Opinion

posted on 17 July 2008 14:18

Steve Willson, Solutions Manager, International, for COPAN Systems, writes:-

There are two approaches to saving power in a storage array. Firstly, you can use the in-built capability of the drive to spin the drive slower, or stop the spinning completely. The drive remains electrically active and most vendors choose only to reduce the spin speed. This results in very modest utility bill savings, as the drive still consumes power in this mode.

The second method is to completely remove power from the disk after a controlled spin down. This is what COPAN Systems does, it saves considerably more power and lengthens the drive life, as it is absolutely inactive.

The Provisioning of power

Bigger savings than the utility bill, comes from infrastructure costs of providing power. Unless the maximum overall power that can be consumed by the unit is reduced, the user must still provision adequate power and cooling for the entire system at 100% loading, so regardless of MAID or spin-down, the maximum power consumption and heat output are the killer metrics. It costs approximately $20,000 to provision 1KwH of power and associated cooling. As COPAN Systems load at 100% is typically 25% or less than that of any other array, this is where the major savings are made today, not in the utility bill. Saying that, most data centres require 40% power over and above the label-plate on every device in the centre to power infrastructure items (UPS, batteries, ancillary office areas etc).

This power is split for chargeback across the label plates on the components in the data center, so even when a device is not spinning, it is allocated a share of total overhead according to its maximum capability to draw power. Spin down does not assist with any infrastructure saving, or as a share of the overall data centre overhead.

Finally, many vendors fit power supplies that far exceed the need of the underlying technology, but this is used to specify power infrastructure and share of the chargeback in the data centre. The more over-rated the PSU is against actual usage, the greater the over-provision affect. This means that whilst at a consumption level, COPAN Systems is probably 4x better than any transactional device, at a practical provisioning and chargeback level this could be more than 10x more efficient.

This is the true cost of datacenter power challenges, not the spinning down of disks for a marginal utility bill saving, which is often negated by the costs of power ancillary equipment specified at a level far above the actual use case.

 ^{[Steve Willson, Solutions Manager, International, for COPAN Systems.]}