Power Management Features

Real-world client storage workloads leave SSDs idle most of the time, so the active power measurements presented earlier in this review only account for a small part of what determines a drive's suitability for battery-powered use. Especially under light use, the power efficiency of a SSD is determined mostly be how well it can save power when idle.

For many NVMe SSDs, the closely related matter of thermal management can also be important. M.2 SSDs can concentrate a lot of power in a very small space. They may also be used in locations with high ambient temperatures and poor cooling, such as tucked under a GPU on a desktop motherboard, or in a poorly-ventilated notebook.

MyDigitalSSD SBX
NVMe Power and Thermal Management Features
Controller Phison PS5008-E8
Firmware E8FM11.4
NVMe
Version
Feature Status
1.0 Number of operational (active) power states 3
1.1 Number of non-operational (idle) power states 2
Autonomous Power State Transition (APST) Supported
1.2 Warning Temperature 81°C
Critical Temperature 85°C
1.3 Host Controlled Thermal Management Not Supported
 Non-Operational Power State Permissive Mode Not Supported

The E8 controller supports two idle power states, implemented as NVMe Power States 3 and 4 (power states 0, 1 and 2 are operational states with successively lower power limits). The firmware version E8FM11.4 used by the MyDigitalSSD SBX doesn't use power state 4 when the NVMe Autonomous Power State Transition (APST) feature is enabled, which is what most systems in the real world rely on for NVMe power management. The drives can be put into PS4 when APST is disabled and the OS manually manages the drive's power states. The relatively new power and thermal management features from the version 1.3 NVMe spec are not implemented.

MyDigitalSSD SBX
NVMe Power States
Controller Phison PS5008-E8
Firmware E8FM11.4
Power
State
Maximum
Power
Active/Idle Entry
Latency
Exit
Latency
PS 0 3 W Active - -
PS 1 2 W Active - -
PS 2 1 W Active - -
PS 3 0.1 W Idle 1 ms 1 ms
PS 4* 0.005 W Idle 400 ms 90 ms

(PS4 not usable by APST with current firmware)

Note that the above tables reflect only the information provided by the drive to the OS. The power and transition latency numbers are often very conservative estimates, but they are what the OS uses to determine which idle states to use and how long to wait before dropping to a deeper idle state.

Idle Power Measurement

SATA SSDs are tested with SATA link power management disabled to measure their active idle power draw, and with it enabled for the deeper idle power consumption score and the idle wake-up latency test. Our testbed, like any ordinary desktop system, cannot trigger the deepest DevSleep idle state.

Idle power management for NVMe SSDs is far more complicated than for SATA SSDs. NVMe SSDs can support several different idle power states, and through the Autonomous Power State Transition (APST) feature the operating system can set a drive's policy for when to drop down to a lower power state. There is typically a tradeoff in that lower-power states take longer to enter and wake up from, so the choice about what power states to use may differ for desktop and notebooks.

We report two idle power measurements. Active idle is representative of a typical desktop, where none of the advanced PCIe link or NVMe power saving features are enabled and the drive is immediately ready to process new commands. The idle power consumption metric is measured with PCIe Active State Power Management L1.2 state enabled and NVMe APST enabled if supported.

Active Idle Power Consumption (No LPM)Idle Power Consumption

The active idle power draw of the MyDigitalSSD SBX is pretty good for a NVMe drive and on par with many mainstream SATA SSDs. The situation when PCIe ASPM and NVMe APST are enabled isn't great, but at least the Phison controllers are no longer freaking out and paradoxically increasing power consumption. The power savings we observed seem to come mostly from PCIe ASPM, and it makes little difference whether the drive is asked to use APST or not. The drive doesn't stay within its own declared limit of 100mW for PS3.

Also important to the E8 platform's overall idle power consumption is how background processing is handled. When idle, the drive will periodically wake up to perform background processing such as garbage collection. For the first few minutes after the drive is powered on, the interval between those wake-ups is 0.8 seconds, then the drive slows to waking up once every 5 seconds. These wake-ups continue whether or not the drive has background garbage collection or SLC cache flushing to do. Since the drive's power spikes to just over 1W during these active periods and they last for about 200ms each, this increases the overall idle power draw by more than 20%.

