Examining Rotational Vibration in Desktop vs. Enterprise
Often we receive the question, “What are the differences between desktop and enterprise drives?” That same person may in fact also be thinking, “Can I use those lower-cost desktop drives in an enterprise environment?”
Seagate engineers design and test enterprise-class drives including Seagate Savvio and Cheetah 15K-rpm families under different stress conditions than desktop drives. Nearline enterprise drives including Seagate Constellation and Constellation ES are also tested under enterprise conditions. All of these drives must be able to operate in 24 x 7 environments, whereas desktop drives (e.g. Barracuda XT) are designed and tested typical 40-hour work week conditions. Additionally, enterprise-class drives must also maintain high performance levels in multi-drive configurations where physical vibrations transmitted through a cabinet occur. This phenomenon is known as Rotational Vibration (RV). Gizmodo recently wrote about a vendor-sponsored study on vibration, although we should note that Seagate enterprise class drives perform quite differently then what’s shown in Gizmodo’s story.
RV itself is a twisting/torqing type action experienced by a hard drive inside a cabinet. RV is measured in radians as the angular rate of change in seconds – in other words, it’s how much angular acceleration movement the drive can handle.
The main sources of RV energy are: 1) the drive’s self-actuation 2) additional drives inside the cabinet accessing data, and 3) external forces acting on the cabinet. If RV is not taken into account in the design of the drive, the force of RV can push the head off track causing missed revolutions and delays in data transfers. Tests on drives not capable of handling RV have shown significant reductions (over 50%) in performance.
Fortunately, Seagate enterprise drives have used numerous technologies within its drives for over a decade to negate the effects of RV. To optimize these densely packed, multi-drive environments, Seagate uses RV sensors as well as linear vibration sensors on its enterprise drives. These sensors enable the drive to compensate for any vibration that occurs from the drive itself or outside of the drive (e.g. cooling fans, poorer-quality chassis, etc.) and still continue to read/write data.
It’s also a good idea by design to reduce the amount of vibration that a drive may generate on its own. Seagate’s larger capacity enterprise drives are built using a Top-Cover-Attach spindle motor that increases rigidity within any drive that uses a 4-disk configuration. Further enhancements of the design are made by optimizing the seek profiles of the drives in firmware to minimize any emitted torque.
And what about our question of using desktop drives in an enterprise environment? The diagram below shows the performance impact of desktop, nearline, and enterprise class drives at different vibration levels. The 6, 12, and 21.5 radians/sec^2 figures represent the specific tolerances of desktop, nearline, and enterprise-class drives respectively in maintaining throuput performance of least 80%. Notice that when the RV’s increase beyond those specified tolerances, the performance of the desktop drives in particular drops dramatically, while the enterprise-class drives maintain their performance at nearly 100% under all but the most extreme conditions. Nearline-class enterprise drives like Seagate’s Constellation fall somewhere in between desktop and enterprise.
The bottom line: customers with multi-drive applications will benefit greatly from actually achieving the specified performance levels made available by integrating Seagate enterprise-class drives. Not all drives are created equal.