Network-attached storage (NAS) is hard disk storage that is set up with its own network address rather than being attached to the department computer that is serving applications to a network's workstation users. By removing storage access and its management from the department server, both application programming and files can be served faster because they are not competing for the same processor resources. The network-attached storage device is attached to a local area network (typically, an Ethernet network) and assigned an IP address. File requests are mapped by the main server to the NAS file server.

A NAS unit is a computer connected to a network that only provides file-based data storage services to other devices on the network. Although it may technically be possible to run other software on a NAS unit, it is not designed to be a general purpose server. For example, NAS units usually do not have a keyboard or display, and are controlled and configured over the network, often using a browser.

A fully-featured operating system is not needed on a NAS device, so often a stripped-down operating system is used. For example, FreeNAS, an open source NAS solution designed for commodity PC hardware, is implemented as a stripped-down version of FreeBSD.

NAS systems contain one or more hard disks, often arranged into logical, redundant storage containers or RAID arrays (redundant arrays of inexpensive/independent disks). NAS removes the responsibility of file serving from other servers on the network.

NAS uses file-based protocols such as NFS (popular on UNIX systems), SMB/CIFS (Server Message Block/Common Internet File System) (used with MS Windows systems), or AFP (used with Apple Macintosh computers). NAS units rarely limit clients to a single protocol.

NAS vs SAN

NAS provides both storage and a file system. This is often contrasted with SAN (Storage Area Network), which provides only block-based storage and leaves file system concerns on the "client" side. SAN protocols are SCSI, Fibre Channel, iSCSI, ATA over Ethernet (AoE), or HyperSCSI.

Despite their differences, SAN and NAS are not mutually exclusive, and may be combined as a SAN-NAS hybrid, offering both file-level protocols (NAS) and block-level protocols (SAN) from the same system. An example of this is Openfiler, a free software product running on Linux.

The key difference between direct attached storage (DAS) and NAS is that DAS is simply an extension to an existing server and is not networked while NAS sits on a network as its own entity; it is easier to share files with NAS. Availability of data might potentially be increased with NAS if it provides built-in RAID and clustering capabilities. 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 on the NAS computers or devices.

NAS Protocols

The two application protocols most commonly associated with NAS are Sun Network File System (NFS) and Common Internet File System (CIFS). Both NFS and CIFS operate in client/server fashion. Both predate the modern NAS by many years; original work on these protocols took place in the 1980s.

NFS was developed originally for sharing files between UNIX systems across a LAN. Support for NFS soon expanded to include non-UNIX systems; however, most NFS clients today are computers running some flavor of the UNIX operating system.

The CIFS was formerly known as Server Message Block (SMB). SMB was developed by IBM and Microsoft to support file sharing in DOS. As the protocol became widely used in Windows, the name changed to CIFS. This same protocol appears today in UNIX systems as part of the Samba package.

Many NAS systems also support Hypertext Transfer Protocol (HTTP). Clients can often download files in their Web browser from a NAS that supports HTTP. NAS systems also commonly employ HTTP as an access protocol for Web-based administrative user interfaces.

The new breed of NAS networking products has succeeded in providing a reasonable alternative to traditional file servers in client/server networks. Entry-level NAS products containing 20-50 gigabytes of storage can be purchased for $500 (USD) or less, whereas mid-range and high-end NAS systems can run in the tens of thousands of dollars. Besides cost, a NAS promises reliable operation and easy management. Look for the Network Attached Storage technology to keep evolving as the field matures over the next several years.