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

December 4, 2007 by Devon
Cooke

Product
Western Digital Caviar Green Power WD7500AACS

750GB, 5,400 RPM Low Power Hard Drive
Manufacturer
Market Price
US$230~280

Most of the time, buying a hard drive comes down to two criteria: Price and
capacity. Performance may also be factored in to the equation on occasion, but
differences in drive performance tend to be small and, outside of a few specialized
applications, very difficult to notice. This is a tough situation for drive
makers, since it means that the best selling drives tend to be the ones with
the best price-to-size ratio — a scenario that leads to price wars and
lower margins.

One way out of this is to find a market segment that will pay extra for certain
features and design a product to fit that niche. Western Digital has done this
before with the Raptor X, and they appear to be doing something similar with
their new Green Power series. It's not hard to guess which sector it targets.
It's main claim to being Green is reduced power consumption — a claim that
Western Digital makes loudly. The marketing for the drive is filled with detailed
numbers about the amount of power you (might) save, the amount of money it (may)
save you, and equivalent amount of carbon it (potentially) saves.

Their most impressive claim is to have reduced power consumption by 40% over
regular drives. That's impressive, but, given that drives aren't terribly power
hungry to begin with, it translates into only 4~5W. That's something, especially
aggregated over thousands of drives in a data center, but it's not a whole lot
for an end user, especially given that most of the environmental cost of the
drive is tied up in the manufacturing process, not the energy it consumes afterwards.

Western Digital Caviar Green Power WD7500AACS

(from Western Digital's web
site
)
FEATURE & BRIEF COMMENT
IntelliPower™
— A fine-tuned balance of spin speed, transfer rate, and caching algorithms
designed to deliver both significant power savings and solid performance.
For each GreenPower™ drive model, WD may use a different, invariable
RPM.
Our 750 GB
sample is 5,400 RPM. Storage Review's 1 TB sample was 5,400 RPM. WD's literature
lists the possible speed range as 5,400~7,200 RPM, but we have yet to hear
reports of any models above 5,400 RPM.
IntelliSeek™
— Calculates optimum seek speeds to lower power consumption, noise,
and vibration.
Just-in-time
seeking that lets the seek head move more slowly when it would otherwise
have to wait for the latency of the spindle. Should be good for reliability
as well.
IntelliPark™
— Delivers lower power consumption by automatically unloading the heads
during idle to reduce aerodynamic drag.
A standard
notebook drive feature migrates to the desktop.
StableTrac™
— Secures the motor shaft at both ends to reduce system-induced vibration
and stabilize platters for accurate tracking, during read and write operations
(750 GB and 1 TB models only).
Presumably,
this is necessary to produce 3- and 4-platter drives.
Preemptive
Wear Leveling (PWL)
— Proactively monitors and prevents magnetic
wear during high read/write duty cycle applications.
Again, should
be good for reliability. Flash drives use a similar technique to best use
the limited number of read/write cycles.
Large capacity
— Up to 1TB of storage—ideal for graphic design, video editing,
gaming, advanced business computing, and other high-end desktop applications.
The current
maximum for most manufacturers.

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.

TEST RESULTS

Our sample was tested according to our standard
hard drive testing methodology
. Our methodology focuses specifically on
HDD noise, and great effort is taken to ensure it is comprehensively measured
and described. Performance is not tested, for reasons discussed in detail in
the methodology article. For comprehensive HDD performance testing results,
we recommend Storage
Review
, who have established a long reputation as the specialist in
this field. Their review of the Western Digital Green Power can be found in
a roundup of
several terabyte drives
.

Our test drive was compared against our reference drives, the Seagate Barracuda
IV and Samsung Spinpoint P80, which are profiled in our methodology article.
To get a good idea of where the drives in this review stand, it is important
to read the methodology article thoroughly. It was also compared against our
current low-noise champ: A 500
GB Western Digital WD5000KS
. A
250 GB Spinpoint P120
was also included in the comparison.

Two forms of hard drive noise are measured:

  1. Airborne acoustics
  2. Vibration-induced noise.

These two types of noise impact the subjective
perception of hard drive noise differently depending on how and where the drive
is mounted.

Both forms of noise are evaluated objectively and
subjectively. Both the subjective and objective analyses are essential to understanding
the acoustics of the drives. Airborne acoustics are measured using a professional
caliber SLM. Measurements are taken at a distance of one meter above the top
of the drive using an A-weighted filter. Vibration noise is rated on a scale
of 1-10 by comparing against our standard reference drives.

