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Serial ATA nets need solid chassis design

Posted: 16 Apr 2004 ?? ?Print Version ?Bookmark and Share

Keywords:serial ata? rvi? storage network? storage chassis? drive?

As serial ATA-based storage networks arrive on the scene, engineers must address system-level reliability. Storage enclosures must be carefully managed to guard against subtle design problems caused by vibration performance degradation, signal integrity-induced data failure and heat-induced catastrophic failure.

One critical imperative when building a storage chassis that supports a large number of disk drives is properly managing vibration effects generated by the drive's actuator system. When placed in an enterprise environment where millions of I/Os are the daily norm, the continuous slamming of an actuator across the disk surface can cripple the drive's ability to relay data quickly and correctly to and from its host. These dynamics not only affect the performance of an individual drive but can also adversely affect the neighboring disk drives within a storage enclosure.

Understanding the magnitude of this performance impact requires a series of tests that compare the maximum I/O rate of a single disk drive securely constrained to that of the same drive mounted into a populated storage enclosure. The difference of the two measured I/O rates constitutes the percentage of performance degradation contributed by the storage enclosure. In storage industry parlance, this measurement is called the rotational vibration index (RVI).

For enterprise-based SCSI and Fibre Channel, drive makers have incorporated mechanical and firmware solutions to counter a lot of the influences of drive vibration. But the enhancements add cost and contribute to the high price normally accompanying such drives. To maintain a low price point for their serial ATA products, some drive makers have opted to leave out pieces of the technology used to counter the effects of drive vibration.

This poses a challenge for the storage enclosure manufacturers that want to offer a serial ATA-based enclosure as a price-sensitive alternative to their SCSI and Fibre Channel products. No longer is it a case of wrapping some sheet metal around a bunch of disk drives. Attention must now be paid to the structural stability of the chassis and to the design of the disk drive carrier and its interface within the assembly.

As serial signaling rates grow, routing and managing the signals as they pass through cables and as traces on boards become critical. In the parallel world, these interfaces were quite forgiving, and many approaches were taken that diverged from accepted specifications. But designers will find that to be an unacceptable strategy for high-speed serial interfaces like serial ATA.

As storage systems migrate out of the controlled environment of a data center and into densely packed data closets, the efficiency of the enclosure's cooling system also becomes crucial. Historically, designers have tried many approaches to manage airflow and heat dissipation within a storage chassis. Most of these systems use axial or radial fans to pull air across the hot components within the box in an effort to keep the components below their maximum operating temperature. Ideally, a well-designed storage enclosure will have a cooling system that produces an unimpeded, uniform flow of air through the chassis. It will provide just enough cooling to achieve maximum mean time between failure of the heat-sensitive components housed within the box.

Cooling requirements for serial ATA drives are no greater than for other drive architectures, so current design practices can be used for serial ATA enclosures. As long as those established design practices are used, the likelihood of premature drive and system failure can be greatly reduced and the system's online availability increased.

- J. Peter Herz, Mike Bell

3ware Inc.

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