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Playing With Power

Watt-ergate: The SSD Power Consumption Conspiracy Exposed
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When you have to work with a hard drive, there are a number of ways to tweak the drive that can affect its power consumption. Unfortunately, these methods are not especially helpful for SSDs. After all, you can’t customize the spindle speed of a drive that has no spindle. It might be possible, if SSD vendors enabled chip-level control of a drive’s NAND, to disable/suspend a portion of the NAND during periods of low activity. This could cut down power consumption, theoretically, but it would likely create as many problems as it solves, fouling up wear leveling, blocking access, and generally wreaking havoc on the drive’s longevity.

One way you can influence the give and take between an SSD’s power consumption and its NAND flash is to simply buy a bigger drive. Here’s why: The SSD’s flash modules and its controller make up over 90% of the drive’s power consumption. A gigantic 1 TB SSD might require 64 NAND chips to reach that capacity, but depending on the drive and its NAND, actively using all of the chips at once may not be possible. The SSD will likely saturate its 9 W SATA power interface by the time only 32 chips are active. In this way, the drive’s full complement of NAND is available, but only half of it is drawing power at any given time.

In this situation, though, be prepared to pay a performance tax. When a drive’s NAND channels are inactive, the SSD has fewer available options for striping data. Don’t be surprised if a drive isn’t as quick on its feet.

In order avoid the performance hit, you could spread the 1 TB across two identical 500 GB drives, each outfitted with half the number NAND chips as the larger SSD. The drives will be able to use all of their NAND without exceeding the 9 W SATA interface power limit, which lets the host controller address all 64 chips simultaneously.

Again, though, there’s a tradeoff. Even though your performance won’t suffer the way it would with a 64-chip, 1 TB SSD, the two 500 GB drives will exact their toll in the form of increased overhead due to the controller having to coordinate data across two drives. There’s also a good chance you’ll pay a slight price premium buying two 500 GB SSDs instead of a single 1 TB SSD. Finally, the cumulative power draw of two drives will probably be 1 W to 3 W higher because of the extra drive controller, according to Seagate.

When it comes to SSDs and power consumption, nothing is as simple as it seems. If you have the good fortune of getting to upgrade your enterprise storage to solid state drives, you have to look beyond one or two specs on a datasheet. It’s far more instructive and beneficial to you in the long run to research a drive’s power consumption when dealing with an enterprise workload, and, if possible, your particular enterprise workload. Duty cycles are important, as is specific power consumption during read and write operations. When you have a full picture of an SSD’s power consumption, performance, and endurance, you’ll have a much better idea of whether it’s right for your organization.