Fibre Channel-Arbitrated Loop: A cost-effective, high-performance hard disk drive interface

The Fibre Channel-Arbitrated Loop (FC-AL) disk drive interface is enhancing a whole new series of applications that need high-speed data storage over long distances. These emerging applications include professional movie editing, collaborative engineering efforts, video-on-demand, medical imaging, and high-volume transaction processing. Thanks to wide industry support, hard disk drives supporting the FC-AL interface are now available from most of the world's computer system manufacturers.

Fibre Channel can be used to connect large amounts of disk storage to a server or cluster of servers. Compared to Small Computer Systems Interface (SCSI), Fibre Channel technology supports greater performance, scalability, availability, and distance for attaching storage systems to network servers. Compared to the Serial Storage Architecture (SSA), a switched Fibre Channel connection can potentially provide higher throughput over longer distances.

Fibre Channel-Arbitrated Loop (FC-AL) is an emerging, powerful technology for businesses that need available, reliable, high-speed storage interconnected over long distances. Its high bandwidth, flexibility, and connectivity make it a wise choice for the next generation in storage interface technology. The following are some of the key benefits of FC-AL storage connections.

• Ease of use: Smaller cables and controllers are needed, which makes it attractive to network planners and administrators.
• Scalability: More devices can be added per system PCI slot, thus protecting and expanding the investment in host adapters.
• Performance: FC-AL is a high-performance serial interface. The ability to transmit data at up to 100 MB/sec (and twice that with dual porting) makes FC-AL an outstanding alternative to other interfaces, especially in long transfer environments.
• Reliability: The FC-AL interface enables highly reliable systems with built-in hot pluggability support and the ability to support long distances with high availability. Enabling a failover system to be as far as 10 kilometers (6 miles) away from the rest of a cluster can improve survivability in the case of a disaster. Work can be transferred to that location without interruption.
• Connectivity: Not only can 126 devices be connected to a fibre channel loop, but in a public loop configuration (with 24-bit IDs compared to a private loop with 8-bit IDs), more than 16 million hard disk drives can theoretically be attached to the fibre channel fabric.

Ultimately, the choice of a storage interface depends on how the system is being used. If top performance or high reliability is important, or if a greater number of devices needs to be connected at a large distance from one another, the FC-AL interface is the logical choice. But, if cost is the most important consideration, the Ultra SCSI interface (including Ultra2 SCSI, Ultra3 SCSI, and others) is still the best interface for low-end servers and workstations.

The origins of FC-AL
Network users have long benefited from Fibre Channel's ability to provide high data transfer rates over long distances. Fibre Channel originated as an optical point-to-point communication method, widely used for high-speed wide-area networks. It consists of an integrated set of standards that defines new protocols for flexible information transfer using several interconnection topologies. These standards have been created by the American National Standards Institute (ANSI) committee, with Hitachi playing a leadership role as it has done before in the development of many new industry-standard technologies.

Hitachi engineers worked on the ANSI committee and helped establish the three basic Fibre Channel topologies: fabric, point-to-point, and arbitrated loop. The FC-AL enhancement added support for copper wire and multi-drop rings with many devices attached.

The advantages of serial interfaces
FC-AL is a serial interface, where data and control signals pass along a single path rather than moving in parallel across multiple conductors as is the case with SCSI. Serial interfaces, including SSA, have many advantages over traditional parallel interfaces, such as SCSI:

• Serial interfaces typically use point-to-point communications, which can increase reliability.
• Serial interfaces offer a dual-porting capability, so data can be transferred over two independent data paths, enhancing speed and reliability.
• Serial interfaces result in simplified cabling and increased connectivity (especially important in multidrive environments) and simplified termination-all of which contribute to ease of use.

A SCSI environment is rarely configured with more than five drives per connection. With FC-AL, up to 126 devices can be supported on one loop. This, combined with FC-AL's high throughput, enables more devices on a given host adapter than traditional SCSI connections.

Another key advantage of Fibre Channel is the large distance that can separate devices on the loop. From 10 meters to 10 kilometers can separate each FC-AL device with no loss in data throughput or reliability. This means that servers and the people who use them can be located miles apart and still share the same storage devices.

If SSA or SCSI were used to connect drives inside a storage subsystem, an FC-AL controller would be required to connect to other storage subsystems or hosts. Because an FC-AL interface can be used both inside and outside of a system, the need for multiple controllers is greatly reduced.

Superior performance of FC-AL hard disk drives
Applications such as video serving and image processing move large blocks of data faster than ever before. Online transaction processing requires extremely fast transfers of small blocks of data at a constant pace. The performance of these types of data-intensive applications can benefit greatly from hard disk drives using the FC-AL interface.

The current generation of FC-AL provides a path to high performance. With dual porting, FC-AL drives can deliver data transfer rates of 200 MB/sec. Today, FC-AL is one of the fastest interfaces available, with future enhancements expected to push performance to 400 MB/sec.

Increased reliability, availability, and serviceability
From the beginning, the FC-AL interface was designed to be robust enough to permit devices to be removed from the loop without interrupting throughput and sacrificing data integrity. If a drive fails, port bypass circuits can quickly route around the problem so all drives on the loop remain accessible.

The FC-AL interface also has a sophisticated error-detection scheme. Several bytes of cyclic redundancy check (CRC) information are transmitted along with each packet of user data. The receiving device then uses the CRC information to check data received and to request a resend if an error is detected. SSA also provides significant link recovery capability. An SSA interface causes data to be retransmitted if errors are detected. Both FC-AL and SSA schemes are more effective than the parity bit scheme used in parallel SCSI transfers where error detection is performed on a byte-by-byte basis. CRC is planned to be added to SCSI in the next-generation Ultra3 SCSI.

Ease of configuration
As businesses grow and become more complex, so do systems. The flexibility and scalability of Fibre Channel technology facilitates easier management of this increased complexity and enables system expansion while protecting the base system investment. This is possible because Fibre Channel technology allows for system expansion by increasing the number of devices that can be attached per PCI slot.

The FC-AL interface also improves ease of use. For example, no jumpers are required to configure the disk drive. Currently, most drives with parallel interfaces are configured via the hardware, but FC-AL drives can be configured in the firmware.

Future trends
Hitachi Fibre Channel solutions will be able to use today's SCSI disks, storage expansion enclosures, and servers. This protects current SCSI hardware investments, which often constitute a high percentage of total system cost.

As the number of vendors producing Fibre Channel hard disk drives and other system components (such as Fibre Channel adapters, RAID controllers, and backplanes) increases in 1999 and beyond, hese new devices should become more cost-effective and available. This will allow solutions based entirely on Fibre Channel to become more prevalent.