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Fibre Channel vs. SCSI: Which is more advantageous for your storage area network?

We help clarify your options

By Ron Levine

March  1999
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As Fibre Channel technology moves into the limelight, it brings with it a host of questions about which technology -- Fibre Channel, SCSI, or a combination of both -- is best for your SAN. Ron Levine sifts the wheat from the chaff and helps you understand the advantages and disadvantages of each. (2,200 words)

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A lot of excitement is being generated over storage area networks (SANs) and Fibre Channel (FC) pipelines. They are being presented as the new industry standard for high-end storage solutions, capable of blasting through the storage bottlenecks that have become common on many distributed networks. Although it's a fact that an all-Fibre Channel (network and storage devices) SAN offers many performance and administrative benefits over SCSI-connected storage, many users combine Fibre Channel storage devices with legacy SCSI devices in a SAN environment using Fibre Channel-to-SCSI bridge products. Both implementations have benefits and drawbacks.

Before we get into the heart of Fibre Channel versus SCSI, let's take a moment to work through a quick backgrounder on SANs for those of you who may not be familiar with this relatively new concept.

A brief introduction to SANs
Let's begin with a definition: A SAN permits information to be accessed, managed, and shared among various storage devices and servers over a dedicated network optimized for storage-related functions. This dedicated network, the SAN, operates independently of the LAN. Thus, LAN overhead and traffic are reduced while overall enterprise network performance is enhanced.

Generally speaking, a SAN is a shared storage repository. SANs aren't necessarily synonymous with Fibre Channel. Today, these storage devices are often RAID, and the channel network can be Fibre Channel, ESCON, or even SCSI. The I/O channel operates on the backend of the server, making file access and data transfers independent of the LAN.

This high-speed channel can be scaled up to a more sophisticated version that enables a mixture of storage devices (for example, RAID and tape libraries) to communicate over common fast, fault-tolerant storage pipelines with multiple hosts and directly with each other. As new software and fabric enhancements become available later this year, SANs will support increasingly complex server-to-storage functions such as fault-tolerant access paths with automatic failover, the dynamic reallocating of storage devices, assignment of dedicated storage space within a device (logical unit masking) for specific hosts or operating environments, and high-availability clusters.

The SAN consists of three basic components: an interface (like, SCSI, Fibre Channel, or ESCON); interconnects (switches, gateways, routers, or hubs); and a protocol (like IP or SCSI) that controls traffic over the access paths that connect the nodes. These three components plus the attached storage devices and servers form an independent storage area network.

While the SAN supports a number of interfaces, Fibre Channel (both Fibre Channel Arbitrated Loop (FC-AL) and Fibre Channel fabrics) has gained the limelight due to its flexibility, high throughput, inherent fault-tolerant access paths, and potential for revitalizing network-to-storage communications.

Fibre Channel benefits
In an FC-AL based SAN, up to 126 nodes can be connected per loop. Switched fabrics support up to 16 million device addresses. This modular scaling capability provides a sound infrastructure for long-term growth. The Fibre Communications Channel supports multiple protocols and has a current bandwidth limitation of 100 megabytes per second (MBps). The Fibre Channel interface can sustain this bandwidth over long distances (up to 10 kilometers) using long wave optical interconnects. Fibre Channel switches can be cascaded to provide dramatic capacity and performance scaling.

In addition, Fibre Channel easily can be configured for high availability environments. Most Fibre Channel disk drives are dual ported. Using both ports in a dual-loop configuration provides a redundant path to and from the device, guaranteeing access should one path fail. Fibre Channel switches provide fault isolation and hot-pluggability at the port. FC connections provide the performance power to tackle an array of bandwidth-intensive storage management functions like backup, remote vaulting, and hierarchical storage, freeing the LAN from these traffic-intensive and bottlenecking chores.

