STORAGE - NAS
Definição - Network-attached storage (NAS) dispositivo de armazenagem de dados dedicado o qual pode ser conectado diretamente a uma rede de computadores.
Utilização - Prover um acesso e armazenagem centralizados aos dados para multiusuários multiplataforma em rede (+ gerenciamento dessas funcionalidades). (diferente do FS tradicional ou DAS). Geralemente com RAID's (redundant arrays of independent disks).
NAS x SAN - The boundaries between NAS and storage area network systems are also starting to overlap, with some products making the obvious next evolution and offering both file level protocols (NAS) and block level protocols (SAN) from the same system. An excellent example of this is Openfiler the opensource product running on Linux. San Magazine did a very informative review of this hybrid functionality.
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, too many I/O operations, 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 its own hardware, which is in most cases not upgradable.
The key difference between DAS and NAS is the reduced CPU and I/O power offered by the latter.
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 . More recently, home NAS devices have incorporated support for the Universal Plug and Play protocol, enabling them to serve the growing number of networked home media players.
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. The term "NAS head" is sometimes also used to refer to the portion of a self-contained NAS system other than its storage. An example would be the ONStor Bobcat.