Examining The Design And Performance Of The Cheetah X15.

15,000RPM enters the market

Within an I/O subsystem, hard disc drives themselves are commonly seen as data bottlenecks. Data transfers are inherently going to be slower with drives that rely on magnetics and mechanical movement, compared with an electrical signal that travels through a bus. Drive engineers have continuously worked on overcoming the challenges of increasing the speed at which data could be moved on and off of a disc drive because of this. The question was and still is fundamentally, "How can we make the drive move data faster while ensuring that data integrity and reliability are not sacrificed?" With the growing need to move data faster due in part to the emergence of the Internet, eBusiness, and the growing number of information being moved within servers in general, answering this question has become critical.

Seagate recently released its Cheetah X15 disc drive, the first drive to rotate at 15,000RPM, as well as deliver more I/Os Per Second (TOPS) than what had been previously accomplished with 10,000RPM drive designs. There were, however, many technical challenges that had to be overcome to reach 15,000RPM while at the same time ensuring that data integrity and drive reliability would not suffer. While manufacturers of microprocessors regularly increase clock speeds, it had been over four years since the first 10,000RPM disc drive was first introduced to the market. This slower implementation time between the 10K and the 15K didn't occur because the research or development itself was a lagging process; quite the opposite, storage areal density demonstrations, for example, have been moving at average rates of 150% per year, a much faster curve than microprocessor clock speed increases. However, the slow implementation cycle to deliver higher speed disc RPMs was rather a testimony of the great challenges that had t o be met to produce this class of disc drive.

The Market Needs

Many areas within server platforms have still not witnessed their largest amount of growth. For example, it has been estimated that the number of appliance servers will grow by as much as 73.1% through 2004. By 2003, there will be 500 million users on the Internet, all of which will need a backbone built by servers incorporating drives that will require the highest level of I/Os per second possible. The emerging Storage Area Network (SAN) market is also poised to explode and, according to International Data Corporation, is estimated to be a $13.8 billion market by 2003.

The common thread between these markets is that storage requirements will also continue to grow in capacity, as well as performance. Seagate's Cheetah XiS disc drive was developed to fill these data intensive needs.

Cheetah X15: Overcoming Challenges, Increasing Performance And Reliability

The three problems that the Cheetah X15 design team had to solve when moving up to a 15,000RPM design were issues with heat, acoustics, and vibration. These three factors, if not addressed properly, would have too much of an impact on overall reliability, so they were the foundation for the basic design of the drive.

The design team solved the issue of thermal heat by using a disc platter that was reduced to a size of 2.5 in. or 65 millimeters in diameter. With the reduced size of discs, a spindle motor design that used less power (in watts) could be implemented. This reduction in power translated to a reduction in heat and, therefore, an increase in reliability. When comparing Seagate's earlier-generation 10K Cheetah design with the X15, the 10K Cheetah uses 13 watts while the X15 only requires 12.5 watts.

Cooling considerations must also be made to the actuator assembly that controls the heads and suspension. The temperature within the actuator's coil will rise considerably with increased movement, especially when used in a heavy data-seeking environment such as a server. As a result, Seagate's design team focused a considerable amount of time developing an actuator that could perform coil-cooling functions.

When dealing with drive vibration issues, the areas of focus are the actuator assembly, the discs themselves inside the drive housing, and the disc's motor. Vibration can be looked at as a problem inside the drive's design, as well as externally, since drive vibration can affect the performance and reliability of other drives placed together within a storage enclosure.

With the actuator assembly, as the arms and heads move across the disc platters, the physical movements of these components themselves cause vibration. This is again seen especially during heavy seeks. As a result, the design team used lighter materials with less mass in the design to reduce vibration. The arm and head assembly also features a shorter stroke and can, therefore, work more efficiently compared to earlier-generation 10K designs.

Utilizing a disc that can withstand problems of disc flutter and windage is also critically important for overall reliability. For the X15, the disc thickness itself was kept the same as compared with larger 3.5 in. diameter disc sizes used previously. This provides even greater rigidity to the 2.5 in. disc, which can experience disc flutter or extra movement if the material used is too thin. Windage (a phrase referring to large volumes of air being pushed within the drive during rotations) will also have a tendency to cause disc flutter problems, especially at higher RPM speeds. In addition to reducing disc flutter and windage problems by use of a smaller diameter disc, additional improvements were made with regard to spindle balance. With a more rigid spindle incorporated into the X15's design, again, overall reliability was increased.

Managing acoustic noise levels in a drive such as the X15 was an additional challenge. Obviously, the X15's improvements incorporated into reducing vibration can directly benefit sound levels. The smaller disc platters also inherently make less noise. The key area, however, that is most responsible for producing noise is the spindle motor itself. Seagate design engineers, therefore, worked to reduce the acoustics by improving the bearing design within the spindle motor. The X15 also uses a four-layer cover with dampening material inserted between layers that reduces overall deciBels (dB). When comparing an earlier-generation 10K Cheetah versus the X15 Cheetah, the overall dB levels of the X15 at idle were actually reduced; the 10K Cheetah is measured at 4.3dBs, while the X15 is rated at 3.9dB.

Performance Compared

As can be seen from Fig 1, when measuring the X15 for performance using random seeks with typical small block sizes of 8K with 7,200RPM and 10,000RPM Seagate designs, the performance improvements are impressive. The performance gains over the 10K Cheetah 18XL is 33% while the X15 outperforms the 7,200RPM Barracuda 18XL by 66%.

Perhaps the best validation of the Cheetah X15s significance in design is through its measured performance in real-world industry transaction tests. The X15 helped Compaq's latest server to garner a new world record TPC-C benchmark.

Mike Nikolaiev, Compaq TPC Director says: "The Cheetah X15 has enabled us to achieve the same level of performance using 42% fewer disc drives."

Fig 2 illustrates the X15's time to data performance as compared to 7,200RPM and 10,000RPM drives. With the release of Seagate's Cheetah X15 into the marketplace, faster transaction processing is available for various server applications. The need for storage solutions to increase in performance while maintaining high levels of reliability will continue to be important because of the expected continued growth in the number of servers being deployed and the increasing number of people accessing the Internet for personal and eBusiness use.

David Szabados is the corporate communications manager at seagate Technology