Why Do We Need Faster SSDs

The claim I've often seen around the Internet is that today's SSDs are already "fast enough" and that there is no point in faster SSDs unless you're an enthusiast or professional with a desire for maximum IO performance. There is some truth to that claim but the big picture is much broader than that.

It's true that going from a SATA SSD to a PCIe SSD likely won't bring you the same "wow" factor as going from a hard drive to an SSD did, and for an average user there may not be any noticeable difference at all. However, when you put it that way, does a faster CPU or GPU bring you any noticeable increase in performance unless you have a usage model that specifically benefits from them? No. But what happens if the faster component doesn't consume any more power than the slower one? You gain battery life!

If you go back in time and think of all the innovations and improvements we've seen over the years, there is one essential part that is conspicuously absent—the battery. Compared to other components there haven't been any major improvements to the battery technology and as a result companies have had to rely on improving other components to increase battery life. If you look at Intel's strategy for its CPUs in the past few years, you'll notice that mobile and power saving have been the center of attention. It's not the increase in battery capacity that has brought us things like 12-hour battery life in 13" MacBook Air but the more efficient chip architectures that can provide more performance while not consuming any more power. The term often used here is "race to idle" because ultimately a faster chip will complete a task faster and can hence spend more time idling, which reduces the overall power consumption.

SSDs are no exception to the rule here. A faster SSD will complete IO requests faster and will thus consume less power in total because it will be idling more (assuming similar power consumptions at idle and under load). If the interface is the bottleneck, there will be cases when the drive could complete tasks faster if the interface was up for that. This is where we need PCIe.

To demonstrate the importance of an SSD from the battery life perspective, let's look at a scenario with a hypothetical laptop. Let's assume our hypothetical laptop has a 50Wh battery and only has two power states: light and heavy use. While in light use, the SSD in our laptop consumes 1W and 3W under heavier load. The other components consume the rest of the power and to keep things simple let's assume their power consumptions are constants and do not depend on the SSD.
 
Our Hypothetical Laptop
Power Consumption Light Use Heavy Use
Whole Laptop 7W 20W
SSD 1W 3W

Our hypothetical laptop spends 80% of its time in light use and 20% of the time under heavier load. With such characteristics, the average power consumption comes in at 9.6W and with a 50Wh battery we should get a battery life of around 5.2 hours. The scenario here is something you could expect from an ultraportable like the 2013 13" MacBook Air because it has a 54Wh battery, consumes around 6-7W while idling and manages 5.5 hours in our Heavy Workload battery life test.

Now the SSD part. In our scenario above, the average power consumption of our SSD was 1.4W but in this case that was a SATA 6Gbps design. What if we took a PCIe SSD that was 20% faster in light use scenario and 40% in heavy use? Our SSD would spend the saved time idling (with minimal <0.05W power consumption) and the average power consumption of the SSD would drop to 1.1W. That's a 0.3W reduction in the average power consumption of the SSD as well as the system total. In our hypothetical scenario, that would bring a 10-minute increase in battery life.

Sure, ten minutes is just ten minutes but bear in mind that a single component can't do miracles to battery life. It's when all components become a little bit faster and more efficient that we get an extra hour or two of battery life. In a few years you would lose an hour of battery life if the development of one aspect suddenly stopped (i.e. if we got stuck to SATA 6Gbps for eternity), so it's crucial that all aspects are actively developed even though there may not be noticeable improvements immediately. Furthermore, the idea here is to demonstrate what faster SSDs provide in addition to increased performance—in the end the power savings depend on one's usage and in workloads that are more IO intensive the battery life gains can be much more significant than 10 minutes. Ultimately we'll also see even bigger gains once the industry moves from PCIe 2.0 to 3.0 with twice the bandwidth.

4K Video: A Beast That Craves Bandwidth

Above I tried to cover a usage scenario that applies to every mobile user regardless of their workload. However, in the prosumer and professional market segments the need for higher IO performance already exists thanks to 4K video. At 24 frames per second, uncompressed 4K video (3840x2160, 12-bit RGB color) requires about 900MB/s of bandwidth, which is way over the limits of SATA 6Gbps. While working with compressed formats is rather common in 4K due to the storage requirements (an hour of uncompressed 4K video would take 3.22TB), it's not uncommon for professionals to work with multiple video sources simultaneously, which even with compressing can certainly exceed the limits of SATA 6Gbps.

