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A storage area network (SAN) is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk array

Disk array

A disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization.Components of a typical disk array include:...

s, tape libraries

Tape library

In computer storage, a tape library, sometimes called a tape silo, tape robot or tape jukebox, is a storage device which contains one or more tape drives, a number of slots to hold tape cartridges, a barcode reader to identify tape cartridges and an automated method for loading tapes...

, and optical jukebox

Optical jukebox

An optical jukebox is a robotic data storage device that can automatically load and unload optical discs, such as Compact Disc, DVD, Ultra Density Optical or Blu-ray disc and can provide terabytes and petabytes of tertiary storage. The devices are often called optical disk libraries, robotic...

es, accessible to server

Server (computing)

In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

s so that the devices appear like locally attached devices to the operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

. A SAN typically has its own network of storage devices that are generally not accessible through the local area network by other devices. The cost and complexity of SANs dropped in the early 2000s to levels allowing wider adoption across both enterprise and small to medium sized business environments.

A SAN does not provide file abstraction, only block-level operations. However, file systems built on top of SANs do provide file-level access, and are known as SAN filesystems or shared disk file systems.

Storage sharing

Historically, data centers first created "islands" of SCSI

SCSI

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...

disk array

Disk array

A disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization.Components of a typical disk array include:...

s as direct-attached storage (DAS), each dedicated to an application, and visible as a number of "virtual hard drives" (i.e. LUNs). Essentially, a SAN consolidates such storage islands together using a high-speed network.

Operating systems maintain their own file system

File system

A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...

s on their own dedicated, non-shared LUNs, as though they were local to themselves. If multiple systems were simply to attempt to share a LUN, these would interfere with each other and quickly corrupt the data. Any planned sharing of data on different computers within a LUN requires advanced solutions, such as SAN file systems or clustered computing.

Despite such issues, SANs help to increase storage capacity utilization, since multiple servers consolidate their private storage space onto the disk arrays.

Common uses of a SAN include provision 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.

SAN and NAS

Network-attached storage

Network-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...

(NAS), in contrast to SAN, uses file-based protocols such as NFS or SMB/CIFS where it is clear that the storage is remote, and computers request a portion of an abstract file rather than a disk block.

SAN-NAS hybrid

Despite the differences between SAN and NAS, it is possible to create solutions that include both technologies, as shown in the diagram.

Benefits

Sharing storage usually simplifies storage administration and adds flexibility since cables and storage devices do not have to be physically moved to shift storage from one server to another.

Other benefits include the ability to allow servers to boot from the SAN itself. This allows for a quick and easy replacement of faulty servers since the SAN can be reconfigured so that a replacement server can use the LUN of the faulty server. While this area of technology is still new many view it as being the future of the enterprise datacenter.

SANs also tend to enable more effective disaster recovery

Disaster recovery

Disaster recovery is the process, policies and procedures related to preparing for recovery or continuation of technology infrastructure critical to an organization after a natural or human-induced disaster. Disaster recovery is a subset of business continuity...

processes. A SAN could span a distant location containing a secondary storage array. This enables storage replication either implemented by disk array controller

Disk array controller

A disk array controller is a device which manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides additional disk cache.A disk array controller name is...

s, by server software, or by specialized SAN devices. Since IP WAN

Wide area network

A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...

s are often the least costly method of long-distance transport, the Fibre Channel over IP

Fibre Channel over IP

Fibre Channel over IP , is an Internet Protocol was created by the Internet Engineering Task Force as storage technology....

(FCIP) and iSCSI protocols have been developed to allow SAN extension over IP networks. The traditional physical SCSI layer could only support a few meters of distance - not nearly enough to ensure business continuance in a disaster.

The economic consolidation of disk arrays has accelerated the advancement of several features including I/O caching, snapshotting

Snapshot (computer storage)

In computer systems, a snapshot is the state of a system at a particular point in time. The term was coined as an analogy to that in photography. It can refer to an actual copy of the state of a system or to a capability provided by certain systems....

, and volume cloning (Business Continuance Volumes or BCVs).

Network types

Most storage networks use the SCSI

SCSI

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...

protocol for communication between servers and disk drive devices. A mapping layer to other protocols is used to form a network:

ATA over Ethernet (AoE)
ATA over Ethernet
ATA over Ethernet is a network protocol developed by the Brantley Coile Company, designed for simple, high-performance access of SATA storage devices over Ethernet networks. It is used to build storage area networks with low-cost, standard technologies.- Protocol description :AoE runs on layer 2...

, mapping of ATA
AT Attachment
Parallel ATA , originally ATA, is an interface standard for the connection of storage devices such as hard disks, solid-state drives, floppy drives, and optical disc drives in computers. The standard is maintained by X3/INCITS committee...

over Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
Fibre Channel Protocol (FCP), the most prominent one, is a mapping of SCSI over Fibre Channel
Fibre Channel
Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
Fibre Channel over Ethernet
Fibre Channel over Ethernet
Fibre Channel over Ethernet is an encapsulation of Fibre Channel frames over Ethernet networks. This allows Fibre Channel to use 10 Gigabit Ethernet networks while preserving the Fibre Channel protocol...

