Guide Helps You Evaluate Storage Architectures


If you're looking for a comprehensive source of information on the main types of storage architectures, David Vellante, a storage analyst and president of ITcentrix, Still River, Mass., says he recommends "A Storage Architecture Guide," published by Auspex, a major storage vendor ( He says, "This report provides a consistent framework, including comparisons, examples, and rules of thumb, for evaluating the three types of storage architecture: DAS, NAS, and SAN. The guide is informative, technically correct, cogently written, and contains information that has never [before] been published." Following is a brief overview of the three storage architectures.

The Direct-Attached Storage (DAS) Model

Think of the DAS model as the way computer systems worked before local area networks. The DAS model contains three basic software layers:

  • Applications software
  • File system software (part of the Unix or Windows NT operations system software)
  • Disk controller software

The elements usually are located close together physically and operate as a single entity. DAS is an appropriate choice for both very low-end PCs and very high-end, high-performance mainframe applications.

The Network-Attached Storage (NAS) Model

The NAS model was made possible because the network file system (NFS) for Unix or CIFS for Windows allows a file system to be located or mounted remotely. It can be accessed over a network, instead of residing on the application server. Compared to DAS, NAS servers offload all the functions of organizing and accessing directories and data on disk and managing the cache. This architecture frees the server's CPU to do additional work, thereby reducing potential CPU bottlenecks.

NAS is the best choice for Unix and Windows NT data sharing applications, consolidated file serving applications, and technical and scientific applications. It is also recommended for Internet and intranet applications and certain types of decision support applications.

The Storage Area Network (SAN) Model

In the SAN model, the file system continues to reside on the application server. As with DAS, the server performs its normal file system functions of organizing and accessing all files and directories on each individual disk partition and managing all caching activity. Reads and writes to the disk controller software, however, must be sent over the SAN. This technique adds latency to the I/O process, and latency reduces performance. Unlike NAS, there is no reduced workload to allow the server processor to offset added network latencies. Because of its current lack of standards, SAN is available only in proprietary configurations, and the long-term interoperability of these schemes is still not apparent. //

Elizabeth M. Ferrarini is a free-lance author based in Arlington, Massachusetts.


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