WD Black SN850X Gen4 NVMe and Linux

Oh, yes! Another NVMe to experiment with in Linux!

This time, let’s look at the Western Digital Black SN850X Gen4 NVMe and how it compares to the Crucial T705 Gen5 NVMe in the same system.

Will there be much of a difference between Gen4 and Gen5?

Does it work with Linux? Yes, indeed!

Note: Nobody sponsors this. Not even the neighbor’s cat that roams around at night sponsors this. After all, the least the furry feline fellow could do is drop an NVMe package on the doorstep, but no. I paid for this out of my own pocket, so you can be sure I am going to provide honest-to-goodness personal opinions. Any links to Amazon are affiliate links to help readers locate the item and to help cover the time spent researching this article since I earn a commission on qualifying purchases at no extra cost to readers.

The NVMe

The T705 Gen5 NVMe is from Crucial, and it is fast. Blazing fast. This one is from Western Digital, and it is fast too for being Gen4, just not as fast. It is available in a number of capacities.

• WD SN850X 1TB
• WD SN850X 2TB
• WD SN850X 4TB (If you have money to burn. Still cheaper than Gen5 4TB.)

I chose the 1TB version, which is single-sided, meaning, all chips are located on the top. The bottom is flat. I chose the model without a heatsink to save money. If I need a heatsink in the future, several third-party options are available.

Box includes everything you see here.

The box reads “up to” 7300 MB/s for reads. We shall see.

CrystalDiskMark

I benchmarked this NVMe in both Windows 10 and Linux. Here are the Windows 10 results using CrystalDiskMark Shizuku Edition…because anime art always makes benchmarks more fun. I also watched HWiNFO to view reported temperatures as the benchmark was taking place.

The SN850X occupied a true PCIe Gen4 x4 M.2 slot on the motherboard.

WD SN850X CrystalDiskMark results. The “up to” clause is correct, but I never saw any read numbers exceed 6500 MB/s with my system. The box technically did not lie. However, 7300 MB/s is about the limit of PCIe x4 Gen4.

Here are the Crucial T705 Gen5 NVMe results in the same system using a Gen5 M.2 slot.

MB/s from CrystalDiskMark. In chart format, the Crucial T705 outperforms the SN850X in everything except the random 4K tests.

The SN850X reported lower temperatures, but one reason might be its location in an M.2 slot on the motherboard that receives better airflow than the T705, which is located in dead airspace below the CPU socket.

Gen5 – Twice as Fast

Gen5 is without a doubt practically twice as fast as Gen4 as long as the NVMe device can support the higher transfer rates. Not all NVMe devices are created equal, and you get what you pay for.

Yes, the PCIe generation does make a difference, and Gen5 is faster. Both of the NVMe devices used in this article are quality products from my experience, so you cannot go wrong with either. Use Gen4 NVMe in a Gen4 slot, and use Gen5 in a Gen5 slot for best results. This might sound obvious, but it is easy to neglect since motherboards do not make this clear without consulting the manual given how some motherboards mix and scatter Gen3, Gen4, and Gen5 M.2 slots together.

Other motherboards take “cheap” to another level by only providing two PCIe lanes in some M.2 slots instead of the full four lanes. It can be easy to get lost and wonder why a Gen4 NVMe is giving only Gen3 speeds because it was inadvertently installed in a slower M.2 slot. I’ve been there.

Linux Disks Benchmark

What does Linux say? I tested the SN850X in Xubuntu 24.04 and Ubuntu MATE 24.04 just to see if the operating system would make a difference. It did not.

Xubuntu 24.04 Disks. WD Black SN850X results. 100x100M test.

Ubuntu MATE 24.04. WD Black SN850X results. 100x100M test. No difference between Linux distributions.

Compare with the Crucial T705 Gen5 NVMe in the same system. Ubuntu 24.04 Disks 100x100M test. The read speeds are much faster, but I still do not know why the write results report 370MB/s when Nemo file writes are fast and report in the multi-gigabyte range.

Temperatures

“Does the SN850X need a heatsink?”

Probably not, but I would install one to be safe. At least use the motherboard heatsink if provided. My tests used the provided motherboard heatsink with a thermal pad, but a better third party heatsink would be a good idea.

The SN850X averaged about 48C and hit a max of 51 to 60C depending upon the file transfer intensity during everyday usage. This is good and well below the HWiNFO threshold of 80-85C, so no danger there. But I prefer to keep parts running as cool as possible, so I would recommend using a heatsink on the SN850X in order to handle extended file transfers. For a Gen5 NVMe, definitely use a heatsink. Gen5 NVMe runs hot.

The location on the motherboard and airflow will affect temperature the most. Since the SN850X was installed in an M.2 slot at the bottom of the motherboard (positioned vertically in a typical design) approximately in line with an intake fan, this helped keep the temperature down. No such line-of-sight airflow was available for the T705, so it reached higher temperatures.

Of course, Gen5 is expected to run hotter than Gen4 to begin with, so always think about airflow within your case. NVMe location matters.

Conclusion

I like the SN850X. Being a part of the WD Black series, it is a higher-performance NVMe, so you cannot go wrong for sustained transfers.

Of course, I like the T705 Gen5 NVMe better. This device is super fast, but I only have one free Gen5 M.2 slot available for it. The other slot is Gen4, and this is where the SN850X fills the need. True, you can use a Gen5 NVMe in a Gen4 slot, but then you are wasting money and not utilizing the full potential of a Gen5 NVMe device. And true, you could always use a cheaper Gen3 NVMe in a Gen4 slot, but then you would wonder, “How fast would Gen4 run in this Gen4 slot? Is this really worth saving a little money?”

My best advice is to use a Gen5 NVMe in a Gen5 M.2 slot as long as the price is reasonable, and use a Gen4 NVMe in a Gen4 slot. If you have any spare Gen3 NVMe devices laying around, then those go in the Gen3 slot or to be repurposed into external 10Gbps NVMe USB devices.

The SN850X fills the Gen4 need brilliantly, and I have not experienced any issues. Best of all, it is 100% compatible with Linux out of the box. Just install and go!

Have fun!