What is Storage Area Network?

A  storage area network ( SAN) is a network which provides access to consolidated, block level data storage. SANs are primarily used to enhance storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear to the operating system as locally attached devices. A SAN typically has its own network of storage devices that are generally not accessible through the local area network (LAN) 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 shared-disk file systems.

Network-attached storage (NAS) was designed independently of SAN systems. In both a NAS and SAN, the various computers in a network, such as individual users' desktop computers and dedicated servers running applications ("application servers"), can share a more centralized collection of storage devices via a network connection such as a local area network (LAN).

Concentrating the storage on one or more NAS servers or in a SAN instead of placing storage devices on each application server allows application server configurations to be optimized for running their applications instead of also storing all the related data and moves the storage management task to the NAS or SAN system. Both NAS and SAN have the potential to reduce the amount of excess storage that must be purchased and provisioned as spare space. In a DAS-only architecture, each computer must be provisioned with enough excess storage to ensure that the computer does not run out of space at an untimely moment. In a DAS architecture the spare storage on one computer cannot be utilized by another. With a NAS or SAN architecture, where storage is shared across the needs of multiple computers, one normally provisions a pool of shared spare storage that will serve the peak needs of the connected computers, which typically is less than the total amount of spare storage that would be needed if individual storage devices were dedicated to each computer.

In a NAS the storage devices are directly connected to a file server that makes the storage available at a file-level to the other computers. In a SAN, the storage is made available at a lower "block-level", leaving file system concerns to the "client" side. SAN protocols include Fibre Channel, iSCSI, ATA over Ethernet (AoE) and HyperSCSI. One way to loosely conceptualize the difference between a NAS and a SAN is that NAS appears to the client OS (operating system) as a file server (the client can map network drives to shares on that server) whereas a disk available through a SAN still appears to the client OS as a disk, visible in disk and volume management utilities (along with client's local disks), and available to be formatted with a file system and mounted.

One drawback to both the NAS and SAN architecture is that the connection between the various CPUs and the storage units are no longer dedicated high-speed busses tailored to the needs of storage access. Instead the CPUs use the LAN to communicate, potentially creating bandwidth as well as performance bottlenecks. Additional data security considerations are also required for NAS and SAN setups, as information is being transmitted via a network that potentially includes design flaws, security exploits and other vulnerabilities that may not exist in a DAS setup.

While it is possible to use the NAS or SAN approach to eliminate all storage at user or application computers, typically those computers still have some local Direct Attached Storage for the operating system, various program files and related temporary files used for a variety of purposes, including caching content locally.