The Basics of SAN Implementation, Part II Page 4 -

The Basics of SAN Implementation, Part II Page 4

Finally, hubs will continue to be a popular and easy method for implementing SANs, for small or simple SANs. In SAN and LAN environments, comparisons can be drawn between use of hubs and switches. In order for a concentrator and a device to simplify cabling, hubs can provide similar functions for both SANs and LANs. The same would apply in a SAN--where a switch would be deployed in a LAN to interconnect various sub-LANs or segments. The choice between switch and hub should not have to be binary. Rather, hubs and switches compliment each other and are both key components in a SAN. In fact, to enable more devices to attach to a given number of ports, to help offset the cost of switch port costs, hubs can also be used to front-end switches.

Summary And Conclusions

Storage Area Networks are the next great leap forward in storage. The ability to locate storage resources on a dedicated gigabit-speed network with shared access and centralized management functions holds the potential to revolutionize enterprise storage. SANs promise higher performance, improved storage reliability and availability, as well as lower total cost of ownership.

Faster, scalable and highly available storage interfaces are needed to meet growing application and enterprise needs, in order to support the ever-increasing amounts of data storage and information retrieval. Fibre Channel is a key enabling technology or the "plumbing" for implementing Storage Area Networks (SANs) that will support tomorrows needs. In order to enhance storage connectivity by overcoming distance, performance, scalability, and availability issues, Fibre Channel and SANs are being used today. SANs will evolve over the next few years from being a storage interface to a robust storage network with enhanced capabilities including LAN-free backup; server-less backup; data and storage sharing; shared file systems; remote data copy and mirroring; and data distribution. Up to now, Fibre Channel has been deployed as loops using hubs. However, to create fabrics over the next year or two, these loops will be interconnected or networked using Fibre Channel switches. Therefore, without the headaches and issues associated with loops, switches will enable many of the promised features of Fibre Channel.

Over the next few years, Fibre Channel can be expected to undergo improvements and enhancements, including faster performance. Today, not all host or system vendors have Fibre Channel products. And, interoperability issues are still being addressed. Some older host systems may not support the industry-standard fibre channel-arbitrated loop (FC-AL) protocol. It is important to verify which Fibre Channel protocol a vendor is talking about. Storage networks consisting of hubs and switches will be needed to ensure adequate performance and redundancy, for all but the smallest systems.

Given the vast numbers of installed parallel SCSI peripherals, it will be a couple of years before Fibre Channel overtakes parallel SCSI as a dominant storage interface. It is safe to say that parallel SCSI will be around for some time to come. However, as a new storage interface, Fibre Channel is here and is gaining momentum. Now is the time to start planning and making decisions regarding storage interfaces.

There are steps that can be taken to prepare for a SAN, whether you are implementing a SAN today, or investigating the technology for a future implementation. First of all, Fibre Channel and SANs should be seen as an enhanced storage interface to replace or supplement existing parallel SCSI interfaces. Although faster than parallel SCSI, Fibre Channel should be treated as an I/O interface and thus it should not be overloaded. For example, like a parallel SCSI interface, it is not a good idea to place too many devices on a single Fibre Channel interface. Instead, spread the I/O over multiple interfaces.

Second, SANs can be implemented in phases and may include some of your existing storage devices. Cost for SAN components are dropping while feature and functions are increasing. This allows you to implement Fibre Channel JBOD today and tomorrow migrate it to Fibre Channel to Fibre Channel RAID controllers.

Third, you can configure your SAN with multiple sub-SANs or switched segments, where certain systems and storage can be isolated and mapped to specific hosts. This is similar to your network environment, which may include sub-nets or switched segments.

Fourth, over the next couple of years, SAN technology developments will include enhancements to performance from 100MB/second to 400MB/second. Interoperability will continue to improve and SAN hardware components will continue to evolve. Also continuing to evolve, will be SAN software for data sharing, file replication, mirroring, SAN backup and other applications.

Finally, the storage industry has the challenge to deliver open interoperable SANs to meet the huge demand for this innovative storage architecture. Overland is convinced that the best way to deliver these new capabilities is with a partnership approach. Open standards are always the preferred option, but the market won't wait for bureaucratic organizations to issue their proclamations. The combination of vendor certification and an adoption of de-facto market standards will speed the development of SANs with a ready supply of hardware and software products and the channel partners to deliver them.

About the Author: John Vacca is an information technology consultant and author. Since 1982, John has authored 36 technical books including The Essential Guide To Storage Area Networks, published by Prentice Hall. John was the computer security official for NASA's space station program (Freedom) and the International Space Station Program, from 1988 until his early retirement from NASA in 1995. John can be reached at

» See All Articles by Columnist John Vacca

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