A final caveat: As with most reviews, our comments
are relevant to the sample we tested. Your sample may not be identical. There
are always some sample variances, and manufacturers also make changes without
telling everyone.

Ambient conditions at the time of testing were 18 dBA and 20°C.

DRIVE NOISE EVALUATION
Drive

Mfg date

firmware version
Vibration

1-10

(10 is best)
Activity State

Airborne Acoustics

Measured Power
Western Digital Caviar Green Power WD7500AACS

October 2007

firmware 01.01B01
7
Idle

19~20 [email protected]

3.3~5.9 W
Seek (AAM)
5.4 W
Seek (Normal)

21 [email protected]

6.7 W
REFERENCE DRIVES
Western Digital Caviar SE16 WD5000KS

March 2006

firmware 07.02E07
4
Idle

21 [email protected]

8.5 W
Seek (AAM)

21-22 [email protected]

8.6 W
Seek (Normal)

23 [email protected]

10.7 W
Seagate Barracuda IV

ST340016A - firmware 3.10
6
Idle

20 [email protected]

6.7 W
Seek (AAM)

23 [email protected]

11.3 W
Seek (Normal)

25-26 [email protected]

11.6 W
Samsung Spinpoint

P120 SP2504C


September 05 - firmware VT100-33
5
Idle

21 [email protected]

7.2 W
Seek (AAM)
9.4 W
Seek (Normal)

23-24 [email protected]

10.3 W
Samsung Spinpoint P80 (Nidec motor)

June 04 - firmware TK100-24
4
Idle

21 [email protected]

6.3 W
Seek (AAM)

23-24 [email protected]

8.3 W
Seek (Normal)

25-26 [email protected]

9.1 W
Samsung Spinpoint P80 (JVC motor)

Feb 05 - firmware TK200-04
6
Idle

21 [email protected]

6.2 W
Seek (AAM)

25 [email protected]

n / a
Seek (Normal)

27 [email protected]

9.3 W

Ladies and Gentlemen, the numbers don't lie: This is the quietest desktop drive
we've ever tested. Well — sometimes they do, but not this time; subjectively,
the drive was just as quiet as its barely-above-ambient objective results suggest.
The biggest difference? A large reduction in air noise. It makes sense —
just like a fan, a slower-spinning drive produces less air turbulence, which
means less noise. The lower amount of air noise uncovered a faint trace of electronic
noise, but this was not audible from a distance, and it would disappear entirely
inside a system.

Seeks were soft, low, and measured very well. However, they were slightly more
obvious than the last Western Digital we looked at — mainly because the
seeks on that drive disappeared under the whoosh of air noise. Without this
broadband noise, the seeks on the Green Power were clearly audible once more.
That said, measurement for AAM seeks was indistinguishable from the idle measurement,
so there's no question they are quiet. Any fan running above ~700 RPM should
generate enough noise to cover the seek noise completely.

Besides idle and seek noise, the Green Power produced one more noise: A pair
of sharp clicks whenever the heads were loaded or unloaded (that is, whenever
the drive went into or came out of idle mode). This noise was louder than the
similar noise made by most notebook drives, but quieter than the infamous head
reset noise made by certain Hitachi models. The noise was quite similar to seek
noise and was roughly the same volume. On close listening, it seemed slightly
sharper than the seek noise.

In addition to low direct noise, the Green Power also tied the best (that is,
lowest) vibration of any desktop drive we've tested. Once again, the lower rotation
speed can be credited. Not only does the lower speed translate into less momentum,
but the resonant frequency is lower and thus harder to hear. On our highly resonant
aluminum test box, the drive produced just enough hum to have a presence, but
it was difficult to pick out the hum on its own. In a real system, it would
be unlikely to be heard at all.

With such good measurements, we were curious to see how the Green Power stacked
up against the legendary Barracuda IV. This drive has stood as the quietest
desktop drive ever since we first saw it, though it has been obsolete for years.
The verdict? The Green Power had slightly more air noise, but it lacked the
slight whine that the Barracuda IV's exhibited. Of the two, the air noise was
slightly easier to ignore, but it's not a huge advantage either way. Seek noise,
on the other hand, was squarely in favor of the Green Power. The Barracuda IV
had short, staccato seeks that drew attention to themselves with their suddenness.
The Green Power sounded far more relaxed (as well as being quieter overall),
and the rumble of the seeks blended easily into the background.

