Network-attached storage ( NAS ) is a file-level computer data storage server connected to a computer network providing data access to a heterogeneous group of clients. NAS is specialised for serving files either by its hardware, software, or configuration. It is most often manufactured as a computer appliance – a purpose-built specialised computer. NAS systems are networked appliances which contain one or more storage drives, most often arranged into logical, redundant storage containers or RAID.
Network-attached storage removes the responsibility of file serving from additional servers on the network. They ordinarily provide access to files using network file sharing protocols like NFS, SMB / CIFS, or AFP. As of 2010, NAS devices began gaining popularity as a convenient method of sharing files amongst multiple computers.  Potential benefits of dedicated network-attached storage, compared to general-purpose servers additionally serving files, include faster data access, easier administration, and simple configuration. 
The hard disc drives with "NAS" in their name are functionally similar to additional drives but might have different firmware, vibration tolerance, or power dissipation to make them more suitable for use in RAID arrays, which are at times used in NAS implementations.  For instance, a few NAS versions of drives support a command extension to allow extended error recovery to be disabled. In a non-RAID application, it might be important for a disc drive to go to great lengths to successfully read a problematic storage block, even if it takes several seconds. In an appropriately configured RAID array, a single bad block on a single drive can be recovered completely via the redundancy encoded across the RAID set. If a drive spends several seconds executing extensive retries it might cause the RAID controller to flag the drive as "down" whereas if it simply replied promptly that the block of data had a checksum error, the RAID controller would use the redundant data on the additional drives to correct the error and continue without any problem. Such a "NAS" SATA hard disc drive can be used as an internal PC hard drive, without any problems or adjustments needed, as it simply supports additional options and might possibly be built to a higher quality standard (particularly if accompanied by a higher quoted MTBF figure and higher price) than a regular consumer drive.
A NAS unit is a computer connected to a network that provides only file-based data storage services to additional devices on the network. Although it might technically be possible to run additional software on a NAS unit, it is most of the time not designed to be a general-purpose server. For instance, NAS units most of the time don't have a keyboard or display, and are controlled and configured over the network, most often using a browser.
A full-featured operating system isn't needed on a NAS device, so most often a stripped-down operating system is used. For instance, FreeNAS or NAS4Free, both open source NAS solutions designed for commodity PC hardware, are implemented as a stripped-down version of FreeBSD.
NAS systems contain one or more hard disc drives, most often arranged into logical, redundant storage containers or RAID.
NAS uses file-based protocols like NFS (popular on UNIX systems), SMB/CIFS ( Server Message Block/Common Internet File System) (used with MS Windows systems), AFP (used with Apple Macintosh computers), or NCP (used with OES and Novell NetWare). NAS units rarely limit clients to a single protocol.
NAS vs. DAS
The key difference between direct-attached storage (DAS) and NAS is that DAS is simply an extension to an existing server and isn't necessarily networked. NAS is designed as an easy and self-contained solution for sharing files over the network.
When both are served over the network, NAS could have better performance than DAS, because the NAS device can be tuned precisely for file serving which is less likely to happen on a server responsible for additional processing. Both NAS and DAS can have numerous amount of cache memory, which greatly affects performance. When comparing use of NAS with use of local (non-networked) DAS, the performance of NAS depends mainly on the speed of and congestion on the network.
NAS is ordinarily not as customizable in terms of hardware (CPU, memory, storage components) or software (extensions, plug-ins, additional protocols) as a general-purpose server supplied with DAS.
NAS vs. SAN
NAS provides both storage and a file system. This is most often contrasted with SAN ( Storage Area Network), which provides only block-based storage and leaves file system concerns on the "client" side. SAN protocols include Fibre Channel, iSCSI, ATA over Ethernet (AoE) and HyperSCSI.
One way to loosely conceptualise 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 disc available through a SAN still appears to the client OS as a disk, visible in disc and volume management utilities (along with client's local disks), and available to be formatted with a file system and mounted.
Despite their differences, SAN and NAS aren't mutually exclusive, and might 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-based systems. A shared disc file system can additionally be run on top of a SAN to provide filesystem services.
