Using iSCSI arrays is already a "green" approach to the data center. By giving each server as much storage as it needs, considerable efficiencies in numbers of hard drives, RAID controllers and power utilization can be achieved.

However, storage arrays can go further in the green arena if their operators pay more attention to how they use hard drives. Several of the vendors whose products we tested gave much lip service about going green generally and adhering to the Massive Arrays of Idle Disks (MAID) initiatives that provide energy savings by putting unused disks into standby mode, but only one, Nexsan Technologies, actually did anything about it.

The Nexsan SATAbeast has settings for three levels of power saving, all activated based on time as set by the system manager. The highest level actually spins disks down; whereas the lower levels either reduce disk speeds or park drive heads. That highest level takes the longest to recover back to full speed, but the others add very little latency. Obviously, a disk subsystem which is fully utilized all the time will never use these features, but it is easy to imagine scenarios in which at least some of the disks (the SATAbeast has 42, the highest density of any array tested) will be idle. The power and heat savings are considerable: our array went from drawing 7.1 Amps to 3.2 Amps after our 15 minute idle timer kicked off in the highest level of energy savings.

Scoring power usage is especially difficult because each of the arrays had a different amount of storage. For example, the Reldata 9240 Unified Storage Gateway had only 3.6TB of storage using 300GB SAS drives and drew 5.4 Amps of current. The StoneFly Storage Concentrator array has 16TB of storage using 1000GB SATA drives and drew 5.6 Amps of current. Therefore, the StoneFly product has a storage density per power unit four times higher than Reldata. However, the 15K RPM SAS drives that Reldata sent aren't substitutable for the 1000GB SATA drives that StoneFly used, so it's not fair to directly compare the two.

Similarly, some storage arrays had room for as few as 12 drives or as many as 42; others consisted of a controller that could be attached to multiple "shelves" of drives, gaining further power efficiencies as the number of drives increases.