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SAN - Storage Area Network


A Storage area network, or SAN, is a high-speed network of storage devices that also connects those storage devices with servers. It provides block-level storage that can be accessed by the applications running on any networked servers. SAN storage devices can include tape libraries, and, more commonly, disk-based devices, like RAID hardware.

SAN Vs. DAS Performance

Organizations often choose to deploy SANs because they offer better flexibility, availability and performance than direct-attached storage (DAS). Because a SAN removes storage from the servers and consolidates it in a place where it can be accessed by any application, it tends to improve storage utilization. Those utilization improvements often allow organizations to defer purchases of additional storage hardware, which saves money and requires less space in the data center.

Recommended Reading: Webopedia's direct-attached storage (DAS) definition.

Thanks to high-speed connections (usually Fibre Channel), SANs often provide better performance than DAS. Also, because SANs usually offer multiple connections to and from the data center's servers, they also improve availability. In addition, separating the storage from the servers frees up the computing resources on the servers for other tasks not related to storage.

SANs Simplify Some Management Tasks

SANs are particularly helpful in backup and disaster recovery settings. Within a SAN, data can be transferred from one storage device to another without interacting with a server. This speeds up the backup process and eliminates the need to use server CPU cycles for backup. Also, many SANs utilize Fibre Channel technology or other networking protocols that allow the networks to span longer distances geographically. That makes it more feasible for companies to keep their backup data in remote locations.

Utilizing a SAN can also simplify some management tasks, potentially allowing organizations to hire fewer IT workers or to free up some IT workers for other tasks. It's also possible to boot servers from a SAN, which can reduce the time and hassles involved in replacing a server.

SAN Alternatives

Before the advent of SANs, organizations generally used direct-attached storage (DAS). As the name implies, direct-attached storage is directly attached to the server, residing either on the server or in a standalone storage device that is not part of a separate storage networking environment. Many smaller organizations continue to use DAS today because it offers lower upfront costs than deploying a SAN. However, for larger companies, the benefits of a SAN often outweigh the costs.

Sometimes people confuse the term SAN with the term NAS, which stands for "network-attached storage." The key to distinguishing the two lies in the last term of each acronym: a SAN (storage area network) is an actual network, while NAS (network-attached storage) refers to a storage device, typically in an IP network. While SANs provide block-level storage for servers, a NAS device provides file-level storage for end users. For example, the mail application on your company servers might utilize a SAN to store all the messages, contacts and other data it requires; by contrast, an end user would use a NAS device to save files, such as word processing documents or spreadsheets. Operating systems see a SAN as a disk, while they see a NAS device as a file server.

Making things somewhat more confusing, some storage systems take a hybrid approach, offering some SAN capabilities as well as some NAS capabilities. It's also possible to include NAS devices within a SAN.

SAN Implementation

To set up a simple SAN, you need only three major components: a SAN switch, a storage device and a server. You'll also require cables to connect the various elements together and SAN management software. In most real-world settings, a SAN will include many different switches, storage devices and servers, and it will likely also include routers, bridges and gateways to extend the SAN over large areas and to connect to other parts of the data center network. The SAN's topology will depend on its size and the needs of the organization.

The process of deploying a SAN requires several steps. First, you need to design your SAN, taking into account your current needs and future scalability requirements. Second, you'll need to select a vendor or vendors to provide the hardware and software you'll need, as well as any related services. Next, you'll install the necessary hardware and then install and configure the software for managing your SAN. Deploying a SAN is a complicated process that often requires specialized knowledge and a great deal of planning, particularly if your SAN is very large.

SAN Vendors

Numerous vendors offer the switches, storage devices, management software and other components necessary for setting up a SAN. They include the following:

Open source projects related to SAN technology include the following:

The SAN switch market is dominated by Cisco and Brocade, while the HBA market is dominated by QLogic and Emulex.

SAN Technology

Several different industry groups have developed standards related to SAN technology. The most prominent is probably the Storage Networking Industry Association (SNIA), which promotes the Storage Management Initiative Specification (SMI-S), as well as related standards. The Fibre Channel Industry Association (FCIA) also promotes standards related to SAN and administers the SANmark Qualified Program.

Fibre Channel is currently the most widely used communication protocol for SANs, but it is by no means the only one. Some SAN networks rely on iSCSI communication, a mapping of SCSI protocol over TCP/IP. SANs can also use ATA over Ethernet (AoE), Fibre Channel over Ethernet (FCoE), ESCON over Fibre Channel, HyperSCSI and some other protocols.

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