A storage area network 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.
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. SANs also tend to enable more effective disaster recovery processes. A SAN could span a distant location containing a secondary storage array.
This enables storage replication either implemented by disk array controllers, by server software, or by specialized SAN devices. Since IP WANs are often the least costly method of long-distance transport, the Fibre Channel over IP (FCIP) and iSCSI protocols have been developed to allow SAN extension over IP networks. The traditional physical SCSI layer could support only a few meters of distance – not nearly enough to ensure business continuance in a disaster.