The Green Power's biggest claim to fame is power consumption, and our measurements
confirmed its efficiency. Measuring the power at idle proved difficult, since
the drive appeared to cycle certain functions on and off apparently at random,
so the power consumption rarely stayed the same for more than a few seconds.
During our measurements, we observed everything from 3.3W to 5.9W at idle, but
the most common level was 3.7W when the heads were unloaded. This is by far
the most efficient drive we've tested, with the next-closest competitor being
a 160 GB model from Hitachi, at 4.7W. Seek power was similarly frugal, with
no other drives coming close.

Western Digital's estimate of a 40% power savings actually seems a little conservative
on the basis of the drives we've tested, as the Green Power actually drew less
than half the power of some of the drives in our database. This was true no
matter what state the drives were in.

AUDIO RECORDINGS

Audio recordings were made of the drives and are presented here
in MP3 format. The recordings below contains 5 seconds of ambient noise, and
10 seconds of idle noise followed by 10 seconds of seek noise with AAM enabled
and 10 seconds without.

Keep in mind that the recordings paint only part of the acoustic
picture; vibration noise is not recorded, and drives often sound different depending
on the angle from which they are heard.

Reference Comparatives:

HOW TO LISTEN & COMPARE

These recordings were made
with a high resolution, studio quality, digital recording system, then
converted to LAME 128kbps encoded MP3s. We've listened long and hard
to ensure there is no audible degradation from the original WAV files
to these MP3s. They represent a quick snapshot of what we heard during
the review. Two recordings of each noise level were made, one from a
distance of one meter, and another from one foot
away.

The one meter recording
is intended to give you an idea of how the subject of this review sound
in actual use — one meter is a reasonable typical distance between
a computer or computer component and your ear. The recording contains
stretches of ambient noise that you can use to judge the relative loudness
of the subject. For best results, set your volume control so that the
ambient noise is just barely audible. Be aware that very quiet subjects
may not be audible — if we couldn't hear it from one meter, chances
are we couldn't record it either!

The one foot recording is
designed to bring out the fine details of the noise. Use this recording
with caution! Although more detailed, it may not represent how the subject
sounds in actual use. It is best to listen to this recording after you
have listened to the one meter recording.

More details about how
we make these recordings can be found in our short article: Audio
Recording Methods Revised
.

We also made a couple of illustrative 3D waveform images from our recording. The first below is a 5-second clip of the WD Greenpower 750 during idle. It may be too small for you to see the details clearly; clicking on it will open a 1008-pixel wide image in a new browser window. Note how smooth the waveform is above the 90Hz fundamental, in frequency and over time. This reflects the smooth quality of the drives noise.



Smooth idle noise reflected in this 3D waveform image.

The second 3D image below is a 5-second clip of the WD Greenpower 750 during seek. It's very similar; most of the difference are at or below the fundamental 90Hz frequency, where there are more changes in amplitude (peaks and valleys) over time. The rest of the frequency / time domain is still very smooth, like in the idle waveform 3D image



Smooth seek noise reflected in this 3D waveform image.


CONCLUSIONS

It's the quietest desktop drive we've ever tested. It's the most
efficient drive we've ever tested. And it comes in capacities up to 1 TB. What
more do you want? As far as SPCR is concerned, this could be the perfect drive
— at least until flash media takes over.

We didn't think we'd ever find a drive to replace our trusty Barracuda
IV — but then, we never expected a manufacturer to release a mainstream
5,400 RPM drive either. We are pleased and surprised to discover we were wrong
on both counts.

The only real question mark hanging over the Green Power is how
much the rotation speed affects performance. And, while no reports show it leading
the pack, it doesn't seem to be far enough back to worry about. Its high capacity
250 GB platters help mitigate the lower spindle speed.

We're not sure how green the Green Power actually is — we're
pretty sure a 4W savings doesn't add up to much, especially given the environmental
cost of manufacturing the drives in the first place. However, we're happy to
see Western Digital pay attention to this market segment, if only because it
means our own neglected segment — silent computing — gets such a fantastic
product to play with. Western Digital has recognized that there's more than
just performance and capacity. We hope they go far with it.

Many thanks to Western
Digital
for the Caviar Green Power sample.

*

SPCR Articles of Related Interest:

SPCR's Hard Drive Testing Methodology

SPCR's Recommended Hard Drives

Western Digital Caviar SE16 500 GB: Big,
Low Noise Champ?


Samsung Spinpoint T Series: Successor to
a Quiet Legacy


Samsung Spinpoint P120 200 & 250 GB Hard
Drives

* * *

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