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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 storageIn computing, direct-attached storage (sometimes abbreviated as DAS is a term to differentiate non- networked storage from SAN and NAS. The term did not exist before the advent of networked enterprise storage, though it is often confused with the mainfram (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.
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 systemIn enterprise storage, a SAN file system is a file system which has been optimized to be shared by multiple clustered servers over a storage area network. SAN file systems are like network attached storage (NAS) protocols (like SMB and NFS) in that they as and clustered computing, SAN storage is still a one-to-one relationship. That is, each device (or LUNIn computer storage, a logical unit number or LUN is an address for an individual disk drive and by extension, the disk device itself. The term originated in the SCSI protocol as a way of differentiating individual disk drives within a common SCSI target) 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.
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 like as NAS.
The most common SAN technology is Fibre ChannelFibre Channel is a serial computer bus intended for connecting high speed storage devices to computers. It started for use primarily in the supercomputer field, but has become the standard connection type for storage area networks in enterprise storage. networking with the SCSI command set. A typical Fibre Channel SAN is made up of a number of Fibre Channel switchA Fibre Channel switch is a computer storage device that allows the creation of a Fibre Channel fabric. This fabric is a network of Fibre Channel devices which allows many-to-many communication, device name lookup, security, and redundancy. Fibre Channeles 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.
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.