WD Green Power: A New Benchmark in HDD Acoustics & Power

Storage
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ROTATION SPEED

So how has Western Digital managed to improve power usage to 40% below its competitors? Some of the savings comes from adopting notebook technologies like parking the head during idle. The IntelliSeek technology that reduces the speed of the seek head when it would otherwise end up waiting for the disk to rotate beneath it also helps. However, the lion's share of the reduction most likely comes from a simple technique that Western Digital mentions but does not explain.

It's quite simple, really. Most of a drive's power is consumed by the motor that spins the disk inside the drive. Reduce the speed of the disk, and you reduce the amount of power required. However, Western Digital doesn't want to say that they're selling 5,400 RPM drives — those became second class in the desktop market years ago. Instead, they rate the drive's speed as "IntelliPower" and take pains to emphasize that there are other factors that affect performance.

Western Digital has caught a lot of flak for withholding the rotation speed of the Green Power, especially when the product was first launched and the marketing material listed the rotation speed as 5,400-7,200 RPM. This led some to speculate that the rotation speed changed dynamically during use — which would have been an impressive engineering feat had it been true. The reality is revealed by a sentence that Western Digital added to the description of IntelliPower: "For each GreenPower™ drive model, WD may use a different, invariable RPM." In other words, Western Digital reserves the right to release both 5,400 RPM and 7,200 RPM drives under the Green Power name — without telling you which are which.

We were able to confirm that our 750 GB Green Power had a spindle speed of 5,400 RPM by analyzing its sound spectrum. Why sound? Sound is vibration; the pitch of the sound corresponds to the frequency of the vibration. Hard drives vibrate at the speed of their motor, so they produce a noise at the same frequency as their rotation speed. Our sample had a sharp spike at exactly 90 Hz (cycles per second). Multiplying that number by 60 (to get cycles per minute) yielded a measured rotation speed of 5,400 RPM.


This little Frequency / Amplitude graph tells us the WD Green drive spins at 5,400 RPM.

It's possible that other Green Power models use a higher spindle speed — but we doubt it. Storage Review tested the 1 TB version of the drive and determined that that model also spun at 5,400 RPM based on a calculation of the drive's latency compared to a previous Western Digital model. That leaves the 500 GB model — which Western Digital says is even lower power than the larger capacity versions. With the majority of the Green Power's efficiency advantage coming from its lower speed, it seems impossible for the 500 GB model to use a higher rotation speed. It's possible Western Digital intends to release a 7,200 RPM version at some point in the future.

Unfortunately, even if they do, you won't be able to tell from the model number. According to Western Digital's model numbering scheme, the second-to-last letter in the model number should designate a specific spindle speed / cache size combination, but the "C" in all of the Green Power models is not listed among the possible letter codes (the reference document hasn't been updated since 2005). It's worth mentioning that the Raid Edition Green Power drives use a different letter ("P") in this position, but The Tech Report has apparently confirmed that the spindle speeds in the RE line do not differ from the regular Caviar models (the two are mechanically identical).


Western Digital has finally done away with the old Molex power connector.

SPECIFICATIONS

The specifications below are specific to model that we examined. Capacity, cache size, platter number, interface, and even performance vary from model to model even within a single product line. Acoustics and power dissipation also vary depending on the number of platters in the drive; smaller capacity drives tend to have fewer platters, and tend to produce less noise and use less power.

Specifications: WD Caviar Green Power WD7500AACS
(from Western Digital's data sheet)
Capacity
750 GB (750,156 MB)
Cache
16 MB
Disks / Heads
3 / 6
Interface SATA 3Gb/s
Spindle Rotation Speed
IntelliPower* (5,400 RPM)
Latency
Not Specified (5.56 ms)
Read Seek Time
8.9 ms
Buffer to Disk Transfer Rate
1160 Mb/s
Weight
1.52 lb. (0.69 kg) ± 10%
Power Requirements: Idle / Seek
4.0W / 7.4W
Acoustics: Idle / AAM / Seek
24 / 25 / 29 dBA

The noise and power specifications are both of interest because of how low they are. 4W at idle is about half of what we expect from a conventional 7,200 RPM drive, though Hitachi's single-platter HDS728080PLA380 is notable for having a 4.7W idle. Likewise, the idle noise level of 24 dBA is significantly lower than other Western Digital drive's we've seen. In addition, the last Western Digital we saw made it onto our recommended list as having some of the quietest seeks we've ever heard. We have high hopes for this one.

A WORD ON PERFORMANCE

As a general rule, our reviews ignore performance in favor of noise and power. In a world where most drives are just dumping places for data, we believe that noise and power (and reliability) are more important than speed. Our specialty is testing noise, not performance.

That said, we can't ignore the fact that our Green Power is a 5,400 RPM drive competing in a 7,200 RPM field. Rotation speed does affect performance, and we'd be remiss if we glossed over this entirely. Performance reviews around the web have been mixed, with the Green Power lagging behind the performance leaders. Reviews at Storage Review and The Tech Report both acknowledged that the performance isn't record-breaking, and — let's be fair — it's not really meant to be. Western Digital is betting that there are people out there that value low power more than performance. We're solidly behind them on that one; we make a similar value judgment every time we recommend a slower, quieter part over a faster, noisier one.

In real terms, the performance difference is minimal. The vast majority of laptops use 5,400 RPM drives, and few people complain that this isn't fast enough. We've been recommending 5,400 RPM notebook drives as a quiet storage alternative for years now. Besides, the large 250 GB platters in the Green Power go a long way to mitigating the performance penalty of the slower rotation speed.



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