(FCoE)
ESCON
ESCON
ESCON is a data connection created by IBM, and is commonly used to connect their mainframe computers to peripheral devices such as disk storage and tape drives. ESCON is an optical fiber, half-duplex, serial interface. It originally operated at a rate of 10 Mbyte/s, which was later increased to...

over Fibre Channel (FICON
FICON
FICON is the IBM proprietary name for the ANSI FC-SB-3 Single-Byte Command Code Sets-3 Mapping Protocol for Fibre Channel protocol. It is a FC layer 4 protocol used to map both IBM’s antecedent channel-to-control-unit cabling infrastructure and protocol onto standard FC services and infrastructure...

), used by mainframe computer
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

s
HyperSCSI
HyperSCSI
HyperSCSI was a computer network protocol for accessing storage by sending and receiving SCSI commands.It was developed by researchers at the Data Storage Institute in Singapore in 2000 to 2003....

, mapping of SCSI over Ethernet
iFCP or SANoIP mapping of FCP over IP
iSCSI
ISCSI
In computing, iSCSI , is an abbreviation of Internet Small Computer System Interface, an Internet Protocol -based storage networking standard for linking data storage facilities. By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage...

, mapping of SCSI over TCP/IP
iSCSI Extensions for RDMA
ISCSI Extensions for RDMA
The iSCSI Extensions for RDMA is a computer network protocol that extends the Internet Small Computer System Interface protocol to use Remote Direct Memory Access...

(iSER), mapping of iSCSI over InfiniBand
InfiniBand
InfiniBand is a switched fabric communications link used in high-performance computing and enterprise data centers. Its features include high throughput, low latency, quality of service and failover, and it is designed to be scalable...

Storage networks may also be built using SAS

Serial Attached SCSI

Serial Attached SCSI is a computer bus used to move data to and from computer storage devices such as hard drives and tape drives. SAS depends on a point-to-point serial protocol that replaces the parallel SCSI bus technology that first appeared in the mid 1980s in data centers and workstations,...

and SATA

Serial ATA

Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...

technologies. SAS evolved from SCSI direct-attached storage. SATA evolved from IDE direct-attached storage. SAS and SATA devices can be networked using SAS Expanders.

SAN infrastructure

SANs often utilize a Fibre Channel fabric topology - an infrastructure specially designed to handle storage communications. It provides faster and more reliable access than higher-level protocols used in NAS

Network-attached storage

Network-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...

. A fabric is similar in concept to a network segment

Network segment

A network segment is a portion of a computer network. The nature and extent of a segment depends on the nature of the network and the device or devices used to interconnect end stations.-Ethernet:...

in a local area network. A typical Fibre Channel SAN fabric is made up of a number of Fibre Channel switch

Fibre Channel switch

In the computer storage field, a Fibre Channel switch is a network switch compatible with the Fibre Channel protocol. It allows the creation of a Fibre Channel fabric, that is currently the core component of most storage area networks . The fabric is a network of Fibre Channel devices which...

es.

Today, all major SAN equipment vendors also offer some form of Fibre Channel routing solution, and these bring substantial scalability benefits to the SAN architecture by allowing data to cross between different fabrics without merging them. These offerings use proprietary protocol elements, and the top-level architectures being promoted are radically different.
They often enable mapping Fibre Channel traffic over IP or over SONET/SDH

Synchronous optical networking

Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an...

.

Compatibility

One of the early problems with Fibre Channel SANs was that the switches and other hardware from different manufacturers were not compatible. Although the basic storage protocols 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 operating systems sharing the same fabric. Many solutions were pushed to the market before standards were finalized and vendors have since innovated around the standards.

SANs in media and entertainment

Video editing workgroups require very high data transfer rates. Outside of the enterprise market, this is one area that greatly benefits from SANs.

Per-node bandwidth usage control, sometimes referred to as Quality of Service

Quality of service

The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...

(QoS), is especially important in video workgroups as it ensures fair and prioritized bandwidth usage across the network, if there is insufficient open bandwidth available. Avid

AVID

AVID stands for:* Advancement Via Individual Determination, a college-readiness system designed to increase the number of students who enroll in four-year colleges in the U.S....

Unity, Apple's Xsan

Xsan

Xsan is Apple Inc.'s storage area network or clustered file system for Mac OS X. Xsan enables multiple Mac desktop and Xserve systems to access shared block storage over a Fibre Channel network. With the Xsan file system installed, these computers can read and write to the same storage volume at...

and Tiger Technology MetaSAN

MetaSAN

metaSAN is Tiger Technology's cross-platform Storage Area Network management software.metaSAN enables one to share one high-speed RAID devices with multiple computers using Fibre Channel, iSCSI, Ethernet, or InfiniBand interconnect...

are specifically designed for video networks and offer this functionality.

Storage virtualization

Storage Virtualization

Storage virtualization or storage virtualisation is a concept and term used within computer science. Specifically, storage systems may use virtualization concepts as a tool to enable better functionality and more advanced features within the storage system.Broadly speaking, a 'storage system' is...

is the process of completely abstracting logical storage from physical storage. The physical storage resources are aggregated into storage pools, from which the logical storage is created. It presents to the user a logical space for data storage and transparently

Transparency (computing)

Any change in a computing system, such as new feature or new component, is transparent if the system after change adheres to previous external interface as much as possible while changing its internal behaviour. The purpose is to shield from change all systems on the other end of the interface...

handles the process of mapping it to the actual physical location. This is implemented in modern disk arrays, using vendor proprietary solutions. However, the goal is to virtualize multiple disk arrays from different vendors, scattered over the network, into a single monolithic storage device, which can be managed uniformly.