Network Attached Storage

Visual differentiation of NAS vs. SAN use in network architecture.

Network-attached storage (NAS) is file-level computer data storage connected to a computer network providing data access to heterogeneous clients.

Description

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.Template:Citation needed

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.

History

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 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 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), ONStor (acquired by LSI in 2009), IBRIX (acquired by HP), Isilon, PolyServe (acquired by HP in 2007), and Panasas, to name a few.

In 2009, NAS vendors (notably CTERA Networks and NETGEAR) began to introduce Online Backup solutions integrated in their NAS appliances, for online disaster recovery.

Benefits

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.

Drawbacks

Because of the need to support multiple protocols and the shared OS layer running on shared hardware, the NAS has its limitations compared to the DAS/SAN 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.

Certain NAS devices 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. One may still use rsync, but through an NFS or CIFS client; that method fails to enumerate huge file hierarchies at the nominal speed of local drives and induces considerable network traffic.

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.

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.

Open source implementations

Open source NAS-oriented distributions of Linux and FreeBSD are available, including FreeNAS, Gluster, Openfiler and the Ubuntu-based TurnKey File Server. Ubuntu-based TurnKey File Server These are designed to be easy to setup on commodity PC hardware, and are typically 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 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.

Clustered NAS

A clustered NAS is a NAS that is 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. The examples of commercial products are GlusterFS and IBM Scale-out File Services.

External links

NAS-Central - A community site detailing information on many NAS devices and associated articles