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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?'
...it's like LCD TVs, people also thought they consumed less power than conventional CRTs. Personally, I can warm my hands if I stick the palms up in front of my 32" LCD which chugs away at 152W when fully "lit" (powersave mode off).
A 32" Philips HD ready CRT was around 100-110 W at the time I looked.
However, this is highly dependent on brand as well.
The thing is that Tom measured battery life with the hard drives constantly working.
He glosses over this with the following statement:
No, Tom, no one turns on their computer and simply waits for the battery to die. However, no one turns on their computer and has their hard drive constantly thrashing either.
Typical usage patterns include document editing, movie watching, music listening, etc, which involve very, very small amounts of hard drive access.
Use a better battery benchmark that leaves the hard drive idle most of the time, then come back and let us know how these drives fare.
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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He makes the claim in the comments about the article that "well who just watches dvds? You have to keep the system busy and test that!" That's about as valid as setting the machine not to sleep and seeing how long it can idle there.
On an ultraportable especially, you aren't going to be churning the CPU with a benchmarking program. You won't be rendering animation frames. Mostly you'll be in a web browser or text processing program, waiting on the user to type something. With occasional spurts of OS and program start/stop. Good gosh it sounds like a MIX of tasks, rather than either extreme.
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Why are we still using swap? We can put, cheaply, more than 4 gigs of ram in a machine. With the differences in SSD, and the concerns about power efficiency, it really makes no sense to me that machines are still being designed to page out memory.
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I have an old flat-panel display (NEC-Mitsubishi NXM76LCD) that takes in 25W at full brightness. It's far less than what current LCD models draw, but it doesn't scream about 2ms timing, either. (Find any benchmark, and a manufacturer will take it to irrelevance. Do you need 2ms if you're not refreshing at 500 Hz?)
Well, since power usage is directly related to heat output, we could benchmark SSD's versus standar drives much more scientifically; we could even use the author's broken benchmarking tool. Put the hard drive into a calorimeter, run the benchmark for 10 minutes and measure the total heat output. Repeat for the drive you are benchmarking against and presto, actual, scientific numbers on power usage for just that device, rather than simply plugging away until the battery runs dry.
I am sure you can get one of those old fashioned hard drives that consume less power, hold much, much more for much less than $1300. It even writes faster and no special software required.
People are rushing to this new technology, I thank them for their beta testing efforts and bringing the costs down. I will probably adopt when:
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.
I'm not so sure your point is quite that significant. There is a graph, towards the end, that seems to be multiplying throughput by total battery life, giving a sort of "megabyte-minutes" rating for the different drives. In terms of simple hard drive throughput, this seems to indicate that the work per watt of the traditional drive was still superior (albeit by a small margin over the most expensive SDD), despite your complaint. But obviously it's not quite that simple--no real life usage would cause non-stop disk access like that.
The claim that the CPU stays more active with the faster drive, while technically true, is a little misleading and not nearly as clear cut as you're making it out to be. The only time the CPU would really be more active with a faster drive is under circumstances where it would be waiting for some kind of blocking I/O from the drive, which in my experience (at least under mundane use) isn't all that much. Most of the time you're much more likely to be dealing with system RAM than hard drive storage during program use (unless you run out of memory and start swapping things out, but then you've got other problems).
In short, while you point out perhaps an interesting oversight, I don't think it is quite as serious as you make it out to be.