The Mainstream Phoenix Rises: Samsung's 970 EVO (500GB & 1TB) SSDs Reviewed

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 Samsung 970 EVO bumps the supported NVMe spec version to 1.3, compared to the 1.2 feature set supported by the PM981 and 960 series. The 970 EVO implements the Host Controlled Thermal Management feature, allowing operating systems to configure the drive to throttle at a lower temperature than it normally would. The (optional) non-operational power state permissive mode feature is not included, so the 970 EVO is not supposed to do background tasks like garbage collection when it is in idle power states (unless they can be done within the power constraints of the idle states, which is unrealistic).

Note that the above tables reflect only the information provided by the drive to the OS. The power and 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 draw of the 970 EVO seems to be about 20% higher than the preceding generation of Samsung drives, but the low-power idle we measured is about the same as most other high-end NVMe drives.

The idle wake-up latency of the 970 EVO is more than twice that of its predecessors and also significantly higher than that of the Samsung PM981. This ~14ms latency exceeds the 8ms that the drive itself claims as its latency to wake up from its deepest sleep state.