Comparing SCSI and Fibre Channel performance

Comparing SCSI and Fibre Channel performance

There are a number of reasons to migrate from SCSI to Fibre Channel, but performance may not be one of them.

Tomlinson G. Rauscher

Fibre Channel technology, with its performance specification of 100MBps, has created great expectations among users. However, first-generation Fibre Channel RAID products have not provided significant performance advantages over SCSI RAID systems, except in relatively expensive products.

Given this fact, when should users migrate from SCSI to Fibre Channel RAID? When the benefits of the new technology are greater than those of the old technology and when the benefits of the new technology outweigh the costs.

Under these guidelines, we examine the performance and cost of Fibre Channel and SCSI RAID controllers and the factors that affect RAID performance.

Performance drives adoption

Better performance is a key reason to move to RAID. Performance is best defined in terms of sustained throughput (in MBps) in reading data from a disk array. While burst rates look good on data sheets, they do not reflect actual performance.

The data being read should also change from I/O to I/O, as in a stream of video data. Repeatedly reading the same data is not a good way to test performance, because RAID systems cache such information, and the performance results represent a memory-to-memory transfer rather than an array-to-memory transfer. Another way to measure performance is to find the maximum speed at which the RAID controller can continuously write data to a disk array.

SCSI is a mature technology that has evolved through several generations (see table). The most recent implementation of SCSI supports speeds up to 80MBps, and next-generation products are specified to provide up to 160MBps. As for Fibre Channel, it`s only in its formative stages.

To determine the state-of-the-art in Fibre Channel RAID controllers, we queried several companies who had integrated them into RAID systems. In most cases, the actual performance of the Fibre Channel RAID systems was lower than vendors` claims; in some cases, SCSI RAID systems actually outperformed the Fibre Channel systems. Why?

Slow controllers. RAID controllers with Fibre Channel connections to host computers and SCSI connections to disk drives are the byproducts of SCSI-SCSI RAID designs. As such, they were developed to sustain Ultra SCSI speeds of 40MBps. Putting a fast Fibre Channel front-end on a slow RAID controller does not make the controller any faster.

Limiting chips. RAID controllers that use Fibre Channel connections to host computers and Fibre Channel connections to disk drives (e.g., "full" Fibre Channel RAID) have been developed with only a few disk channels using current Fibre Channel chips, which lack the general-purpose memory interface capabilities of SCSI chips. These design limitations typically preclude simultaneous reading and writing of more than two disks, as well as zero-overhead real-time hardware parity generation. As a result, full Fibre Channel RAID systems may run substantially slower than advanced SCSI RAID systems.

Figure 1a shows a theoretical model for estimating the maximum performance of a RAID system. Note the three parameters in this model:

- The amount of data in each I/O operation (I).

- The overhead of the RAID system in processing I/O operations (OH).

- The burst transfer rate of the bus (B).

Let`s examine the effect of each of parameter. Figure 1b shows how I/O size can affect performance. Note that performance depends significantly on I/O size. An interesting corollary is that Ultra SCSI generally provides sufficient bandwidth for transfers using small I/O operations.

Figure 1c shows how overhead affects RAID performance. Overhead includes factors such as I/O command processing, host adapter driver, and disk latency. There is significant overhead in most Fibre Channel RAID systems.

Figure 1d shows typical sustained bus rates. Note that both Ultra2 SCSI and Fibre Channel provide enough bandwidth to support the throughput of most RAID implementations.

Therefore, based on performance alone, the theoretical bus rate is not the key factor to consider when deciding whether to transition from SCSI to Fibre Channel. Rather, one must consider design limitations that affect performance, including:

- Some RAID implementations can only support Ultra SCSI bandwidth of 40MBps

- Some Fibre Channel chip designs preclude the parallel operations of transferring data and computing parity to independent disks.

Cost comparison

Another important issue is cost. To determine the cost of a Fibre Channel versus a SCSI RAID implementation, consider the price of electronic chips. Fibre Channel controller chips can be as much as three times more expensive than Ultra2 SCSI controller chips. And performance has not been an incentive to switch from SCSI to Fibre Channel. As a result, Fibre Channel adoption has been slower than expected.

So, what about the future? Will Fibre Channel RAID fulfill its promise of higher performance and play a major role in the market?

Fibre Channel technology has no inherent problems that prevent achieving higher performance. Cost and design issues will be resolved, at which time Fibre Channel RAID will consistently outperform current SCSI RAID. But for now, the benefits of a switch to Fibre Channel are longer cable lengths, simplified connectivity, and the ability to attach more devices. Performance is often the least important reason to migrate from SCSI to Fibre Channel.

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Figs. 1b, 1c and 1d represent theoretical results based on the formula in Fig. 1a.

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Tomlinson G. Rauscher is president of Digi-Data Corp., in Jessup, MD.

This article was originally published on April 01, 1999