Power-efficient drives are drives that slow down and go into a standby or idle mode and do exactly what they say they will do -- they save power. The challenge with these drives is that many manufacturers are putting these drives into standard array shelves, typically with the same power supplies and the same fans. The array shelf still has to be designed to assume that the drives will spin up at full power, because at some point they probably will.

The bigger problem is the intelligence that is used to write data to these drives. To the array controllers, these are just another set of drives in a cabinet. The power management happens in the background. If there is no intelligence in how data is written to these drives, then their ability to spin down or go idle on a consistent basis is limited. If you create logical unit numbers (LUNs) as you would traditionally across five or more of these drives, any write to any LUN across those drives will cause ALL the drives to spin up.

The newer data is, the more likely it is to be accessed; the older it becomes, the less likely it is to be accessed. As data became older you would want to move that data to these power-efficient drives. For them to be optimized, you or the storage controller would need to ensure that this data is grouped together by age and by likelihood of being accessed again.

The storage administrator could manually manage the process by creating array groups with a low drive count, only allowing one LUN per RAID group, and then manually knowing to start moving old data to subsequent RAID groups. This would allow you to limit how often the drives spin up. Invoking another manual process into the storage administrators day isn't what is needed. This would be very complex, and a nightmare to figure out what data is where.

What is needed is intelligent data layout. In primary storage, companies like 3Par and Compellent will have this capability built in because of their ability to understand data at a block level. Moving data to power efficient drives could be just another tier in their strategy. Copan Systems is delivering this type of intelligence today for backup and archive data. It has the ability to intelligently lay data out so the old archive data doesn't get mixed with new archive data.

Before you race out to add power-efficient drives to your environment, make sure that your supplier has thought through these issues and can provide automation to the data layout process. If it can't, look elsewhere, because in green, you can't wait.

George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualization and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integrators. An industry veteran of more than 25 years, Crump has held engineering and sales positions at various IT industry manufacturers and integrators. Prior to Storage Switzerland, he was CTO at one of the nation's largest integrators.