nVMe x4 vs x2 slot SSD: Differences, Test and Results

Recently, SSDs have become so cheap that almost everyone can afford a SATA model of at least 250 – 500 GB. But if you buy, say, a terabyte solid-state drive, so that there is enough space for years ahead, then it makes sense to pay a little extra for a model with a fast PCI-Express interface, better known as M.2 NVMe. Only the mysterious symbols x2 and x4 in the model name can cause some confusion. What they mean and how important, we will try to explain in understandable language. how NVMe X4 Vs X2 Slots are different.

NVMe X4 Vs X2 Slot SSD

PCI-Express slots

Modern solid-state drives of the M.2 form factor are divided into three types of connection interface: SATA, PCI-E x2, and PCI-E x4. They differ not only in data transfer speed, but incompatibility (or vice versa, not compatibility) with various models of laptops and motherboards for desktop PCs. Also, SSD M.2 may differ in size, or rather in length (4, 6, and 8 cm). Let’s consider each of the three connection interfaces in more detail.

SATA3 is the usual slow SSD (up to 560 MB / s), only made in the modern compact M.2 form factor instead of the old-school 2.5-inch. Compatible with absolutely all motherboards and laptops with an M.2 connector.

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NVMe X4 Vs X2 Slot: NVMe 3.0 x2 vs 3.0 4x Vs 2.0 x4 Vs 4.0

NVMe 3.0 x2 – budget high-speed SSDs running on the PCI-E 3.0 bus. A single PCI-E 3.0 lane has a theoretical throughput of 8 GT / s (Giga transactions per second), which equals 985 MB / s. The x2 marking implies two lines, that is, the speed is up to 1970 MB / s. In practice, however, the speed is usually slightly lower. Legacy motherboards, as well as many modern gaming laptops and ultrabooks, have an M.2 NVMe x2 slot.

NVMe 3.0 x4 is the fastest M.2 SSD yet, utilizing four PCI-E 3.0 lanes (up to 32 GT / s or 3940 MB / s). The use of such a fast solid-state drive, at times, imposes restrictions on other connectors on the motherboard. For example, some of the PCI-E x1 / x4 or SATA slots can be deactivated.

NVMe 2.0 x4 – SSDs running on the old PCI-E 2.0 protocol are no longer on sale. But there are Intel LGA1151-v2 motherboards on the H310 chipset, which are equipped with an M.2 PCI-E 2.0 x4 slot (500 MB / s per lane). An NVMe 3.0 x2 SSD installed in such a slot will be limited to 1000 MB / s, and x4 – 2000 MB / s.

NVMe 4.0 – the first motherboards based on the AMD X570 chipset with support for the fourth generation of the PCI-E bus should go on sale in the near future. Compared to its predecessor, its throughput has doubled – up to 16 GT / s. For SSD manufacturers who have hit the ceiling of PCI-E 3.0 speed, this will again free their hands. The announcement of solid bodies with a speed of over 4000 MB / s, I think, will not have to wait long.

Test for NVMe X4 Vs X2 Slot

WD Black SN750 NVME SSD WDS100T3XHC 1TB with heatsink 

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WD Black SN750 NVME SSD WDS100T3XHC 1TB with heatsink 

Very high read and write speed even after exceeding the SLC array, triple-core controller, large RAM cache, 5 years warranty, optional heatsink.  There are no significant drawbacks.

It’s time to move from theory to practice: we will conduct tests using the example of one of the fastest SSD M.2 at the moment – WD Black SN750 (NVMe x4).

The WD Black SN750 is the flagship M.2 NVMe SSD from the American brand. Recall that Western Digital, widely known primarily for its HDD, took over SanDisk several years ago and began to actively develop its own solid-state bodies. The SN750 is the second iteration of the Black SSD and is not only faster but also cheaper. There are four sizes to choose from: 250GB, 500GB, 1TB, and 2TB.

