The Western Digital WD Black SN850 Review: A Very Fast PCIe 4.0 SSD
by Billy Tallis on March 18, 2021 12:50 PM ESTBurst IO Performance
Our burst IO tests operate at queue depth 1 and perform several short data transfers interspersed with idle time. The random read and write tests consist of 32 bursts of up to 64MB each. The sequential read and write tests use eight bursts of up to 128MB each. For more details, please see the overview of our 2021 Consumer SSD Benchmark Suite.
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The WD Black SN850 turns in excellent scores on almost all of the burst IO tests. For random reads, it edges out the Intel SSD 670p to set a new record for flash-based SSDs, and even when testing beyond the bounds of any possible SLC caching it is only 2% slower than the MLC-based Samsung 970 PRO. For random writes the WD Black SN850 is slightly slower than the Phison E16 drive, but otherwise is s clear step up in performance from the rest of the field. When testing sequential transfers on a small slice of the drive, the SN850 is substantially faster than everything else, but when testing across 80% of the drive its sequential read performance drops dramatically and is beat by the Samsung 980 PRO and several of the faster PCIe 3.0 drives.
Sustained IO Performance
Our sustained IO tests exercise a range of queue depths and transfer more data than the burst IO tests, but still have limits to keep the duration somewhat realistic. The primary scores we report are focused on the low queue depths that make up the bulk of consumer storage workloads. For more details, please see the overview of our 2021 Consumer SSD Benchmark Suite.
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| Sequential Write | Throughput | Power | Efficiency | ||||||
On the longer random read test, the WD Black SN850 doesn't quite stand out from the best performance offered by other drives with newer flash. But on the other three workloads the SN850 is clearly superior, with significant performance leads over the rest of the competition. Its power consumption is consistently on the high side and in some cases it is drawing more than any of the other drives, but the performance is high enough that the efficiency scores are all good.
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For random reads, the SN850 eventually ramps up to around 4GB/s or 1M IOPS at the end of the test, which is significantly faster than any other drive that we've tested so far on this new test suite. However, when testing across 80% of the drive instead of just a 32GB slice, the random read performance falls to roughly the same level as the Samsung 980 PRO.
For random writes, the SN850's performance scales up a bit quicker than the 980 PRO, but it hits a throughput limit sooner and the 980 PRO ends up being much faster for random writes to the SLC cache at high queue depth.
For sequential reads, the SN850 ends up slightly faster than the 980 PRO, but when testing across 80% of the drive the Samsung reaches full performance with a lower queue depth. For sequential writes the SN850 is again a bit faster than the 980 PRO and this time it doesn't need higher queue depths to reach full speed, but it also starts running out of SLC cache before the test is over while the 980 PRO maintains full performance through the end of the test.
Random Read Latency
This test illustrates how drives with higher throughput don't always offer better IO latency and Quality of Service (QoS), and that latency often gets much worse when a drive is pushed to its limits. This test is more intense than real-world consumer workloads and the results can be a bit noisy, but large differences that show up clearly on a log scale plot are meaningful. For more details, please see the overview of our 2021 Consumer SSD Benchmark Suite.
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The WD Black SN850 starts off this test with good random read latency, but around 80k IOPS it shifts gears and latency spikes alarmingly. It actually improves a few times later in the test so by the time the drive is approaching its throughput limit, it is only a bit slower than the Samsung 980 PRO.
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Samus - Tuesday, March 23, 2021 - link
eva: true. SATA hasn't been updated in over a decade (unlike SAS) and it'll be some time before consumer-class drives can saturate a 6Gbps link (currently almost none can even saturate a 3Gbps link.)With MAMR, HAMR, etc coming to market, performance is finally going to increase where areal density was historically the only way sequential transfers went up, so drives might start cracking SATA2 bandwidth. I suspect when drives near SATA3 bandwidth, it'll either be so long from now that hard disk technology in the consumer space will be dead (replaced by cheap NAND storage) as hard disk technology seems to be focusing on data centers where SAS is common and already capable of 12Gbps+, or consumers that wish to actually use magnetic disk storage will adopt SAS.
Molor1880 - Thursday, March 18, 2021 - link
2.5 in drives and U.X won't make a comeback outside a server room, which is what that combination is designed for. The trend for personal devices is smaller and lighter, not bigger and bulkier. I would expect M.2 and gum stick drives to evolve, in step with PCIe, but it's not going away for at least another decade.Tomatotech - Thursday, March 18, 2021 - link
2.5” is dead for casual home use. I used to think it had a place in the office, but with the rise of laptop-powered WFH and the popularity of space-saving small SFF computers for the office I don’t see it as having a future.Your point about cost makes no sense. 2 TB+ of SSD chips is expensive. It makes no difference whether it’s on an m.2 stick or in a half empty U.3 case, it costs the same either way. With U.3 there’s a (small) extra cost for the packaging, plus the extra wires and extra ports required and extra assembly steps. Might be worth it in the datacentre but not for price-sensitive home or office market where 99% of drives are never swapped.
Tomatotech - Thursday, March 18, 2021 - link
Ninja’d by Molor1880!WaltC - Thursday, March 18, 2021 - link
I think M.2 is here to stay. You are looking for economies of scale in NVMe M.2 drive capacity--that will happen as time goes on. It's remarkable to me how fast M.2 drives have ratcheted up in performance and capacity already. But, hey, if you need the economic capacity there's always the old 7200 rpm standby, right? These super-capacity drives will be around for a long while--but eventually M.2 will supplant them, imo.My older PCIe3 960 EVO M.2 boot drive would throttle regularly in large tasks, like doing a full AV Defender scan on C:\. The drive always crashed and never completed a full C:\ scan. This doesn't happen with the 980 Pro at all, and it's running in the same mboard and in the same slot the 960 ran in--using the same heatsink--just a flat sink that came with the mboard. Things are improving rapidly on the NVMe M.2 front, imo.
damianrobertjones - Friday, March 19, 2021 - link
"Threw is the past tense of the verb throw. It’s the word you use to say that something threw you for a loop or threw you off. Through is an adverb and a preposition. It’s used to say that you entered on one side of something and exited on the other."Not sure if 2.5" drives have gone anywhere?
twotwotwo - Wednesday, April 21, 2021 - link
There are a few 4 and 8TB m.2 drives out already, so a stick with more than 2TB might be practical for you before any switch to the mostly-enterprise u.3 form factor. Not that there's anything wrong with holding on to your current stuff! :)Makaveli - Thursday, March 18, 2021 - link
"Later this year we're expecting another wave of Phison E18 drives to arrive using 176L 3D TLC NAND"This is what i'm waiting to see.
I don't like that all the new generation drives also all took a reduction in TBW and all seem to have smaller SLC caches minus this WD drive.
ozzuneoj86 - Thursday, March 18, 2021 - link
With the SK Hynix P31 performing so well for the money, especially in efficiency, I'll be keeping an eye out for PCI-E 4.0 offerings from them.I'm currently booting from a 2.5" MX500 1TB. Since I have an X570 board, it feels like my next drive purchase should be PCI-E 4.0. Thankfully, I doubt these things provide any appreciable difference in performance over a good SATA SSD for the vast majority of applications I use, so I can stand to wait for the prices to come down. Given the choice between buying a 1TB SSD with blistering fast performance for $200, or one that generally benchmarks lower but uses less power, runs cooler and provides an almost identical experience for $135 (with sales often much lower)... its hard to justify the more expensive one.
lmcd - Thursday, March 18, 2021 - link
Imo in a laptop it's impossible to justify a faster SSD that consumes more power.In a desktop, though, I can see it making sense for certain workloads.