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Are SSDs Really More Power Efficient?
Bakasama writes "Tom's Hardware compared the power performance of several available SSD cards with a Rotating HDD that was chosen specifically for its poor power efficiency.
The results seem to fly in the face of current wisdom.
'Flash-based solid state drives (SSDs) are considered to be the future of performance hard drives, and everyone seems to be jumping on the bandwagon. We are no exception, as we have been publishing many articles on flash-based SSDs during the last few months, emphasizing the performance gains and the potential power savings brought by flash memory. And there is nothing wrong with this, since SLC flash SSDs easily outperform conventional hard drives today (SLC = single level cell). However, we have discovered that the power savings aren't there: in fact, battery runtimes actually decrease if you use a flash SSD.'"
I believe that much of the problem is that SSD's are still a new technology compared to rotating disks. Right now, engineers are more concerned with increasing capacity and just making the damn things work. These are much more important than efficiency. As time goes on and the technology gets more mature, efficiency will get more attention from engineers.
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I wish they would measure power usage under conditions that many notebook computer users actually use them in, which does not include running synthetic benchmarks on their computer 100% of the time it's running. Of course, if you keep the machine writing to the ssd constantly then it's not going to show power savings. But how many mobile users' usage patterns include constant reading from and writing to disk?
I believe this comment from the article could explain some of this away.
'There could be a systematic error in the benchmarks shown: if the flash based "disks" are faster then the whole system CPU/MEM/Chipset would draw much more power with flash "disks" compared with conventional disks - just because the benchmarks could run more often in the same time.
Maybe one should compare something like playing video from disk where it is assured that the systems do precisely the same work?'
Exactly... The thing about spinning platters is that it takes energy to start up _and_ keep it spinning. So obviously doing read/write 100% of the time would bias towards the conventional hard drives.
Hell, even read/write 10% of the time is too much for normal usage.
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In TFA, there is a graph on page 14 with power consumption measurements for the 5 drives tested.
The SanDisk SSD shows 1.0 watt active, 0.5 watt idle.
The Hitachi drive (magnetic) shows 3.2 watts active, 1.1 watts idle.
So even if the SanDisk drive spent 100% of its time in active mode and the Hitachi drive was always idle, the SanDisk drive should still provide longer runtime.
However, their runtime test (page 12) shows 7:03 runtime with Hitachi, 7:02 with the SanDisk.
All they have to say about this is:
Most of the power consumption measurements are in line with our results in Mobilemark 07. However, it has become clear that idle and maximum power do not provide the full picture when we talk about flash SSDs.
Well, something clearly is wrong here.
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Unfortunately, the article comes to miserably faulty conclusions, and nobody seems to have noticed it. One thing everybody needs to note is this: the test that Tom's ran was designed to run a program to completion over and over again. All that Tom's came up with, thanks to that test, is that the computer with the SSD ran out of power faster. What they SHOULD have noticed, and what any sane and logical tester would have done, is counted the number of times the program RAN. A system with an SSD will run the program a significantly higher number of times because of its increased speed and lack of moving parts. With the higher number of runs, the CPU STAYS MORE ACTIVE, sucking more power out of the system and thus causing it to power down earlier. The only reason the HDD system died later is because the CPU idles waiting for it to retrieve data. The SSD may have chewed up your battery faster, but it did, comparatively, run through that program a BUNCH more times. I am willing to bet the work per watt was much better out of the SSD system than the HDD - and once again, Tom's Hardware's scientific testing system proves itself anything but.