Power-Sipping Desktop Hard Drives
The hard-disk drive (HDD) is the most important, mostly mechanical component in regular use on computer systems. Due to their highly mechanical nature, it is easy to believe that disks consume the most amount of power, especially when thrashing (that is, rapidly moving the heads to various locations -— a process that generates an easily recognized sound).

This is easy to believe, but incorrect. Rarely do HDDs consume more power than the processor. The disk industry has been remarkably effective over the years at keeping power consumption low.

A typical mid-range HDD today uses somewhat under 10 watts. Actual consumption depends on the type of drive, capacity and usage. Let's first look at how to measure consumption for this one component, and then see what the figures tell us.

Measuring HDD Power Usage

In my previous column, I discussed how to measure the power consumption of a system. In all cases, you want to measure consumption at the plug. That is how much power is being consumed in total. I pointed to one inexpensive means of doing this, which is to use the P3 Kill-A-Watt Electricity Usage Meter from P3 International.

The problem that immediately comes to mind is how to use this meter to test the power consumption of a single disk. No one is going to swap disks in and out of a desktop system to ascertain wattage. The solution is a product from the USB 2.0 Universal Drive Adapter from Newertech, which sells for $29.95 retail.

This kit consists of a small power supply to drive an HDD and a USB cable that can hook the HDD to a system. This cable supports ATA drives, including notebook and 3.5-inch drives, plus SATA drives. You plug the USB cable into your system and into the hard drive and attach the drive to its power supply. Suddenly the HDD is running. It appears to your system as a plain old USB drive. By plugging the adapter's power supply into your watt meter, you can see power consumption by this one drive. And by having the HDD hooked to your computer, you can make the drive active or let it spin undisturbed.

Apart from the power measurement considerations, this set-up is useful for peering at the contents of old HDDs without having to mount them into your system. This use is the principal use intended for this adapter.

I recently ran some parallel ATA (a.k.a IDE) drives through this set-up, and they registered around 5 watts when the disk was spinning but not doing I/O, and at around 8 watts during I/O. When compared with the 40 watts to 100 watts consumed by the processor, it's clear that despite being mechanical devices, HDD are remarkably power efficient, even in the bad old days before power consumption was on anyone's radar drive.

I pulled out an old HDD from 1995 and it consumed only 13 watts when just spinning. If you measure consumption in watts/terabyte rather than pure watts, then the progress vendors have made in the last ten years is off the charts, although this is mostly due to their abilities to increase capacity.

SATA drives run at roughly 1 watt more in quiet and I/O states, and SCSI drives run at typically 2 watts more than the ATA numbers.

To come up with at unified number for a given disk drive, the formula that is used is:

Typical power consumption = idle * .90 + write * .025 + read * .075

Since you probably won't be able to distinguish read from write in terms of power consumption (although these numbers are available from vendor specifications), you can use this formula as a basis:

Typical power consumption = idle * .90 + I/O * .10

While it's clear that hard drives should rarely be the principal focus for energy conservation on desktop systems, servers and storage boxes, their figures become more important.

Because of this, HDD vendors are stepping up their energy conservation efforts. Part of the motivation for this is the release of ENERGY STAR 4.0, which establishes low-power consumption thresholds and no vendor wants to be rejected because its components push a system over the tight baseline requirements.

In this regard, Western Digital -- one of the leading drive makers —- is moving to a new product line called GreenPower. The company claims that its 1GB SATA drive runs at 4 watts when idle and 7.5 watts during I/O (I am expecting a drive to look at and will report on this if there are variances). This would raise the bar for power efficiency on desktop drives. I expect SCSI drives will eventually be on that same power curve as well.

Save Power: Hibernate

All of this means that power consumption is rarely the correct primary factor for upgrading HDDs. Secondly, if you do upgrade your HDDs, you'll get a small lift in absolute power savings and an enormous lift in watts per GB. The best way to save power on your disks, especially on desktops, is to use the built-in power reduction technology in your operating system that puts HDDs into hibernate mode following a given portion of inactivity. Per vendor specs, this move will do the most to save energy by cutting it roughly in half for the time spent asleep.

