Ultra SCSI To Fibre: the Preferred Performance Path
Designers and system integrators specifying disk drive interfaces for computers are faced with a choice between evolutionary and revolutionary technologies. One path leads to adequate performance improvements with minimal integration effort and no additional costs. The other path promises great performance gains in exchange for more implementation efforts and cost.
For many designers, the best path is to take full advantage of the performance improvements offered by the evolutionary technology. Others will forge ahead, recognizing the advantages of the revolutionary path and choose to be the early adopters of the new interface. In the disk drive industry today, the evolutionary technology is represented by the Ultra SCSI interface, and the revolutionary technology is represented by the serial interfaces.
Hewlett Packard, Quantum, and Seagate Technology believe that Ultra SCSI is the cost-effective choice to the performance gains required by some systems such as low-end servers and desktop workstations. For those systems who have substantial performance or stringent reliability requirements, the Fibre Channel Arbitrated Loop (FC-AL) interface is the best choice.
The Switch to Serial Interfaces
Serial interfaces allow data and control signals to pass along a single path rather than moving in parallel across multiple conductors. Command, status, and data information is encapsulated for transmission in packets. All serial SCSI protocols attempt to maintain command set compatibility with parallel SCSI, but the hardware layer protocol is different.
Figure 1. Data Packet
The type of interface that a customer (systems integrators and OEMs) chooses will depend on his objective. The new serial interfaces have a lot to offer over the traditional parallel SCSI interfaces. For example, serial interfaces typically have point- to-point interconnections which can both increase reliability and decrease cabling complexity. With a point-to-point interconnect, a single wire connects only two devices. This contrasts with parallel interfaces in a typical bus environment in which the demands on drivers can vary depending on the number of devices on the bus and the cable lengths. Serial interfaces also offer a dual porting capability, so data can be transferred over two independent data paths, enhancing reliability.
If customers desire higher performance, serial interfaces provide transfer rates as high as 200 MB/second with the FC-AL interface. Serial interfaces also offer simplified cabling and increased connectivity (especially important in multi-drive environments), and simplified termination — all of which contribute to ease-of-use.
Today, three serial interfaces exist:
- FC-AL with data transfer rates as high as 200 MB/second, 30-meter copper lengths, and support for 126 devices
- SSA with data transfer rates up to 80 MB/second, 20-meter cable lengths and support for 126 devices
- P1394 with data transfer rates starting at 12.5 MB/second and a low cost connector (This interface will likely be more prominent in consumer electronics rather than the drive industry.)
Hewlett Packard, Quantum, and Seagate Technology believe that FC-AL holds greater long-term potential over other serial interfaces. The FC-AL interface is a subset of the Fibre Channel network systems interconnection standard adopted by the American National Standards Institute (ANSI). Fibre Channel is already in use at the system level, and is rapidly gaining momentum in the disk drive industry. This means that customers who have already invested in Fibre Channel as their box-to-box solution can leverage their current investment inside the box as well. The SSA and P1394 interfaces will never be used as a host-to-host interconnect solution because they were not designed to run multiple protocols (e.g., SCSI, networking, etc.). While leveraged investment is a key benefit offered only by the FC-AL interface, there are a number of other benefits of that interface, depending on the customers’ objectives.
One key overall objective is to decrease connectivity cost.
- Because of higher throughput, customers can put more devices on a given host adapter. Fibre Channel enables up to 126 devices per interconnect.
- Because that same interface is used inside and outside of the box, many
storage subsystems will not require an expensive dedicated controller. With SSA
or any parallel interface, a controller would be required in the storage
subsystem if FC-AL is used as the box-to-box solution. (See Figure 2.)
Figure 2. FC-AL: Minimizing Controller Costs
In a SCSI environment where interfaces are rarely configured with more than five drives per interconnect, a proliferation of controllers can easily occur in any given cabinet or subsystem. FC-AL greatly decreases the number of controllers required to handle numerous drives and decreases the complexity of cabling required for multiple controllers.
- Development expenditures for box-to-box interconnections are now directly applicable to storage peripheral interconnections as well.
- Though the name Fibre Channel seems to imply the use of expensive fibre optic cabling, which is indeed used in some cases outside of the box, the preferred medium for use inside the box is much less expensive: copper transmission cabling.
For many customers, the primary goal is to obtain the highest performance possible. This makes serial interfaces all the more necessary. The need to move large blocks of data faster than ever before is required for applications in the video server market or image processing. On-line transaction processing requires extremely fast transfers of small blocks of data at a constant pace. For those systems aimed at these types of applications, FC-AL offers the best performance path.
- The first generation of FC-AL provides a path to much higher performance than other serial storage interfaces. Currently, with dual porting, FC-AL drives are capable of delivering buffer-to-host data transfer rates of 200 MB/second. As such, it is the fastest interface available.
