At the moment with the current NVMe SSD speeds and the newer PCIe 4.0 version, the M.2 slot uses a maximum of 4 PCIe lanes. However, depending upon your motherboard configuration the amount of PCIe lanes the M.2 slot actually uses may differ.
M.2 is an interface widely used today for high speed SSDs. It offers a very small form factor and is thus considered almost a necessity for the modern setup. However, unlike the older M.2 SATA SSDs, which use the slower SATA 3 interface, the M.2 NVMe SSDs utilize PCIe lanes.
And hence, the NVMe SSDs are multiple folds faster than even the fastest Hard Disk Drives and SATA SSDs out there. However, the amount of PCIe lanes on the M.2 slot AND its PCIe version has a direct impact on its performance.
In the following write up we will explore how many PCIe lanes does M.2 slot use, how it is impacted by the version and the lane count and also touch on the best practices for getting the right SSDs.
Understanding PCIe Lanes, Version and SATA 3 Speeds
Very briefly, the amount of PCIe lanes directly affect the overall throughput rate (speed) of the slot.
The higher the amount of lanes, the faster would the connected device perform.
In addition to that, the version of the PCIe also has a significant impact on the speeds.
So much so that every successive PCIe version doubles its transfer speed compared to the previous version. The following table summarizes this:
The key points to note are as follows for the purpose of this article:
- SATA 3 interface has max throughput rate (speed) of 750 MB/s or 0.75 GB/s. Therefore, an M.2 SATA slot will have a max theoretical throughput rate of 750 MB/s.
- The PCIe version 4.0 has per lane throughput of 1.969 GB/s
- A PCIe V4.0 M.2 NVMe Slot using 4 lanes has a theoretical max transfer speed of 7.877 GB/s
- A PCIe V4.0 M.2 NVMe Slot using 2 lanes has a theoretical max transfer speed of 3.938 GB/s
- A PCIe V3.0 M.2 NVMe Slot using 4 lanes has a theoretical max transfer speed of 3.938 GB/s
- A PCIe V3.0 M.2 NVMe Slot using 2 lanes has a theoretical max transfer speed of 1.969 GB/s
In reality though, SSDs do not reach the theoretical max throughput rate of the slots. So for instance, SATA 3 SSD can have a typical transfer rate of 560 MB/s.
So How Many PCIe Lanes Does M.2 Slot Use at Max?
As mentioned earlier, the maximum amount of lanes an M.2 slot uses is 4 at the moment. 4x PCIe lanes are capable of supporting the fastest NVMe SSDs out there.
However, some motherboards may also feature M.2 slots utilizing only 2 x PCIe lanes. