Network-attached storage
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Network-attached storage (NAS) is file-level computer data storage connected to a computer network providing data access to heterogeneous network clients.
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[edit] Description
A NAS unit is essentially a self-contained computer connected to a network, with the sole purpose of supplying file-based data storage services to other devices on the network. The operating system and other software on the NAS unit provide the functionality of data storage, file systems, and access to files, and the management of these functionalities. The unit is not designed to carry out general-purpose computing tasks, although it may technically be possible to run other software on it. NAS units usually do not have a keyboard or display, and are controlled and configured over the network, often by connecting a browser program to their network address.
The alternative to NAS storage on a network is to use a computer as a file server. In its most basic form a dedicated file server is no more than a NAS unit with keyboard and display and an operating system which, while optimised for providing storage services, can run other tasks; however, file servers are increasingly used to supply other functionality, such as supplying database services, email services, and so on.
A general-purpose operating system is not needed on a NAS device, and often minimal-functionality or stripped-down operating systems are used. For example FreeNAS, which is open source NAS software designed for use on standard computer hardware, is just a version of FreeBSD with all functionality not related to data storage stripped out. NASLite as the name suggests is a highly optimized version of Linux running from a floppy disk for the sole purpose of a NAS. Likewise, NexentaStor is based upon the core of the NexentaOS, an open source hybrid operating system with an OpenSolaris core and a Linux user environment.
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) or SMB (Server Message Block) (used with MS Windows systems). NAS units rarely limit clients to a single protocol.
NAS provides both storage and filesystem. This is often contrasted with SAN (Storage Area Network), which provides only block-based storage and leaves filesystem concerns on the "client" side. SAN protocols are SCSI, Fibre Channel, iSCSI, ATA over Ethernet, or HyperSCSI.
Despite differences SAN and NAS are not exclusive and may be combined in one solution: SAN-NAS hybrid
The boundaries between NAS and SAN systems are 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. However a SAN device is usually served through NAS as one large flat file, not as a true filesystem. An example of this is Openfiler, a free product running on Linux.
[edit] History
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 in the early 1990s to create the integrated NetApp filer, which supported both Windows' CIFS and UNIX'es NFS, and had superior scalability and ease of deployment. This started the market for proprietary NAS devices.
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 (now acquired by NetApp), Exanet, IBRIX, Isilon, PolyServe (acquired by Hewlett-Packard in 2007), to name a few.
[edit] Benefits
Availability of data might potentially be increased with NAS if it provides built-in RAID and clustering.
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 (RAM) on the NAS computers or devices.
It should be noted that NAS is effectively a server in itself, with all major components of a typical PC – a CPU, motherboard, RAM, etc. – and its reliability is a function of how well it is designed internally. A NAS without redundant data access paths, redundant controllers, redundant power supplies, is probably less reliable than Direct Attached Storage (DAS) connected to a server which does have redundancy for its major components.
[edit] Drawbacks
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 upgradeable.
NAS will also fail to expose well-known services that are typical of a file server, or enable them in a way that is not efficient. Examples are: ability to compute disk usage of separate directories, ability to index files rapidly (locate), ability to mirror efficiently with rsync. You may still rsync, but through an NFS client; that method fails to enumerate huge file hierarchies at the nominal speed of local drives and induces considerable network traffic.
The key difference between 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. NAS typically has less CPU and I/O power compared to DAS.
[edit] NAS uses
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 [1] and Linksys NSLU2 [2]. 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.
[edit] NAS heads
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, NexentaStor, from [Nexenta]. EMC Celerra, also functions in this capability and a developer edition is available at no cost.
[edit] NAS operating systems for consumer PCs
Open source NAS-oriented distributions of Linux and FreeBSD are available, including FreeNAS, NASLite and Openfiler. They are easy to configure via a Web-based Interface and run on low-end conventional computers. They can run from a Live CD, bootable USB flash drive (Live USB), or from one of the mounted hard drives. They run Samba, NFS daemon, and FTP daemons which are freely available for those operating systems. An article has been published on building a cheap NAS device from a general-purpose computer and cheap PATA drives: Build a Cheap and Fast RAID 5 NAS.
NexentaStor, built on the NexentaCore 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 also contains most of the features of enterprise class NAS solutions, such as snapshots, management utilities, tiering services, mirroring, and end to end check summing due, in part, to the use of ZFS.
