Demystifying SAN , NAS And DAS

 

SAN Described:
A storage area network is, quite simply, a network dedicated to storage. More precisely, the technical dictionary published by the Storage Networking Industry Association (SNIA)1 defines a storage area network as:

A network whose primary purpose is the transfer of data between computer systems and storage elements and among storage elements. Abbreviated SAN. A SAN consists of a communication infrastructure, which provides physical connections, and a management layer, which organizes the connections, storage elements and computer systems so that data transfer is secure and robust.
Unlike the traditional DAS direct attach storage model, a SAN attaches storage devices to servers in a networked fashion, using hubs, switches, routers and bridges to build the topology (see Figure 1). Both the systems and the storage devices can, in theory, be heterogeneous in nature, though today interoperability concerns limit some customers to building homogeneous SANs. Although the network could conceivably be built with any networking technology, Fibre Channel has emerged as the technology of choice for SANs.

Figure 1: Typical Current SAN Connectivity Requirements

SANs provide a number of advantages over direct attached storage. They provide any-to-any connectivity between servers and storage devices, making possible the sharing of storage resources between multiple servers and thus enabling IT managers to consolidate storage on a few large storage platforms. They also provide any-to-any connectivity between the storage devices themselves, opening the way for direct movement of data between storage devices, vastly improving efficiency of data movement and processes such as data backup or replication. The use of Fibre Channel, or most any other networking technology proposed for SANs, enables longer connectivity distances and higher performance than currently possible with SCSI technology. Over time, SAN technology will ease the task of centralized storage management and drive the adoption of remote management and data protection strategies, storage consolidation, system clustering and cross-platform data sharing.
The SAN market is made up of the vendors of Fibre Channel interconnect technology, as well as the vendors of the systems and storage devices that attach to the network. The Fibre Channel vendors are primarily new, relatively small companies such as Brocade, Vixel, Gadzoox and Crossroads. The storage companies are the same ones that have been providing direct-attach storage for years, such as EMC, Hitachi, Sun, HP and Compaq; and it is no exaggeration to say that every storage company is involved in the SAN market.

NAS Described:
Network attached storage, on the other hand, describes file storage attached to a network. The SNIA Technical Dictionary defines network attached storage as:

A term used to refer to storage elements that connect to a network and provide file access services to computer systems. Abbreviated NAS. A NAS storage element consists of an engine, which implements the file services, and one or more devices on which data is stored. NAS elements may be attached to any type of network. When attached to SANs, NAS elements may be considered to be members of the SAS (SAN attached storage) class of storage elements.

A class of systems that provide file services to host computers. A host system that uses network attached storage uses file system device driver to access data using file access protocols such as NFS or CIFS. NAS systems interpret these commands and perform the internal file and device I/O operations necessary to execute them.
Note that the SNIA definition says that a NAS system may be connected to any type of network. This is an important future consideration, which will be discussed shortly. Today, however, NAS systems are generally connected to a local area network (LAN).
In common usage, a NAS system is a special- purpose device that is designed to serve files to clients over a LAN (see Figure 2). The clients request access to files using standard network file system (NFS) or common Internet file system (CIFS) commands. NAS devices typically contain embedded processors hosting a specialized operating system, or microkernel, and a highly optimized file system both designed to enable the NAS device to serve up files to clients with very high performance. Because they can serve multiple heterogeneous clients, NAS devices provide a form of heterogeneous data sharing.
 

Figure 2: NAS system serves files over a LAN.

Although the attributes of specific NAS products vary, NAS vendors generally attempt to adhere to the "appliance" model of computing. That is, NAS devices are designed to do one thing ­ file serving ­ and to do it very well. Moreover, they are typically designed to be very simple to install and configure. The storage they provide is often housed within the device's enclosure, though some NAS devices allow for the attachment of external storage.
The NAS market was pioneered by companies such as Network Appliance and Auspex, which provide NAS systems for workgroup and enterprise customers. As the NAS market has grown, new vendors such as Connex and CDS, are attempting to stake out niches in the mid-range and low end, while system and storage vendors such as HP, Sun and EMC have also entered the market.

NAS devices advantages:
Independence: - A NAS can sit anywhere on the network, independent of servers, and serve files to any network connected PC or server. If a server or PC goes down, the NAS is still functional. If power goes down, there's no need for complex reconfiguration. With its simple architecture and setup, a NAS can be up and running again in minutes.

Ease of Use: - NAS devices typically come as preconfigured, turnkey solutions. There's no need to install a host adapter or operating system. You simply plug the NAS into the network and, depending on the ease of use of the user interface, you do some very light configuration using a Web browser. There may be a little more configuration to do on PC's and servers accessing the device, but in most cases you're up and running in minutes. Compared to traditional servers, NAS units requires little maintenance, few updates, and little troubleshooting. Whatever administration is necessary can usually be done via a simple Web browser interface.