SCSI limitations
We're all familiar with the venerable SCSI and its numerous reincarnations (Wide SCSI, Fast SCSI, and SCSI-3). SCSI (Small Computer System Interface) is and has been the interface of choice for computer-to-storage high-speed connectivity for Unix and Windows NT. But its restrictive characteristics limit the ability of IT administrators to enhance LAN storage performance.

For example, SCSI has never supported multiple host-to-storage device connections well. Its forte is point-to-point, directly attached computer-to-storage device interfacing. The traditional SCSI data throughput rate of 40 MBps quickly bottlenecks with today's database applications and high-volume, large file size data and multimedia information transfers. The SCSI limitation of 15 devices maximum per channel is also daunting to those creating multiple server-to-multiple storage device networking schemes. This limitation on device hookups is further reduced as bus bandwidth increases.

With SCSI, there is a 25-meter point-to-point connection cap between devices along the channel. With Low Voltage Differential (LVD) SCSI, this drops to 12 meters, requiring storage units to be located in close physical proximity to the server, often within the same enclosure. This configuring of server and storage within a single "box" results in an expensive interrelationship between scaling of server capacity and storage capacity. Also, the short cable length limitation often prevents LAN administrators from physically locating storage devices in a centralized environment and limits their ability to perform remote mirroring or remote vaulting operations.

Because of SCSI's point-to-point nature, backup traffic from server to server must travel over the LAN. This places additional strain on network resources and robs already narrow bandwidth from online users, further exasperating throughput and device connection limitations. "What's happening is that in a typical client/server network configuration, we're trying to push ever-growing volumes of data through a small, narrow pipeline, and like too much water running down too small a drain pipe, it's backing up and overflowing," exclaims Scott Robinson, vice president of engineering at Minneapolis, MN-based Datalink Corporation, a leading independent information management solutions provider.

All Fibre or SCSI-Fibre Channel solutions
Fibre Channel was designed specifically to address server-to-storage interface limitations. A basic server-to-storage device I/O connection over a single Fibre Channel bus (without a SAN) can vastly improve overall network and storage access performance. Fibre Channel's high bandwidth greatly enhances information transfer throughput at any site where high-end applications like imaging, video streaming, OLTP, databases, and CAD/CAM are outpacing storage system performance. And, the extended connectivity distances possible with Fibre Channel encourage the implementation of remote backup, archiving, and mirroring for disaster recovery purposes.

At active SAN installation sites, Fibre Channel interfaces are already delivering immediate and measurable operational benefits not previously possible with SCSI connections. For example, by connecting RAID to the backend of a server over a Fibre Channel bus, higher bandwidth results in quicker I/O transfers over longer distances than are possible with a standard SCSI interface. The RAID-Fibre Channel combination improves storage subsystem reliability through fault-tolerant storage array operations and redundant pipeline data paths.

Fibre Channel switches and hubs provide for simplified storage device scalability, hot-plugging of storage devices, and isolation between functions. This translates into easily scalable bandwidth and improved subsystem availability.

"Fibre Channel provides an unprecedented combination of bandwidth, performance, high availability, and configuration flexibility for servers and disk subsystems. Incorporating complete Fibre Channel connectivity (frontend and backend) throughout a RAID subsystem surpasses SCSI or Fibre Channel-to-SCSI hybrid storage solutions," claims Peter Gibbs, marketing director at CLARiiON, a manufacturer of SCSI and FC-AL RAID storage solutions for enterprisewide applications. The company cites the following to support its viewpoint:

  1. Fibre Channel takes the best of SCSI -- the command set -- and links it to a high-speed interface. The Fibre Channel physical layer is very fast. Current Fibre Channel speeds range up to 100 MBps, with plans to increase these speeds to 200 MBps and 400 MBps in the future. Arbitration (deciding who owns the bus) is faster and equal, not prioritized as it is with SCSI.

