Making the Transition from SCSI to Fibre Channel

Posted on January 01, 1998

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Making the Transition from SCSI to Fibre Channel

For most enterprises, there`s no need to rush into Fibre Channel. Taking advantage of SCSI extensions and hybrid solutions may be the most prudent course.

By Steven Paulhus

As network storage demands for bandwidth, availability, scalability, and connectivity continue to increase, many network administrators and system integrators are contemplating the implementation of Fibre Channel to meet their storage performance objectives. While Fibre Channel technology can certainly overcome many of SCSI`s limitations, for now it remains more of a revolutionary approach than an evolutionary one. A number of evolutionary technologies on the market not only meet current objectives, but they provide a cost-effective and smooth transition to Fibre Channel--if Fibre Channel capabilities are ultimately required.

Given the migration to storage-centric networks, two factors are driving companies to seek alternatives to parallel SCSI technology: demand for greater bandwidth for new applications and the need for longer cable lengths to meet changing configuration requirements and to support additional devices.

Pushing the bandwidth envelope is the growing need to support multiple storage pools on a distributed network running data- and I/O-intensive enterprise applications. Now commonly referred to as storage-area networks (SANs), these environments represent a new server storage interconnection architecture that combines storage and networking attributes to provide tremendous increases in the scalability of both capacity and performance.

Cable distance is also important. The 1.5-meter real-world limitation of Ultra SCSI is fast becoming unacceptable due to new configurations that rely on external storage to meet bandwidth and redundancy needs. The use of clustered architectures--in which multiple servers share a common storage pool comprised of multiple devices to optimize throughput and fault tolerance--also necessitates greater cable lengths.

But Fibre Channel is by no means the only interface in town. Next-generation SCSI solutions not only meet today`s needs, but also provide seamless migration to Fibre Channel.

So, which technology should you choose? The first question to ask is, Do current Ultra SCSI solutions meet your performance requirements? If additional cable length is required to support additional devices beyond the capacity of a 1.5-meter cable, Ultra SCSI investments may still be protected. The key is to implement a storage system that features an extension of the Ultra SCSI bus, thereby extending the cable length. These systems allow you to daisy chain multiple storage subsystems comprised of separate storage devices, resulting in a proportional increase in capacity and performance.

Ultra SCSI cable length can be extended even farther by using a high-voltage differential SCSI (HVDS) solution. For implementations that offer a differential- to single-ended conversion, cable lengths of up to 25 meters can be achieved with existing Ultra SCSI devices--without having to purchase higher-priced HVDS devices.

While Ultra Extender buses and HVDS can resolve cable length issues, they will not help you extend bandwidth beyond 40MBps. But again, Fibre Channel is not the only option, thanks to an enhanced version of parallel SCSI called Low-Voltage Differential SCSI (LVDS), also known as Ultra-2 SCSI. LVDS not only supports a 12-meter cable length--and up to 15 devices--but it also features data transfer rates of 80MBps.

To gain the full 80MBps, new LVDS devices have to be implemented. However, since vendor solutions support both Ultra SCSI and LVDS, the best of both worlds is possible without compromising investments: HVDS can be implemented today to extend cable length, and LVDS devices can be integrated later when additional bandwidth is required.

Therefore, LVDS may be a better choice than Ultra SCSI for networks that have an immediate need for greater cable length (or more devices) but also anticipate increased bandwidth requirements down the road. Alternatively, if increased cable length and bandwidth are needed and bandwidth does not have to exceed 80MBps, HVDS may be the best option. It is important to note that, as with Ultra SCSI, LVDS solutions also support extender technology, which increases LVDS cable lengths beyond 12 meters.

Again, the transition from SCSI to Fibre Channel does not have to be an either/or decision. However, if you are convinced that Fibre Channel is what you need, then keep in mind that it represents a new paradigm in terms of how enterprise-wide storage is implemented. An attractive entry-point into the Fibre Channel world, in terms of lower cost and complexity of implementation, involves hybrid storage solutions.

Hybrid storage solutions employ parallel SCSI on the target or drive side while providing a low latency, point-to-point Fibre Channel link back to the host. This type of implementation not only preserves investments in legacy SCSI hardware, but also offers some of the connectivity and performance benefits of Fibre Channel. For scalability, multiple SCSI channels can be implemented on the target side to maximize bandwidth usage on the Fibre Channel loop.

Fibre Channel-Arbitrated Loop (FC-AL) is the most commonly implemented topology for Fibre Channel disk storage. FC-AL transfers data at 100MBps per loop, or 200MBps in a dual-loop configuration. These data rates provide a level of I/O performance commensurate with the processing power found in networked and clustered server environments. Additionally, FC-AL supports up to 126 devices per loop, or 252 devices in a dual-loop configuration.

FC-AL`s dual-loop capability also provides fault tolerance. If a single loop fails or becomes unavailable, data availability is ensured via the second loop. This fault- tolerant capability can be further complemented by vendors who combine dual-loop Fibre Channel with subsystems that include dual power supplies, fans, and RAID controllers. Of course, achieving these benefits comes at a cost. However, if this investment is the precursor to a much larger enterprise shift to Fibre Channel, the cost can be easily justified.

Another benefit of Fibre Channel is unprecedented scalability. By linking Fibre Channel loops to switches and hubs, extended fault tolerance and enormous amounts of storage and network bandwidth can be supported.

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Steven Paulhus is director of marketing at nStor Corp. in Lake Mary, Florida. He can be reached at (407) 829-3534 or at steven.paulhus@nstor.com.


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