The MyDigitalSSD SBX SSD Review: NVMe On The Cheap

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.

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.

(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.

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%.

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.