Built Black SN750 on proprietary WD / SanDisk triple-core controller and 64-layer SanDisk 3D TLC BICS3 flash memory (the number and density of chips depends on the size of the SSD). Additionally, a DDR4 buffer memory chip is provided, the volume of which is equal to 256 MB for every 250 GB of SSD capacity (for example, 1 GB for a 1-TB SSD model). In addition, there are two configurations available – non-radiator and with a passive cooler designed by EKWB, a renowned manufacturer of LSS.

For the terabyte version, the highest sequential read and write speed is declared: 3450 and 3000 MB / s, respectively. This automatically makes the WD Black SN750 one of the fastest SSDs on the market. The 500 GB and 2 TB versions are only slightly inferior in linear speeds to the terabyte version, but the 250 GB version is already noticeably lagging behind (3100/1600 MB / s). However, it is still ahead of most competitors of the same size.

Like other SSDs based on TLC memory, the Black SN750 drops the speed when sequentially writing very large files (more than 12 GB), but only by half (up to 1500 MB / s), while many competitors sag by three to five times. The disc then needs a short rest time to regain its original speed. In general, the WD Black SN750 is one of the most interesting M.2 NVMe x4 SSDs at the moment. Nice bonuses are a 5-year warranty, a high declared rewritable resource (600 TB for a 1 TB model) and a functional proprietary utility WD SSD Dashboard.

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Components involve in Test bench

  • processor – AMD Ryzen 5 Pinnacle Ridge 2600X BOX
  • water cooling – SilverStone Tundra TD02-RGB
  • motherboard – ASRock B450 Steel Legend 
  • RAM – Apacer DDR4 1x16Gb EL.16G2V.GNH
  • video card – INNO3D GeForce RTX 2060 TWIN X2
  • solid state drive – WD Black SN750 NVME SSD WDS100T3XHC 1TB with heatsink
  • HDD – WD NasWare Red WD40EFRX 4 TB cache 64 MB 
  • power unit – Cougar CMX CMX850
  • frame – Cougar Turret RGB black 

Test results

Our experimental motherboard ASRock B450 Steel Legend has two M.2 slots with NVMe 3.0 support. True, only one of them (the top one, we will use it) is a full-speed x4 and with a metal heatsink, while the second has a half-bandwidth x2 and without cooling.

Test results

The following benchmarks were used to test the WD Black SN750 solid-state drive: Crystal Disk Mark for measuring sequential read and write speeds, AS SSD Benchmark for measuring access times, Anvil’s Storage Utilities for displaying IOPS results, and AIDA64 Disk Benchmark for testing virtual SLC caching.

So, in Crystal Disk Mark, the linear read speed turned out to be even higher than the declared one (almost 3500 MB / s), and the linear read speed almost exactly corresponds to the declared 3000 MB / s (several megabytes per second missing to a round number can be easily attributed to a statistical error) … An additional retest of the sequential write speed was carried out in the AIDA64 Disk Benchmark and showed a decrease in the write speed from 3000 to 1500 MB / s after exceeding the volume of the virtual SLC array.

In turn, AS SSD Benchmark and Anvil’s Storage utility applications traditionally underestimate line rates compared to CDM. But the first showed ultra-fast access time to files on the SSD – only 0.02 ms, and the second – the processing speed of the smallest files as much as 168 thousand IOPS (Input / Output Operations Per Second or the number of input/output operations). Simply put, this SSD turns and turns small files as lightning fast as an experienced thimble player.


The final comparison diagrams allow you to visually evaluate the speed performance of the WD Black SN750 with NVMe x4 bus against the background of other SSD formats M.2 SATA and NVMe x2. The advantage of four PCI-E lanes over two and, moreover, over the SATA3 bus is significant and indisputable. Buying a slow M.2 SSD in 2019 is at least somewhat justified only because of an outdated motherboard or laptop. If your PC is equipped with an NVMe 3.0 x4 slot, then it is wiser to make a choice in favor of such a high-speed drive, since the gradual price reduction contributes to this. You will definitely be satisfied!