Easy Upgrades: - Adding storage to a server usually requires shutting down the server, replacing a drive or adding a new one and then firing up the server again. To get more storage with NAS, you simply plug another NAS device into the network and are up and running with additional shared file storage in minutes. Or some NAS devices allow swapping of hard drives or adding internal or external storage while they are in operation (commonly known as "hot swap").

Flexibility: - Many NAS devices can share their files easily among Windows, Mac, Unix, and Linux-based PCs. Some are also flexible enough to be used as a NAS, as DAS for a single server, or,as a storage device on a SAN. Many come with capabilities for sharing printers.

Easy Backup: - NAS devices can be a great storage medium for PC-based backups. Many of these devices come with backup software that is easy to configure and use, both for backing up user PCs to the NAS and backing up the NAS to another storage device, tape, or an external backup service. When all your files are in one place, backup is inherently easier than when they are spread around the office. Some NAS's also come with easy tools for migrating data to the device and replicating data over the network from storage device to storage device.



Can SAN and NAS work together?
SAN describes a networked storage topology and NAS describes a highly optimized network file server. The questions asked by the IT managers, then, typically come down to some variation of the following:

Can SAN and NAS be used together or must I choose to base my infrastructure on one or the other?
When do I choose which technology?
The first question arises because, just as NAS provides high-performance shared access to (file system) data, one of the promises of SAN is also to provide high-performance storage and data sharing. The good news is that the choice between SAN and NAS is not an either/or decision. SAN topologies and NAS devices do, in fact, peacefully co- exist in many data centers. For example, a SAN in the data center may network database and application servers with a number of large storage devices on which their data resides, while one or more NAS devices are attached to the LAN providing file access to clients.


The choice of which technology to use is driven mainly by the requirement being addressed and partly by timing. If the requirement is to provide shared file access to a number of clients, NAS is generally the answer. NAS devices meet this need today with great efficiency. Because NAS systems are built on existing LAN and file system protocols, NAS technology is relatively mature in comparison with SANs. While a few SAN file sharing solutions exist, they are generally aimed at specialized markets such as video editing. Generalized SAN file sharing solutions will probably require a distributed SAN file system, which could be years away from appearing and maturing.
On the other hand, many IT managers are grappling with the need to consolidate data used by large databases or applications such as Microsoft Exchange onto a small number of shared storage platforms to improve centralized management. Or, they want to take advantage of device-to-device data movement for applications such as backup or data replication. In this case, SAN topologies can provide unique capabilities to address these requirements.

And for small business you can use Direct-Attached Storage (DAS):

Direct attached storage refers to the storage attached directly to a PC or server. You can share files stored on one of your PC's hard disks or buy a server running Windows Server Foundation or Windows Small Business Server and share its internal storage. As discussed earlier, you can also add storage to an internal bay of your server or add external storage via a USB or FireWire.

These are viable solutions, but if you haven't yet made the leap to the world of servers, consider your other options carefully. Why?

Complexity: - You have to do some research and investigation to find the right server for your needs. Then you must purchase, install, and configure the hardware and operating system for your network of PC users. If you're new to server technology this can take a long time with the potential for a high level of frustration. Alternatively you can hire someone to do all this for you-for a fee.

Once your server is installed, its loosely integrated collection of hardware, operating system, and software require ongoing tuning and troubleshooting. The server operating system and software are likely to require frequent patching and updates for continued security and performance.

Availability: - DAS storage can only be accessed through the server or PC to which it is attached. If that server goes down or is turned off for any reason, the storage will not be available to the network.

Upgrades: - If you run out of storage you'll probably have to shut down the server to install a new hard disk. This requires downtime and staff resources. Some servers and external storage solutions let you swap hard disks in and out while the server is up and running, but these tend to be at the high end for medium and large business use.

Performance: - The typical server operating system (OS) is designed to run many different applications, provide many different types of services, and carry out many different tasks simultaneously. A full-fledged OS can have an unnecessary impact on performance if all you really want to do is share files.

Flexibility: - You can run into similar inefficiencies with server-attached DAS drives just as you did with your PC-attached DAS drives. As your business grows and you add servers to your network, heavily used servers and DAS units will run out of storage frequently, requiring upgrades, while less-used servers will have storage to spare, but none to share with their storage-strapped brethren.

Despite these caveats, DAS can be an inexpensive viable solution for many networks, particularly those that also want to run server applications like email, CRM, and other database solutions.
 

 

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