- Future products will push that level of performance to 400 MB/second, while increasing the level of chip integration to improve costs.
- The FC-AL interface allows data to travel over longer distances at higher speeds than parallel (single-ended) SCSI.
For the first time, 3.5-inch drives are beginning to make in-roads in the mainframe, minicomputer, and superserver markets. These systems fit numerous drives in confined cabinets where they are exposed to lots of electrical noise. Yet, in these types of environments, customers are storing multiple terabytes of data on multiple small form factor drives. Never before has the need for reliable storage been more important. Today's 3.5-inch drives must be designed specifically to handle the reliability and data integrity requirements of these challenging environments.
The FC-AL interface itself heralds some attributes that contribute to the drive's overall reliability. From the beginning, the interface was designed to be robust enough to permit multiple devices to be removed from the loop at one time with no interruption in throughput and without sacrificing data integrity. The interface also provides for a highly sophisticated error detection scheme. In this scheme, several bytes of error detection code (EDC) information are transmitted along with each packet of user data. The receiving device then uses the EDC information to check data received, and to request a resend if a discrepancy arises. This scheme is inherently more robust than the parity bit scheme used in parallel transfers in which error detection is performed on a byte-by-byte basis.
The FC-AL interface also helps to solve some of customers’ ease-of-use objectives.
- For example, no jumpers are required to configure the drive. Currently, most drives with parallel interfaces are configured via hardware, but FC-AL drives can be configured in firmware.
- The FC-AL interface is the only interface designed for simultaneous hot plugability. The FC-AL interface's loop redundancy circuit allows the removal or insertion of multiple drives from an active loop without impacting data throughput. This also provides for better data availability.
Summing up FC-AL's Advantages
FC-AL is the highest-performance solution of all serial interfaces. It also provides the best value relative to performance gains. The FC-AL interface gives users: the ability to build highly reliable systems and subsystems with built-in hot pluggability support; improved connectivity; the capability to send data fast over long distances, and high data availability.
Finally, Fibre Channel is already accepted as a network interface standard — even by IBM. The fact that Fibre Channel is the chosen link for host-to-host communication enables companies to leverage their investments inside boxes, using fibre as an I/O interface as well.
Table 1 below summarizes the benefits of FC-AL over today's SCSI
|Table 1. Solving SCSI's Limitations|
|SCSI Today||FC-AL Solution|
|Cable Distances||1.5 to 3 meters||30 meters between|
each device, using
coaxial cable 1
|Data Rates||5 MB/second
|Addressing||15 devices||126 devices|
|Disk Array Support||Parity||Improved|
For those who don't require the performance or functionality of FC-AL, Ultra SCSI is a solution that offers improved performance over the current parallel SCSI interface at no additional cost. For OEMs, systems integrators, and VARs who want to improve throughput, Ultra SCSI is the lowest-cost, easiest to implement path to higher performance.
Ultra SCSI doubles the burst data transfer rate of SCSI disk drives: up to 20 MB/second for the 8-bit implementation and up to 40 MB/second for the 16-bit option. The performance boost benefits both servers and single-user desktop systems. For servers requiring maximum performance, Ultra SCSI helps to alleviate bus contention problems that can occur when numerous high-performance hard disk drives and other peripherals are added to the bus. For single-user systems, the 8-bit implementation increases data transfer rates to 20 MB/second.
Ultra SCSI is backward compatible down to the connector level. For example, the 50-pin implementation of Fast SCSI can transfer data from the drive's buffer to the host at 10 MB/second while Ultra SCSI, using the same 50-pin connector, can transfer data up to 20 MB/second. Its backward compatibility enables Ultra SCSI drives to simply plug into regular SCSI interfaces. As with Fast SCSI, the Ultra SCSI interface negotiates with the host controller to determine the actual transfer rate, thus ensuring that every existing SCSI controller is a potential home for Ultra drives.
To ensure both compatibility of the technology and rapid market acceptance of Ultra SCSI, Hewlett Packard, Quantum and Seagate Technology are working with several leading controller companies, including Adaptec, BusLogic, NCR Microelectronics Products Division, and QLogic.
While the implementation of serial interfaces is imminent, further upgrades to parallel SCSI are now in development. The ANSI XT310 Standards Committee is working on an Ultra SCSI interface with longer cables capable of delivering data transfer rates up to 160 MB/second.
Figure 3. Interface Road Map for the Future
When it comes to choosing a new or improved interface, the customer must evaluate his objectives. If the customer requires top-of-the-line performance and wants to protect his current investment in Fibre Channel, the FC-AL interface is the logical choice. But, if cost is the most important consideration, the Ultra SCSI interface is the best choice.