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(Storage Area Network) An array of disk drives in a self-contained unit. In large enterprises, SANs serve as pools of storage for the servers in the network. Compared to managing disks attached to each server, SANs improve system administration. Treating all storage as a single resource makes disk maintenance and routine backups easier to schedule and control. To support disaster recovery, redundant SANs are deployed in separate locations, each a copy of the other.

The SAN transfers data between servers and disks at the same fast peripheral channel speeds as if directly attached, and Fibre Channel has been the traditional interface. Some SANs perform backup procedures without any processing overhead at the host computers. See Fibre Channel.

Centralized or Distributed
A centralized SAN connects the disk array to local servers, whereas a distributed SAN uses one or more Fibre Channel or SCSI switches to connect nodes within buildings or campuses. For long distances, SAN traffic is transferred over ATM, SONET or dark fiber. There are also protocols for sending data to a SAN over local IP/Ethernet networks and the Internet (see IP storage).

Channel Attached Vs. Network Attached
SANs are channel attached. However, a related technology is the less-costly network attached storage (NAS) device. The NAS is a specialized file server that connects to the network like any other workstation or server and is thus subject to the variable traffic and erratic behavior of the network.

Although the SAN and NAS are both storage arrays, they operate differently. The channel-attached SAN extends the server's peripheral reach, whereas the NAS is just another node on the network. For more details, see block level. See NAS, SCSI switch and SNIA.

A SAN is like attached disk storage, while the NAS is a file server. They both fall under the "storage network" umbrella.

Channel Attached
EMC has been a pioneer in SANs for the enterprise, and its Symmetrix systems support up to 128 channels and petabytes of data. (Image courtesy of EMC Corporation.)

Network Attached
Containing only an on/off switch and Ethernet port, Adaptec's Snap Server provides instant storage by merely plugging it into the network. (Image courtesy of Adaptec, Inc.,

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