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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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