Storage disk arrays pose challenges for chassis design

The need to measure drive array cost-performance in terms of Input/Output Operations per second (IOPs) per Rack Unit or Megabytes per Rack Unit is forcing drive array vendors to allocate as many square centimeters of rack space to drives as possible. The emergence of SAS, SATA and 2.5" drive form factors is changing drive array packaging from an "implementation detail" to an art form. As drive packaging density continues to soar, the product feature list doesn't get any shorter. Redundant power delivery, redundant cooling and reducing chassis resonance due to rotational vibration from the drives are all still required.

Many drive arrays based on 3.5" form factors are 3U in height and contain 14 to 16 drives. The drives are either vertically or horizontally oriented in a matrix of 4 drives high by 4 drives wide. New drive arrays based on 2.5" form factors have up to 30 drives in an array of 5 high by 6 wide in a 2U chassis. Power consumption is an issue: An array of 30 2.5" drives will consume over 30 percent more power than an array of 16 3.5" drives. Drive manufacturers have reduced power consumption, but the increased density of drivers will demand smaller power supplies and increased output current.

Smaller chassis also have less space available for the robust drive carriers typically required to handle rotational vibration. Packaging 30 15,000 rpm SAS drives in a 2U enclosure while keeping resonances low enough to meet drive read and write success rates will be challenging. Disk drive reliability is directly impacted by the temperature of the "HDA" (Hard Drive Assembly) which needs to be kept at 50C during continuous operation.

Typically, the air intake is in the front of a chassis and is completely blocked by front access hard drives. The tiny spaces between the drives provide the only air inlet to cool everything—devices, midplane, disk array controller card and power supplies. That said, any fan failure, combined with a partially loaded chassis at a high ambient, can easily result in an HDA of 50C.

Connectors on disk array backplanes need to be extremely low profile so as not to block midplane airflow and have the flexibility to handle SAS and FC future data rates of >6Gbs, low-speed single ended signals and +12V or +5V current. 2U enclosures with 30+ drives will require more traces to route within less space due to more cutouts required in the midplane. Disk drive connectors are typically low profile without much wipe or guidance. This will continue to challenge the reliability of blind mating drives into midplanes.

Chris Heard is a Hardware System Architect at Teradyne, which had some interesting product demonstrations at DesignCon in January. Chris designs backplanes, chassis and cooling systems and holds patents on chassis level cooling, EMI containment and disk drive packaging.