A Storage Area Network (SAN) is a type of a high-speed local area network (LAN) designed to handle large data transfers. A SAN typically supports data storage, retrieval and replication on business networks using high-end servers, multiple disk arrays and Fibre Channel interconnection technology.

SAN technology is similar but distinct from network attached storage (NAS) technology. While SANs traditionally employ low-level network protocols for transfering disk blocks, a NAS device typically works over TCP/IP and can be integrated fairly easily into home computer networks.

A SAN's architecture works in a way that makes all storage devices available to all servers on a LAN or WAN. As more storage devices are added to a SAN, they too will be accessible from any server in the larger network. In this case, the server merely acts as a pathway between the end user and the stored data.

Because stored data does not reside directly on any of a network's servers, server power is utilized for business applications, and network capacity is released to the end user.


Network types

Most storage networks use the SCSI protocol for communication between servers and disk drive devices. They do not use SCSI low-level physical interface (e.g. cables), however, as its bus topology is unsuitable for networking. A mapping layer to other low-level protocols is used to form a network:
  • ATA over Ethernet (AoE), mapping of ATA over Ethernet,
  • Fibre Channel Protocol (FCP), the most prominent one, is mapping of SCSI over Fibre Channel (FC),
  • Fibre Channel over Ethernet (FCoE),
  • mapping of FICON over FC, used by mainframe computers,
  • HyperSCSI, mapping of SCSI over Ethernet,
  • iFCP or SANoIP mapping of FCP over IP,
  • iSCSI Extensions for RDMA (iSER), mapping of iSCSI over InfiniBand (IB),
  • iSCSI, mapping of SCSI over TCP/IP.


Storage sharing

Essentially, a SAN connects storage islands (SCSI disk arrays)together using a high-speed network, thus allowing all applications to access all disks. These local file systems, which cannot be shared among multiple operating systems/hosts, are the most reliable and most widely used. If two independent local file systems resided on a shared LUN (logical unit number), they would be unaware of this fact, would have no means of cache synchronization and eventually would corrupt each other. Thus, sharing data between computers through a SAN requires advanced solutions, such as SAN file systems or clustered computing. Despite such issues, SANs help to increase storage capacity utilization, since multiple servers share the storage space on the disk arrays. The common application of a SAN is for the use of transactionally accessed data that require high-speed block-level access to the hard drives such as email servers, databases, and high usage file servers.

In contrast, NAS allows many computers to access the same file system over the network and synchronizes their accesses. Lately, the introduction of NAS heads allowed easy conversion of SAN storage to NAS.




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