Storage Basics: Network Attached Storage


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In this, the next article in our 'Storage Basics' series, we take a look at a technology called Network Attached Storage (NAS).

The most widely used method of storage on today's networks is direct attached storage (DAS). In a DAS configuration, storage devices such hard disks, CD-ROM drives and tape devices are attached directly to the system through which they are accessed. These systems are normally a network server running an operating system such as Microsoft Windows, Novell NetWare, Linux or Unix. The term 'direct' is used to describe the connection between device and server because it is exactly that. The connection is normally achieved using a storage device interface such as Integrated Drive Electronics, or more commonly in a server environment via the Small Computer Systems Interface (SCSI). DAS devices can be physically inside the server to which they are attached or they can be in external housings that are connected to the server by means of a cable. Of the two device interface standards discussed, only SCSI supports external devices in this way.

DAS is a very mature technology that can be implemented at relatively low cost. It does, however, have it's drawbacks. First is that the storage devices must be accessed via the server, thereby requiring and using valuable system resources. There is also the issue that, by placing the information on a server system, a licensed connection is needed to access the data and last, and perhaps least, it restricts the amount of disk space that can be used.

The solution to all of these problems is to take the storage devices away from the server and connect them directly to the network media. This is where NAS comes in. Before we continue our discussion, however, we should just point out that you must not confuse NAS with a storage area network (SAN). While both allow storage devices to be moved away from the server, SANs are mini networks dedicated to storage devices, while a NAS device is simply a storage subsystem that is connected to the network media.

NAS devices are very streamlined and dedicated to a single purpose; make data available to all clients in a heterogeneous network. Because NAS devices are dedicated to a single purpose, their hardware components, software and firmware are tightly integrated leading to more reliability than a traditional file server. With NAS devices, application incompatibilities that can cause a system to crash are a thing of the past, and with fewer hardware devices there is less to go wrong on that front as well.

NAS devices operate independently of network servers and communicate directly with the client, this means that in the event of a network server failure, clients will still be able to access files stored on a NAS device. The NAS device maintains its own file system and accommodates industry standard network protocols such as TCP/IP and IPX/SPX to allow clients to communicate with it over the network. To facilitate the actual file access, NAS devices will accommodate one, a couple, or all of the common file access protocols such as SMB, CIFS, NCP, HTTP and NFS.

One of the major considerations for file serving is of course file system security. NAS devices tackle this issue by either providing file system security capabilities of their own, or by allowing user databases on NOS to be used for authentication purposes. By providing multiple authentication measures, the flexibility of NAS solutions is further enhanced.

Apart from those already discussed, NAS also has other benefits. NAS allows devices to be placed close to the users who use them. Not only can this have the effect of reducing overall network traffic, it also allows users to physically access the NAS device if appropriate. Perhaps the best example of such a situation might be a CD Jukebox NAS system. Users could swap CD's in and out of the jukebox to make them available on the network. Although there may still be security issues that need addressing, such a situation is far safer than giving a user access to the server room to change the contents of a CD jukebox.

Perhaps one of the biggest advantages of NAS is that it offers platform independent access, an increasingly common consideration in today's heterogeneous networking environments. Because of the fact that many environments now use more than one operating system platform, NAS provides a mechanism that allows users to access data irrespective of what NOS is used to authorize them on the network.

One common misconception about NAS is that it is considerably faster than DAS, which is not the case. In terms of data retrieval from a storage device, the bottleneck is rarely the speed of storage devices or the server to which they are attached. Far more likely is that the speed of the network is the restricting factor. Consider that storage device throughput is generally measured in Megabytes per second whereas network media is measured in Megabits per second and you'll see what we mean.

So how easy is it to start using NAS? Well, easier than you might think. NAS devices from a number of companies now offer a great way for you to expand your storage infrastructure. Starting from just a few hundred dollars and going up from there, NAS devices are available as free-standing or rack-mounted units that can accommodate storage devices of all types and all capacities. Many NAS devices also incorporate technologies such as RAID and provide UPS capabilities as well. When you need a storage solution that is 'outside of the box' but you don't want to get involved with the technology and expense of a SAN, NAS is the way to go.

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