With the WD Black SN770, WD is modernizing their DRAM-less M.2 "Black" SSD offering. While the previous models used PCI-Express 3.0 with controllers from WD/Phison, and older 64 or 96-layer Toshiba NAND flash, the SN770 uses a brand-new controller that supports PCI-Express 4.0, and the flash memory has been updated to the latest 112-layer 3D TLC from WD's collaboration with Kioxia/Toshiba. This gives WD the rare capability of manufacturing all components on the SSD in-house, which not only helps with cost, but also makes them more independent from various supply-chain issues. What's not changed is that the SN770 is a 4-channel design, which of course limits the maximum transfer rates compared to an 8-channel design like the SN850, but this is mitigated by the more modern 112-layer flash (vs. 96-layer on the SN850). Also, and as expected, a DRAM cache chip is not included in the SN770 for cost reasons. Higher-end drives have a separate DRAM cache chip that stores a copy of the mapping tables of the SSD. This table helps the controller figure out where a piece of data is located; like most DRAM-less SSDs, the SN770 does use 64 MB of the host system's memory, though.

Synthetic test results of the SN770 are decent, certainly above the various DRAM-less alternatives. While 4K random IOPS are near the top, 512K sequential transfer rates are lower due to the more narrow 4-channel flash interface. If we only used the standard synthetic benchmarks and our review ended here, I'd say the SN770 is a decent value SSD that doesn't stand out in any way.

Good that we also do extensive real-life testing with actual applications (not disk traces) and the drive at 80% full, which is much harder to optimize for. Here, the WD Black SN770 achieves unbelievable results that match or exceed the more expensive and higher-positioned WD Black SN850, Samsung 980 Pro, and Kingston KC3000. In the match-up SN850 vs. SN770, the SN770 does have a little bit of a hardware advantage because it uses faster 112-layer flash, whereas the SN850 uses the slower 96-layer NAND, which apparently is enough to make up for the lack of DRAM, and the 4-channel interface. Another important win is that the SN770 beats the DRAM-less Samsung 980 non-Pro by 4%, and the best PCIe 3.0 drives are 3-5% behind. Once again, the SN770 is a DRAM-less SSD design, which usually have terrible random write performance, especially when the writes are spread over a larger area, which is due to the DRAM-cache being emulated in flash, which is much slower. Seems WD figured out a magic bullet to overcome this problem. If you showed me the real-life performance results of the SN770, there's no way I'd come to the conclusion that this is a DRAM-less drive.

We dug deeper and specifically tested random write performance across a wide range of sizes and uncovered that the SN770 barely loses much performance with a larger data area—it's almost like magic. What's astonishing is just how much better WD's algorithms work—300K IOPS in a test where other DRAM-less drives get 30K IOPS, so 10x as high—no wonder this drive is fast. I only wish everybody could learn more about the secret sauce and license it for their own controllers—it would start a revolution for SSDs, making DRAM cache chips obsolete for all but the most high-end designs. However, I'm quite positive that WD is keeping this a closely guarded secret, so they can sell more of their own product, which is well-deserved, of course—coming up with such algorithms isn't easy or it would have been done a long time ago.

The pseudo-SLC cache size of the WD Black SN770 is "big." The drive fills all its capacity in SLC mode first, which gives it the ability to soak up large bursts of write activity. Once the SLC cache is exhausted though, write speeds go down considerably, by almost 90%. You're still getting around 500 MB/s, which is important if your source of data is a SATA SSD, but this low write performance is a clear reason why you'd want to spend more on one of the high-end PCIe 4.0 SSDs. You've got to ask yourself how often you'll exhaust the SLC cache (probably never), though, and whether this is worth paying $50, or almost +50%, more for. The Samsung 980 non-Pro, the strongest DRAM-less competitor, doesn't do any better in this test either. Of course, momentarily stopping the write activity will have the SLC cache free up capacity immediately, so full write rates are available as soon as you give the drive a moment to settle down.

WD doesn't include a heatsink with the WD Black SN770, which is a reasonable decision given the cost-sensitive positioning of the product. In our thermal stress test, which hammers the drive non-stop with writes, we could get the drive to thermally throttle just a little bit, but the throttling is very well-behaved. Going from 4.7 GB/s to 4.1 GB/s when throttled really is a total non-issue, as you'll probably never notice it during daily usage, if you can even get the drive warm enough to throttle in the first place—we had to pummel it with 400 GB of incoming data in 80 seconds.

The 1 TB WD Black SN770 is currently listed for $115 at various online stores, which is an extremely competitive price given the performance we're seeing. If you want a fast PCIe 4.0 SSD and don't want to spend $160+, this is pretty much the only drive you should consider. Given my results I find it hard to justify spending $170 (WD Black SN850), $150 (Samsung 980 Pro), $170 (Kingston KC3000), or $180 (Corsair MP600 Pro LPX). Maybe if you're building a crazy $10k gaming PC, but for everyone else, it seems DRAM-less is making a big push. The next-best alternative to the SN770 is the ADATA XPG Atom 50, which is just marginally slower, but a bit more expensive at $120, and only available in the States. If money is tight, you might want to include PCIe 3.0 SSDs on your shopping list, too. The performance difference is very small unless you transfer huge files all day. Interesting options here are the ADATA SX8200 Pro for $100 and Kioxia Exceria SSD for $85—these also include a DRAM cache, but are still slower overall than the WD Black SN770. Another alternative is the WD Blue SN570, which is only PCIe Gen 3, but has very similar DRAM-cache algorithms as the SN770, making it an outstanding value option.