Western Digital (WD) is a market leader in mechanical hard drives, but on the SSD front, they still have a lot of catching up to do. The WD Black SN850 is a huge step in the right direction and will no doubt help them grab the attention of the enthusiast SSD storage community. Based on a new eight-channel flash controller called just "WD G2," but the SanDisk 20-82-10035, and paired with 96-layer 3D TLC flash from Toshiba/Kioxia, the SN850 achieved very impressive results in our testing. The biggest innovation with the SN850 is support for the PCI-Express 4.0 interface, which has become mainstream because of AMD's Ryzen processors. Intel will support PCIe 4.0 with their upcoming Rocket Lake CPUs, which should help the push for PCIe 4.0 SSDs. You can also pair these drives with "Ice Lake" or "Tiger Lake" notebooks to work as intended.

Just a few days ago, I reviewed the ADATA XPG Gammix S50 Lite, which supports PCIe 4.0, too, but its controller is bottlenecked due to the four-channel design. No such limitations exist on the WD SN850. Thanks to eight channels on the controller, we see stunning transfer rates of up to 7 GB/s read and 5 GB/s write. Random IOPS are also high, but fall a little bit behind the Phison E18 on the Corsair MP600 Pro. Overall, these real-life results are really good, but not the best.

Things change when we look at our real-life performance numbers. Here, the SN850 is the fastest drive we ever tested, slightly faster than the Samsung 980 Pro and Corsair MP600 Pro. The SN850 is also one of the first TLC SSDs I would pick over the MLC-based Samsung 970 Pro. The first PCIe 4 SSDs were released in 2019, but their performance wasn't that impressive, just matching PCIe Gen 3 drives in real life. On the SN850, this isn't the case and many of our tests show significant improvements. On the other hand, the gains are not "double," which is what the transition from PCIe Gen 3 to Gen 4 could suggest to the uneducated reader. The underlying reason is that almost no application is limited by sequential transfer rates. Most workloads are a mixture of low queue depth random and sequential IO, with working sets as big as a few gigabytes. It's also important to mention that our real-life testing runs the actual applications, not disk traces that assume an infinitely fast computer system. Replaying a disk trace will skip all the "think time" the program needs to perform its task—almost nothing is just pure disk IO. We also test application performance with 80% of the capacity filled to closer represent real-life usage scenarios.

This "80% disk full" test setup is possibly the reason why the MP600 Pro wins synthetic tests, but only placed third on a volume that's not completely empty. Still, the top drives in our test group are very similar in performance unless you pick specific test cases, which you could if most of your workloads consist of similar activity. When compared against SATA drives, the SN850 is 50% faster, even almost twice as fast as the Crucial BX500. I guess you can imagine what a huge revelation the upgrade from a mechanical HDD to the SN850 will be. Against typical midrange M.2 NVMe SSDs, the performance difference is 10%, and PCIe 3.0 high-end drives are 5% behind the SN850.

Sequential write performance of the WD Black SN850 is very good, filling the whole drive completes at around 1.6 GB/s on average, with peaks starting out at over 5 GB/s. What's important is that when the SLC cache is exhausted, the drive will continue writing at 1 GB/s until it is completely full. I criticized WD big time for the tiny 8 GB SLC cache on the WD Black 2018; this time, the cache is sufficiently large with 277 GB, which makes sure the drive can absorb larger spikes of write activity with ease. Of course, when you momentarily stop the write activity, the SLC cache will free up capacity immediately, so full write rates are available as soon as you give the drive a moment to settle down.

The WD Black SN850 comes without a heatsink, but WD offers a variant with a heatsink preinstalled—at higher pricing, of course. In our thermal testing, we saw temperatures rise up to 89°C, which is not uncommon for a high-performance M.2 SSD. Actually, these temperatures are better than many competing drives. If you've read other sites' reviews of the WD Black SN850, you'd have noticed that several reviewers complain that the drive gets hot. The problem here is that they are trusting software-only readings, which are much lower on some other drives. These drives basically cheat by reporting much lower temperatures despite running much hotter, which puts the more accurate thermal sensor on the WD Black at a disadvantage. That's why it's important to perform physical temperature measurements on SSDs—only those report actual drive temperatures, which has us use a FLIR thermal camera.

Priced at $230 for the tested 1 TB version, the WD Black SN850 is expensive. For that amount of money, you can get a pretty decent 2 TB M.2 NVMe SSD (PCIe Gen 3)—just not one that's as fast as the SN850. The two strongest competitors to the WD Black SN850 are the Corsair MP600 Pro and Samsung 980 Pro, which cost exactly the same—is anyone surprised? If I had to pick between these three drives, I would opt for the WD Black SN850 every time. But unless you are building a super-high-end gaming PC, I'm not sure if doubling the storage cost for a little bit of extra performance is worth it. On the other hand, with graphics card prices above $1000 becoming the norm, spending $100 extra on the SSD is a path I could recommended to ensure maximized system performance.