NAS differs from the traditional file serving and Direct Attached Storage in that the operating system and other software on the NAS unit provide only the functionality of data storage, data access and the management of these functionalities. Furthermore, the NAS unit does not limit clients to only one file transfer protocol. NAS systems usually contain one or more hard disks, often arranged into logical, redundant storage containers or RAIDs (redundant arrays of independant disks), as do traditional file servers. NAS removes the responsibility of file serving from other servers on the network and can be deployed via commercial embedded units or via standard computers running NAS software.
NAS uses file-based protocols such as NFS (popular on UNIX systems) or SMB (Server Message Block) (used with MS Windows systems). Contrast NAS's file-based approach and use of well-understood protocols with storage area network (SAN) which uses a block-based approach and generally runs over SCSI over Fibre Channel or iSCSI. (There are other SAN protocols as well, such as ATA over Ethernet and HyperSCSI, which however are less common.)
Minimal-functionality or stripped-down operating systems are used on NAS computers or devices which run the protocols and file applications that provide the NAS functionality. A "leaned-out" FreeBSD is used in FreeNAS, for example, which is open source NAS software meant to be deployed on standard computer hardware. Commercial embedded devices and consumer "network appliances" may use closed source operating systems and protocol implementations.
Network-attached storage was introduced with the early file sharing Novell's NetWare server operating system and NCP protocol in 1983. In the UNIX world, Sun Microsystems' 1984 release of NFS allowed network servers to share their storage space with networked clients. 3Com's 3Server and 3+Share software was the first purpose-built servers (including proprietary hardware, software, and multiple disks) for open systems servers, and the company led the segment from 1985 through the early 1990s. 3Com and Microsoft would develop the LAN Manager software and protocol to further this new market. Inspired by the success of file servers from Novell, IBM, and Sun, several firms developed dedicated file servers. While 3server was among the first firms to build a dedicated NAS for desktop operating systems, Auspex Systems was one of the first to develop a dedicated NFS server for use in the UNIX market. A group of Auspex engineers split away to create the integrated Network Appliance "filer", which supported both Windows and UNIX, in the early 1990s, starting the market for proprietary NAS arrays.
Availability of data can potentially be increased with NAS because data access is not dependent on a server: the server can be down and users will still have access to data on the NAS. Performance can be increased by NAS because the file serving is done by the NAS and not done by a server responsible for also doing other processing. The performance of NAS devices, though, depends heavily on the speed of and traffic on the network and on the amount of cache memory (the equivalent of RAM) on the NAS computers or devices. Scalability of NAS is not limited by the number of internal or external ports of a server's data bus, as a NAS device can be connected to any available network jack. NAS can be more reliable than DAS because it separates the storage from the server. If the server fails, there is unlikely to be file system corruption, although partially-created files may linger. However, if the power source or OS of the NAS fails, corruption is still possible.
Due to the multiprotocol, and the reduced CPU and OS layer, the NAS has its limitations compared to the DAS/FC systems. If the NAS is occupied with too many users or too many I/O or CPU processing power that is too demanding , the NAS reaches its limitations. A server system is easily upgraded by adding one or more servers into a cluster, so CPU power can be upgraded. While the NAS is limited to it's own hardware, which is in most cases not upgradable.
NAS is useful for more than just general centralized storage provided to client computers in environments with large amounts of data. NAS can enable simpler and lower cost systems such as load-balancing and fault-tolerant email and web server systems by providing storage services. The potential emerging market for NAS is the consumer market where there is a large amount of multi-media data. Such consumer market appliances are now commonly available. Unlike their rackmounted counterparts, they are generally packaged in smaller form factors. The price of NAS appliances has plummeted in recent years, offering flexible network based storage to the home consumer market for little more than the cost of a regular USB or FireWire external hard disk. Many of these home consumer devices are built around ARM, PowerPC or MIPS processors running an embedded Linux operating system. Examples include Buffalo's TeraStation and Linksys NSLU2.
A NAS head refers to a NAS which does not have any on-board storage, but instead connects to a SAN. In effect, it acts as a translator between the file-level NAS protocols (NFS,CIFS,etc.) and the block-level SAN protocols (Fibre Channel, iSCSI). Thus it can combine the advantages of both technologies.