InformationWeek SMB TechEncyclopedia

(Storage Area Network) A network of storage disks. In large enterprises, a SAN connects multiple servers to a centralized pool of disk storage. Compared to managing hundreds of servers, each with their own disks, SANs improve system administration. By treating all the company's storage as a single resource, disk maintenance and routine backups are easier to schedule and control. In some SANs, the disks themselves can copy data to other disks for backup without any processing overhead at the host computers.

High Speed
The SAN network allows data transfers between computers and disks at the same high peripheral channel speeds as when they are directly attached. Fibre Channel is a driving force with SANs and is typically used to encapsulate SCSI commands. SSA and ESCON channels are also supported.

Centralized or Distributed
A centralized SAN connects multiple servers to a collection of disks, whereas a distributed SAN typically 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. To guarantee complete recovery in a disaster, dual, redundant SANs are deployed, one a mirror of the other and each in separate locations.

Over IP
Another SAN option is IP storage, which enables data transfer via IP over fast Gigabit Ethernet locally or via the Internet to anywhere in the world (see IP storage). See LAN free backup.

Channel Attached Vs. Network Attached
A related storage device is the network attached storage (NAS) system, which is a file server that attaches to the LAN like any other client or server in the network. Rather than containing a full-blown operating system, the NAS uses a slim microkernel specialized for handling only file reads and writes (CIFS/SMB, NFS, NCP). However, the NAS is subject to the variable behavior and overhead of a network that may contain thousands of users. See block level.

The Terminology
SAN-NAS terminology is confusing (storage area network vs. network attached storage). They both fall under the "storage network" umbrella, but operate differently: the channel-attached SAN extends the disk channel, whereas the NAS is another node on the network. See NAS, Fibre Channel, SCSI switch, AoE, iSCSI, IP storage, SCSI and SNIA.








SAN Vs. NAS
A SAN is extended disk storage, while the NAS is a file server. They all fall under the "storage network" umbrella.






Channel Attached
EMC has been a pioneer in channel-attached storage networks, especially in the mainframe arena. Its Symmetrix storage systems support up to 32 ports (channels) and hold up to 6TB. Network-attached options are also available. (Image courtesy of EMC Corporation.)





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