  2. Depending on the media used, Fibre Channel can connect nodes up to 10 kilometers (over 6 miles) apart. SCSI cable distance is limited to 25 meters (80 feet). Most interfaces support one transmission medium or mutually exclusive transmission media. With Fibre Channel, you can use copper as well as fiber optic cable; however, the transmission medium used affects the distances supported between nodes.

  3. SCSI is limited to a single shared bus cable, whereas Fibre Channel can attach devices through a number of methods like loops, hubs, and switches. Lower costs result from a channel supporting a large number of devices with a single connection:

  4. SCSI connections can be hard to manage in large systems, especially when trying to diagnose intermittent problems. When configuring for high availability (multiple hosts, multiple arrays), connecting hardware components is a complex and time-consuming task. Fibre Channel eliminates Y cables and terminators.

  5. SCSI cables have 68 wires; Fibre Channel has 4. While wire practically never fails, connectors do. Each Fibre Channel connector has about 1/16 fewer connections on it than does SCSI. Because the Fibre Channel cable is thin, lightweight, and flexible, less stress occurs at the connector. In contrast, SCSI cable is thick and inflexible. In tight configurations, bending stress may be applied to the connector.

  6. Fibre Channel supports several fault-tolerant features not available or practical with SCSI bridge solutions. By combining Fibre Channel's dual-looping capability with dual-ported Fibre Channel drives, you can easily implement complete I/O subsystem redundancy with no single point of failure.

  7. Fibre Channel drives and Ultra-2 SCSI drives are equally priced. However, installation of SCSI drives is more time-consuming and error prone. Fibre Channel SANs can be completely reconfigured and reallocated online; SCSI installations require more complex adjustments for reassigning devices.

However, not all users are sold on end-to-end all-Fibre Channel connectivity; some have opted for integrating SCSI and Fibre Channel on the fabric, while others have chosen combination frontend fibre and backend SCSI channels. Their reasons vary from economics to performance needs. As Robinson points out, "Sites have spent a lot of money on SCSI devices, and they want to protect that investment. Most existing SCSI disks will remain directly attached to servers outside the SAN. But, there are also products on the market now that provide seamless integration between SCSI and FC on the SAN fabric that will leverage that investment, especially for devices like tape libraries."

Some sites have settled on installing Fibre Channel between the server and storage device controllers while maintaining SCSI on the backend -- between a RAID controller and individual disk drives, for example. "The benefits of FC behind the RAID controller are not as apparent; in fact, the RAID architecture is often more important than the device channel. By installing FC on the frontend it's still possible to communicate with the storage unit over a long distance because the host and controller are talking FC -- the SCSI is hidden behind the controller. This configuration enables a site to gain many of the benefits of Fibre Channel while leveraging investments in existing RAID architectures. Moving forward, however, full Fibre Channel implementations will become more predominant." explains Robinson.

With full end-to-end Fibre Channel connectivity, users receive all of the benefits in scalability, throughput, configuration options, and robustness that this high-powered I/O pipeline offers. And, at the same time, they are investing in infrastructure technology for the future. But you can get many of these same gains with a combination of Fibre Channel-SCSI topology.

Robinson believes that an independent storage management integrator, like Datalink Corporation, that isn't a manufacturer of equipment or a software developer, can best provide an unbiased analysis of the cost-to-benefit ratio and an in-depth review of whether Fibre Channel or Fibre Channel-SCSI is the better investment. "Unlike vendors who sell FC or SCSI devices and components, it makes no difference to us which technology the user selects. Therefore, we can do an honest evaluation as to the advantages [and/or] disadvantages of an all-Fibre, all-SCSI, or combination FC-SCSI methodology and implement the solution which best meets the site's needs."

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About the author
Ron Levine is a freelance writer based in Carpinteria, CA. He specializes in networking, storage device, and emerging technology applications. His most recent features for SunWorld were "Adding network hard disk storage -- painlessly" (March 1998) and "Choosing the best type of high-end tape backup" (January 1998).

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