk drives. For example, a server would ask for "Block 6000 from disk drive 4". For comparison, if a file storage access method was used (like SMB or NFS), the server would request "file /home/smith/foo". Most SANs actually use the SCSI communications protocol to access data on the network, though they do not use SCSI's lower-level physical interfaces.

A SAN can be thought of as an extension of direct attached storage (DAS). Where DAS was a point-to-point link between a server and its storage, a SAN allows many computers to access many storage devices over a shared network.

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 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.

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, 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 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 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.

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.

This problem began to be solved during 2002 and 2003 through the combined efforts of the members of the Storage Networking Industry Association (SNIA). 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.

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.

See also

External links

  • nas-san comparison (http://www.nas-san.com/differ.html) - including NAS and SAN whitepapers.


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