The Western Digital WD Black SN850 Review: A Very Fast PCIe 4.0 SSD

Mixed IO Performance

For details on our mixed IO tests, please see the overview of our 2021 Consumer SSD Benchmark Suite.

Mixed Random IO	Throughput	Power	Efficiency
Mixed Sequential IO	Throughput	Power	Efficiency

The WD Black SN850's overall performance on the mixed random IO test is just behind the Samsung 980 PRO, but still very good for a flash-based SSD. However, its power efficiency on that test is only second-tier, behind the 980 PRO and the SK hynix Gold P31.

On the mixed sequential IO test, the SN850's performance is better than any of the 1TB drives, and almost as fast as the 2TB 980 PRO. It's still not quite as efficient as the 980 PRO and during this test it averages about 6.6W, which is definitely getting up to where a heatsink would be of use (for similarly long-running workloads).

Mixed Random IO	WD Black SN850 1TBSamsung 870 EVO 1TBSamsung 970 EVO Plus 1TBSamsung 970 PRO 1TBSamsung 980 PRO 1TBSamsung 980 PRO 2TBKingston KC2500 1TBIntel SSD 670p 2TBSK hynix Gold P31 1TBWD Blue SN550 1TBWestern Digital SN730 1TBWD Black SN750 1TBSilicon Power US70 1TBIntel Optane SSD 905P 1.5TB
Mixed Sequential IO	WD Black SN850 1TBSamsung 870 EVO 1TBSamsung 970 EVO Plus 1TBSamsung 970 PRO 1TBSamsung 980 PRO 1TBSamsung 980 PRO 2TBKingston KC2500 1TBIntel SSD 670p 2TBSK hynix Gold P31 1TBWD Blue SN550 1TBWestern Digital SN730 1TBWD Black SN750 1TBSilicon Power US70 1TBIntel Optane SSD 905P 1.5TB

On the mixed random IO test, the SN850 starts out with a lead over the 980 PRO for the most read-oriented mixes but then the 980 PRO takes a small lead for the rest of the test while always using less power. On the mixed sequential IO test, it seems like the larger SLC cache may be helping the SN850 get a performance boost relatively early while there are still more reads than writes, and it maintains leading performance as the mix gets more write-heavy. That means the SN850 ends up having a considerable performance advantage over the 1TB 980 PRO for a 50/50 mix that would be expected from a workload like copying files within the same SSD.

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 WD Black SN850 implements the full range of power and thermal management features. It's specced for quick transitions in and out of its low-power sleep states. The drive indicates that it may use up to 9 W while active; it probably gets close at peak, but the highest sustained power draw we saw during our synthetic benchmarks was in the 7-8W range. Constraining this drive to either of its lower-power active states would definitely throttle performance by a lot.

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, and depending on which NVMe driver is in use. Additionally, there are multiple degrees of PCIe link power savings possible through Active State Power Management (APSM).

We report three 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. Our Desktop Idle number represents what can usually be expected from a desktop system that is configured to enable SATA link power management, PCIe ASPM and NVMe APST, but where the lowest PCIe L1.2 link power states are not available. The Laptop Idle number represents the maximum power savings possible with all the NVMe and PCIe power management features in use—usually the default for a battery-powered system but rarely achievable on a desktop even after changing BIOS and OS settings. Since we don't have a way to enable SATA DevSleep on any of our testbeds, SATA drives are omitted from the Laptop Idle charts.

Typically for Western Digital's NVMe controllers, the active idle power consumption from the SN850 is high at over 1W, and the desktop idle state only drops that by 35%. But the SN850's deepest idle state gets power draw down to the appropriate range for use in a laptop. Wakeup from the desktop idle state is almost instant, but waking up from the deepest idle is quite a bit slower than on Samsung's drives. The SN850 still wakes up several milliseconds faster than indicated by its firmware, and it's not slow enough to be a serious concern for system responsiveness.