Sunday, December 14, 2008

Smaller is Bigger – why 2½” drives will replace 3½” drives in enterprise array architectures.

While it may not be intuitively obvious the latest generation of 2½”, small form factor disk drive outperforms its larger 3½” cousin by delivering a much improved volumetric capacity, higher IOPS and greater energy efficiency. That is why 2½” drives are on track to replace the 3½” drives as the incumbent technology in enterprise storage arrays.

Let’s look at some of the arguments in its favor.

1. Drive Capacity: 2½” drives now support 500GB with 1TB expected in the 2010 timeframe. Not yet on par with the 3½” drive but the gap is closing.

2. Volumetric Efficiency: How many drives can be packaged per unit volume? Each 3½” drive consumes 391³ cm compared to the slimmer 2½” at 66.6³ cm for a consumer class and 104³ cm for the enterprise class drive. This translates to a 5.8 and 3.76 space utilization advantage respectively. Note: at least one high performance array vendor is successfully using the consumer class 2½” drives.

3. Volumetric Density: How much storage capacity is available per unit volume? Using the above efficiency ratios and based on current drive capacities, volumetric storage density is increased by 2.9 and 1.88 respectively when using 2½” consumer or enterprise class drives. While the lower mass of the 2½” drive helps to minimize rotary random-operating vibration (RROV) and enables much denser packaging, limitations will temper the magnitude of the actual advantage realized in packaging density. Greater volumetric density translating to more drives per unit volume which equates to more available spindles driving greater access density. Bottom line is higher IOPS and aggregate bandwidth..

4. Energy Efficiency: How many TB can be supported by 1W of energy? Idle power consumption for a 2½” drive is approximately 0.7W with an average power consumption of about 1.3W. In comparison a 3½” drive consumes about 8W on idle and 12W on average. This means that a 2½” drive uses 11.4 times less power in idle and consumes 9.2 times less average power. However 3½” drives have greater maximum capacity, 1TB as against 500GB for the smaller 2½” form factor. I know there are now 1.5TB drives available but not yet in storage arrays. To normalize this difference the drive power consumption numbers were compared for a 5PB storage array. This reduced the 2½” advantage to 5.7 and 4.6, still significant considering that drives are at least 80% of the power hogs in a storage array.

A sharper comparison is to compare the TB/W metric.
a. 3½” drives support approximately 84TB/W
b. 2½” drives support approximately 384TB/W.

5. Performance: Bandwidth and IOPS. Seagate claim that their 2½” drive has twice the IOPS when compared to a 3½” drive. When this unit advantage is multiplied by an increased access density (available actuators/capacity) significantly higher IOPS can be realized. Bandwidth is another positive recipient of a higher spindle count to capacity ratio and when combined with miserly power demands, 2½” architectures are very power conscious, bandwidth solutions.

The bottom line is that a storage array based on 2½” drive technology can deliver higher storage density, more IOPS, higher aggregate bandwidth and with a lower energy consumption than an equivalent solution using current 3½” technology. The remaining challenges however are cost and reliability. While the cost of the 2½” is rapidly declining its current price point excludes it from applications where capacity is the driving requirement, particularly now that 1.5TB capacities are available in the 3½” form factor. However, if high performance and capacity are the requirements then 2½” technology is a very viable option. The second issue impacting adoption is the high duty cycle reliability rating (MTBF) of the 2½”. Not yet comparable with 3½” technology and is probably why the use of this technology is currently limited to solution architectures that can manage this current shortcoming.

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