In the early 1980s, the "Newcastle Connection" by Brian Randell and his colleagues at Newcastle University demonstrated and developed remote file access across a set of UNIX machines.  Novell's NetWare server operating system and NCP protocol was released in 1983. Following the Newcastle Connection, Sun Microsystems' 1984 release of NFS allowed network servers to share their storage space with networked clients. 3Com and Microsoft would develop the LAN Manager software and protocol to further this new market. 3Com's 3Server and 3+Share software was the first purpose-built server (including proprietary hardware, software, and multiple disks) for open systems servers.
Inspired by the success of file servers from Novell, IBM, and Sun, several firms developed dedicated file servers. While 3Com was amongst 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 in the early 1990s to create the integrated NetApp filer, which supported both the Windows CIFS and the UNIX NFS protocols, and had superior scalability and ease of deployment. This started the market for proprietary NAS devices now led by NetApp and EMC Celerra.
Starting in the early 2000s, a series of startups emerged offering alternative solutions to single filer solutions in the form of clustered NAS – Spinnaker Networks (acquired by NetApp in February 2004), Exanet (acquired by Dell in February 2010), Gluster (acquired by RedHat in 2011), ONStor (acquired by LSI in 2009), IBRIX (acquired by HP), Isilon (acquired by EMC – November 2010), PolyServe (acquired by HP in 2007), and Panasas, to name a few.
The way manufacturers make NAS devices can be classified into three types:
- Computer based NAS – Using a computer (Server level or a personal computer), instals FTP/SMB/AFP... software server. The power consumption of this NAS type is the largest, but its functions are the most powerful. Some large NAS manufacturers like QNAP, Synology and ASUStor make these types of devices. Max FTP throughput speed varies by computer CPU and amount of RAM.
- Embedded system based NAS – Using an ARM or MIPS based processor architecture and a real-time operating system (RTOS) or an embedded operating system to run a NAS server. The power consumption of this NAS type is fair, and functions in the NAS can fit most end-user requirements. Marvell, Oxford, and Storlink make chipsets for this type of NAS. Max FTP throughput varies from 20 MB/s to 120 MB/s.
- ASIC based NAS – Provisioning NAS through the use of a single ASIC chip, using hardware to implement TCP/IP and file system. There is no OS in the chip, as all the performance-related operations are done by hardware acceleration circuits. The power consumption of this type of NAS is low, as functions are limited to only support SMB and FTP. LayerWalker is the sole chipset manufacturer for this type of NAS. Max FTP throughput is 40 MB/s.
NAS is useful for more than just general centralised storage provided to client computers in environments with large amounts of data. NAS can enable simpler and lower cost systems like 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's a large amount of multi-media data. Such consumer market appliances are now commonly available. Unlike their rackmounted counterparts, they're ordinarily 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.
Open-source server implementations
Open-source NAS-oriented distributions of Linux and FreeBSD are available, including FreeNAS, NAS4Free, CryptoNAS, NASLite, Gluster, Openfiler, OpenMediaVault, EasyNAS, Rockstor and the Debian-based TurnKey File Server.  These are designed to be easy to set up on commodity PC hardware, and are ordinarily configured using a web browser.
They can run from a virtual machine, Live CD, bootable USB flash drive ( Live USB), or from one of the mounted hard drives. They run Samba (an SMB daemon), NFS daemon, and FTP daemons which are freely available for those operating systems.
NexentaStor, built on the Nexenta Core Platform, is similar in that it is built on open source foundations; however, NexentaStor requires more memory than consumer-oriented open source NAS solutions and additionally contains most of the features of enterprise class NAS solutions, like snapshots, management utilities, tiering services, mirroring, and end-to-end checksumming due, in part, to the use of ZFS.
List of network protocols used to serve NAS
- ATA over Ethernet (AoE)
- Andrew File System (AFS)
- Apple Filing Protocol (AFP)
- Server Message Block (SMB, aka. CIFS)
- File Transfer Protocol (FTP)
- Hypertext Transfer Protocol (HTTP)
- Network block device ( nbd )
- Network File System (NFS)
- SSH file transfer protocol (SFTP)
- Universal Plug and Play (UPnP)
A clustered NAS is a NAS that's using a distributed file system running simultaneously on multiple servers. The key difference between a clustered and traditional NAS is the ability to distribute (e.g. stripe) data and metadata across the cluster nodes or storage devices. Clustered NAS, like a traditional one, still provides unified access to the files from any of the cluster nodes, unrelated to the actual location of the data.