Idle Wake-Up Latency

The idle wake-up latency test shows no significant difference in performance for the SBX between having all the power management features enabled or disabled—sometimes the measurements are faster with power management enabled, but still within the margin of error, so we're showing the latency as just zero in those cases. It is clear that no deep power saving measures are being taken within the SSD in this configuration, so there is nothing that would impose a significant wake-up delay.

Mixed Read/Write Performance Conclusion
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  • MajGenRelativity - Tuesday, May 1, 2018 - link

    The price premium of lower-end NVMe SSDs forms a big part of their problem in my head. When I build a computer for somebody, I usually use an M.2 SATA SSD if I include an SSD, because SATA SSDs are the cheaper alternative, and most people won't use the performance of an NVMe SSD. If they *do* need the performance of NVMe, I find it a little hard to justify stopping at the low end when they can pay a bit more and bump up the performance by quite a lot. My thoughts are that while products like the SBX are a step in the right direction, they really need to match SATA drives in price to become fully worthwhile. Reply
  • Dribble - Tuesday, May 1, 2018 - link

    The other thing they have going for them is size. I can see a cheap nvme being used quite a bit in mid range laptops that came with a SATA disk but have a spare nvme slot. For desktops not only does the drive sit flush with the motherboard mostly, but it requires no cabling and no hd cage to put it in. I could see myself getting one as a second drive - it's not like I'd really be able to tell it's any slower then a high end drive for standard desktop usage. Reply
  • Dribble - Tuesday, May 1, 2018 - link

    Ah didn't read the previous comment carefully enough "M2 SATA SSD" not "SATA SSD", never mind. Reply
  • MajGenRelativity - Tuesday, May 1, 2018 - link

    Yeah. I can see 2.5" SATA going away entirely / being replaced by 2.5" U.2, but M.2 SATA still has a place for me Reply
  • eek2121 - Wednesday, May 2, 2018 - link

    My current machine no longer has any type of SATA drive in it. I have a single 1TB Samsung 960 EVO. I thought about going going for the pro or a 2 TB EVO, but this fits my needs perfectly. Now if we could just get a mini-itx threadripper board... ;) Reply
  • MajGenRelativity - Thursday, May 3, 2018 - link

    For me, I need bulk storage, so SATA isn't going away any time soon. However, I don't think Mini-ITX Threadripper will happen, as the socket is too big Reply
  • Ratman6161 - Monday, May 7, 2018 - link

    Price wise, for a lot of people in your situation, you could get the 500 GB 960 EVO for $200 and also the 1 TB MX500 for $249. That would give you a total of 1.5 TB for the price of a 1 TB 960EVO. Then EVO could then be used for OS and programs with some data while the cheap but still pretty good MX500 could provide the big storage. Reply
  • gglaw - Saturday, May 19, 2018 - link

    The popular budget Micron 3D TLC 2TB drives are on sale all the time from $280-$300 range. They don't have much marketing or even a fancy name associated with them, come in a bare OEM type box but the few reviews out are all favorable. I have one as a secondary storage drive with a 960 EVO 500GB boot drive but tbh when I moved my Steam library over to it, I can't even tell the difference in performance between it and the NVMe EVO.

    https://www.amazon.com/Micron-1100-SATA-2-5-inch-M...

    that's the drive but it goes on sale lower than that pretty much every week.
    Reply
  • wumpus - Tuesday, May 1, 2018 - link

    Why eat the slot? Do your clients get grumpy if they see and "old fashioned" 2.5" drive? I'd rather leave the M.2 slot waiting for a card that really needs it, and still have the SATA drive connected. Reply
  • MajGenRelativity - Tuesday, May 1, 2018 - link

    I sell to a variety of clients (I co-own a small business so we cover all sorts), and usually they don't have enough technical knowledge to know the difference between a 2.5" drive and M.2. I use the M.2 slot because it allows me to either disconnect the SATA cable (for a modular PSU), or tuck it out of the way to increase airflow. Most of my customers won't need the performance boost of upgrading to an NVMe drive, so it hasn't been a problem before. In the cases where that is a viable upgrade path, I discuss it ahead of time, and do use a 2.5" drive in situations where necessary/desired. Reply

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