As storage capacity continues to grow at a compounded annual growth rate that exceeds 60%-90% for Windows NT-demands for external storage under centralized management persist. With these growing needs, continuous availability, ease of management, scalability, and resource sharing-ultimately in a heterogeneous environment-also receive increased emphasis. Two specific architectures are prime candidates to fulfill these requirements: Storage Area Networking (SAN) and Network Attached Storage (NAS).

Neither SAN nor NAS concepts are new, but their evpolution into the open system environments represents one of the most important changes affecting the storage industry in more than a decade. SAN and NAS are about to revolutionize the industry with ramifications extending beyond storage into network and systems management and architecture. SAN and NAS no longer couple storage directly to the server; this architectural change has important repercussions on the management of data in an enterprise.

This Paper covers NAS and compares NAS to SAN. Look for other Papers on SAN at http://www.periconcepts.com/

Network-attached storage (NAS)

Network-attached storage (NAS) is a fully integrated and dedicated storage solution. It can quickly and easily attach to a network topology, becoming immediately transparently available as a network resource for all clients. NAS is platform- and OS-independent, and appears to the application as another server. You can plug it in without shutting down the network, and it requires no changes to existing file servers.

NAS consists of a simplified server that performs only one function-and performs it well-not having to meet the conflicting requirements of a general-purpose OS. NAS can only run limited applications, but its ease-of-use and price/performance are unmatched. Implementations vary in which storage technologies and devices they support: Some may restrict themselves to CD-ROM, others to optical technologies, and still others may provide access to any storage technology including tape and magnetic disk.

NAS devices contain embedded processors running some sort of OS or microkernel that understands networking protocols and is optimized for I/O services. In contrast to the dedicated networks utilized with the storage area networking (SAN) architecture , NAS devices directly attach to the standard messaging network, and are addressable via standard file-system protocols-for example, Network File System (NFS) or Common Internet File System (CIFS). These devices are typically optimized for particular tasks, such as file serving, and tend to accomplish these tasks at very high performance levels.

To applications running on the network, NAS looks like an ordinary server. To any client, it looks like a large storage device. If the NAS does little more than allow the network connection, it is called network-ready storage or a thin server. When NAS provides additional processing power to perform file and storage management tasks, it is called a network-attached storage server (NASS). NASS provides autonomy from a central server or CPU and optimizes the cost/performance ratio for a wide range of products covering multiple applications. It consolidates the storage of many application servers into a single storage service offered by one manageable pool of physical storage (Figure 1).

NASS may address the interoperability issue among different major platforms. Interoperability is an important feature that helps facilitate server consolidation in today's diverse operating environment. NASS usually takes advantage of NFS, CIFS, and NetWare Core Protocol (NCP) to be operational under different UNIX, Windows, and Novell NetWare platforms while providing simultaneous access to the same files from different platforms. The downside is that NASS systems move a great deal of data across the messaging network, potentially degrading overall network performance.

Major participants in the NAS market include Creative Design (CDS), Meridian, and Axis Communications at the low end; Network Appliance (NetApp), Unisys, LSI, Procom, and Artecon in the mid-range; and EMC and Auspex at the high end. Hewlett-Packard (HP) offers NAS jukebox solutions. Some software vendors offer a complete OS for NAS that can turn any hardware storage system into a multiprotocol file server for both UNIX and Windows clients. CrosStor is one of the leading vendors in this market.

A More Elegant Solution

Traditionally, the approach to solving network storage problems has been to add new drives and more processing power to available general-purpose/application servers or to add a new server. This approach does little to address the real problems of degraded response time, increased complexity, and availability of data. The advantages of NAS over conventional server-attached (or bus-attached) storage are performance and connectivity (see Figure 2). In addition, if you need an additional server to handle the new storage, a NAS solution is simpler and cheaper. NAS can produce improved file-access performance at a substantially lower cost than a general-purpose network server can. When factoring in the additional cost savings generated with a simple installation process, which literally takes minutes instead of hours or days, and ongoing reduced management costs, NAS is a better, more elegant solution.

New applications have created new requirements for high-speed transfer of very large files. General-purpose servers and most OSs were developed for fast processing in multiuser, multitasking environments and are a poor match for handling large files at high speeds. Performance problems and traffic bottlenecks develop when one server transmits data to another, or more accurately, when one storage device attached to a server transmits data to another storage device connected to another server on the other side of the network. NAS is optimized to move the data to users efficiently without the overhead and complexity of general-purpose servers. The controller's ability to connect anywhere on the network lets you balance network performance by placing the storage close to the users who need the data. This architecture is particularly effective when bridges, routers, or switches segment the network.

Both storage area networking (SAN) and network-attached storage (NAS) technologies involve externalizing storage from the server and adding flexibility to network storage. With SAN technology, storage devices reside on their own networks with all the flexibility and performance benefits associated with networking. NAS technology involves employing a networking interface on storage devices, making them fully active nodes on the existing network. Both technologies come with comparative benefits and drawbacks.

Pros and Cons

The advantages of SAN reside in its superior performance, reliability, and connectivity. SANs offer a high-bandwidth link capable of growing incrementally and are better suited to transferring very large blocks of data. By contrast, the bandwidth properties of the data network characterize NAS networks, which are suited to efficiently move data in moderate-size segments. SAN delivers data reliability in a predicted time, while LANs (and NAS) retransmit data when the network is congested or fails for any reason. SAN offers a very high level of connectivity via cascading hubs and switches.

NAS devices typically see storage as files; SANs usually see blocks of data (see Table 1). This difference represents one of the major advantages of NAS configurations. Its other big advantages are ease-of-installation and its ability to offer low-cost entry product configurations. NAS doesn't require any significant initial investment, and the technology is available and proven.

NAS is not always a good idea for database applications because it's file-oriented. It works well, however, for document management and knowledge management applications. Because NAS does nothing but hold files for the network, it is flexible; however, it can also be inefficient at peak times due to network slowdowns. NAS devices work well for workgroups with high storage demands and in clustered server environments.

Each architecture has advantages depending on the application and existing infrastructure. The advantages of NAS include leveraging the current networking infrastructure and the enormous amount of development invested in this industry. NAS has a lead over SAN in heterogeneous data-sharing environments. SANs are at least two years away from offering this feature on a widespread commercial basis.

SANs and NAS are likely to coexist for some time and are, in many respects, complementary technologies. It appears likely that NAS functions will eventually migrate to the SAN. One way to think of NAS is as an important evolutionary step on the path to SAN.

By: Farid Neema

PERIPHERAL CONCEPTS, INC.
351 Hitchcock Way, Suite #B-200
Santa Barbara, California, 93105
Tel: (805) 563-9491
fneema@silcom.com

This article was published in the May 1999 issue of Windows NT Magazine