What is NAS

Traditional direct attached storage (DAS) is connected directly to a general purpose file server and is accessed through the IP address of the hosting server. A SAN is a set of switches that is used to connect central storage devices to servers.

Network-attached storage (NAS) is a combination of disk storage possibly in a RAID configuration, and operating system and software for configuration and file mapping. NAS also provides centralised file system functions. NAS is attached to the local area network with its own IP address. So NAS is a device, not a network infrastructure, and shared storage is either internal to the NAS device or attached to it, it is not attached to the file servers.

The advantage of NAS is that it concentrates on managing the file requests and storage; it is not sharing processor power with applications and general users. This also means that it is faster to reboot if problems occur. Because Network speeds are now as fast as internal server processing, NAS is as fast as DAS.

NAS components

NAS Head or Gateway

The hardware that performs the NAS control functions is called a NAS head or NAS gateway. The clients always connect to the NAS head, as it is the NAS head is addressable on the network. A NAS head is usually a discrete hardware device that is independent of the storage devices and contains an imbedded operating system that does not need a keyboard, mouse or monitor. A storage administrator accesses the appliance and manages the disk resources from a remote console. Disks and in some cases tape drives are attached to the NAS head for capacity. NAS Heads are also sometimes called NAS appliances, based on the ideas that NAS is a commodity item like a toaster or washing machine.

NAS file systems and protocols

The most popular NAS protocols are

• NFS (Sun Network File System) which was developed for UNIX but also supports other systems
• CIFS (Common Internet File System) which was developed Windows operating systems
• HTTP (Hypertext Transfer Protocol) which supports web browsers and is commonly used for administrator interfaces

NAS devices support true file sharing between NFS and CIFS, which covers the majority of computers today. The NAS vendors can either implement these protocols with their own software, or use a freeware program like SAMBA.

NAS or SAN

At a functional level SAN and NAS both supply data storage capability to other network devices. SANs work on a private, usually fibre channel network and connect storage devices to servers with switches. NAS devices usually connect directly to the LAN with Ethernet and so are simpler network storage solutions.
However this simple definition does not work these days as SANs can use Ethernet or iSCSI for connection, and NAS devices can use fibre channel and can connect to, and so be part of a SAN. The only real difference is one of network protocol. SANs transfer data in fixed size blocks as the file systems reside on the servers, while NAS systems contain the file systems, so data transfer works at file level.

The bottom line is that you do not have to choose between NAS and SAN, they can co-exist in your environment

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