Yes, you could use RAID to at least partially overcome the SATA bottleneck but that add costs (a single PCIe controller is cheaper than two SATA controllers) and especially with RAID 0 the risk of array failure is higher (one disk fails and the whole array is busted). While 4K is not ready for the mainstream yet, it's important that the hardware base be made ready for when the mainstream adoption begins.

What Is SATA Express? NVMe vs AHCI: Another Win for PCIe
POST A COMMENT

131 Comments

View All Comments

  • Khenglish - Thursday, March 13, 2014 - link

    That 2.8 uS you found is driver interface overhead from an interface that doesn't even exist yet. You need to add this to the access latency of the drive itself to get the real latency.

    Real world SSD read latency for tiny 4K data blocks is roughly 900us on the fastest drives.

    It would take an 18000 meter cable to add even 10% to that.
    Reply
  • willis936 - Thursday, March 13, 2014 - link

    Show me a consumer phy that can transmit 8Gbps over 100m on cheap copper and I'll eat my hat. Reply
  • Khenglish - Thursday, March 13, 2014 - link

    The problem is long cables is attenuation, not latency. Cables can only be around 50M long before you need a repeater. Reply
  • mutercim - Friday, March 14, 2014 - link

    Electrons have mass, they can't ever travel at the speed of light, no matter the medium. The signal itself would move at the speed of light (in vacuum), but that's a different thing.

    /pedantry
    Reply
  • Visual - Friday, March 14, 2014 - link

    It's a common misconception, but electrons don't actually need to travel the length of the cable for a signal to travel through it.
    In layman's terms, you don't need to send an electron all the way to the other end of the cable, you just need to make the electrons that are already there react in a certain way as to register a required voltage or current.
    So a signal is a change in voltage, or a change in the electromagnetic fields, and that travels at the speed of light (no, not in vacuum, in that medium).
    Reply
  • AnnihilatorX - Friday, March 14, 2014 - link

    Just to clarify, it is like pushing a tube full of tennis balls from one end. Assuming the tennis balls are all rigid so deformation is negligible, the 'cause and effect' making the tennis ball on the other end move will travel at speed of light. Reply
  • R3MF - Thursday, March 13, 2014 - link

    having 24x PCIe 3.0 lanes on AMD's Kaveri looks pretty far-sighted right now. Reply
  • jimjamjamie - Thursday, March 13, 2014 - link

    if they got their finger out with a good x86 core the APUs would be such an easy sell Reply
  • MrSpadge - Thursday, March 13, 2014 - link

    Re: "Why Do We Need Faster SSDs"

    You power consumption argument ignores one fact: if you use the same controller, NAND and firmware it costs you x Wh to perform a read or write operation. If you simply increase the interface speed and hence perform more of these operations per time, you also increase the energy required per time, i.e. power consumption. I your example the faster SSD wouldn't continue to draw 3 W with the faster interface: assuming a 30% throughput increase expecting a power draw of 4 W would be reasonable.

    Obviously there are also system components actively waiting for that data. So if the data arrives faster (due to lower latency & higher throughput) they can finish the task quicker and race to sleep. This counterbalances some of the actual NAND power draw increases, but won't negate it completely.
    Reply
  • Kristian Vättö - Thursday, March 13, 2014 - link

    "If you simply increase the interface speed and hence perform more of these operations per time, you also increase the energy required per time, i.e. power consumption."

    The number of IO operations is a constant here. A faster SSD does not mean that the overall number of operations will increase because ultimately that's up to the workload. Assuming that is the same in both cases, the faster SSD will complete the IO operations faster and will hence spend more time idling, resulting in less power drawn in total.

    Furthermore, a faster SSD does not necessarily mean higher power draw. As the graph on page one shows, PCIe 2.0 increases baseline power consumption by only 2% compared to SATA 6Gbps. Given that SATA 6Gbps is a bottleneck in current SSDs, more processing power (and hence more power) is not required to make a faster SSD. You are right that it may result in higher NAND power draw, though, because the controller will be able to take better advantage of parallelism (more NAND in use = more power consumed).

    I understand the example is not perfect as in real world the number of variables is through the roof. However, the idea was to debunk the claim that PCIe SSDs are just a marketing trick -- they are that too but ultimately there are gains that will reach the average user as well.
    Reply

Log in

Don't have an account? Sign up now