For more information on power consumption by specific drives, go here.

manufacturers simply don't publish this data. In the case of PC, server, and system vendors, there is arguably some justification for this policy, as each configuration is different, and so no fixed usage number would be a true reflection. A few hardware vendors provide basic power calculators for some PC and server models (here is a calculator for PCs from Dell), but rarely do these provide the comprehensive information you need for an informed decision. The absence of data from vendors of components, where configuration is far less of a factor, is an equal pain and one that I expect will improve as reductions in power consumption increasingly become a competitive factor.

The upshot is that many IT managers have no real feel for how much power systems and components actually consume. For example, how much do all those desktops running late at night cost? How about workgroup servers? Blades vs. pizza-box servers? High-end PCs vs workstations? And so on.

One way to start getting a handle on this is to use a simple wattmeter and begin walking around the enterprise testing various configurations. The results, I assure you, are eye-opening. For starters, pick up an inexpensive meter such as the Kill-A-Watt Electricity Usage Meter from P3 International.

As you can see, the meter has a three-prong outlet into which you plug the device you want to test, and then you choose the metric you're trying to quantify from the front row of buttons: volts, amps, watts, watt-hours (for long-lasting tests), and so on. The data is presented on a large four-digit display from which you will begin to get some sense of what's eating up your power budget.

Let's begin with desktops. Last year, our firm purchased a Dell Dimension E521 PC with a dual-core AMD processor, 2GB RAM, two hard-disk drives, and a mid-range graphics card that can drive two large flat panel monitors. (Today, this machine, called the Dell Inspiron E531, retails in this configuration for under $700.) When plugged in and running, but performing no particular task, the system consumes 110 watts, according to the watt meter.

This machine replaced a dual-Xeon Dell workstation with 3GB of RAM, two disks, and a lesser graphics card. That system, per the meter, used 191 watts at rest. But that difference is only part of the story. We ran benchmarks on both systems, using the Sandra benchmark software and found a wide difference in platform capabilities. The tests rated the new system at 29.4 (where higher is better), while the old system came in at 22.7. Combining the two data points revealed that the old system delivered less than half performance per watt of the new PC. How then to translate the performance boost into further energy savings?

Maximizing That Benefit

Looking only at the power savings, the gains are modest: 1.9KWh per day. If your electrical utility charges $0.10/KWh, this number translates into savings of roughly $70/year. Multiplied by each desktop, however, the sums start to add up.

The economics become better when you include performance in the mix. One PC can do the work of two. If you were to use virtualization, you could run two PC instances on the new system-and the savings would now be considerably more. You would have the previous savings plus the entire power consumption of the second PC. In our case, if we saved the full consumption of the old Dell workstation, our new system would reduce daily consumption by 6.5KWKh and save us $238 per year. At this rate, we would have pay back on the new system in less than three years on power savings alone.

On the surface, the math might seem incorrect, because two virtualized PCs on a single system would generate twice as much activity, so using the savings numbers calculated from a PC at rest are not reflective of the real world. This is a valid point, but it obscures another factor. Most PCs use very little of their computing power, even when they are so-called "busy." This is particularly true on dual-core or multiprocessor systems, where many applications can only run on one core; so running two applications at the same time presents little additional performance load, although admittedly it does consume more power.

The comparatively little stress applications place on PCs is meaningful in assessing PC (and server) consolidation. Reports from enterprises that employ virtualization for consolidation consistently point to an unexpected pattern: they are generally able to consolidate more systems on to one host than they originally expected-again because most PCs and many servers spend much of their time idle. In the example I am developing, the reality is that you probably could consolidate three or four PC loads onto the new system-and now the ROI is very quick and the savings become important.

None of this would be visible to a manager, however, without the ability to measure power consumption. And for this you need a tool, even as primitive as a simple watt meter to start to learn basic energy-consumption patterns.

As I will discuss in future columns, the ability to track consumption at fairly low levels is an important part of being able to intelligently plan the purchase and deployment of hardware. In the words attributed in multiple variations to Lord Kelvin: "You can't manage what you don't measure."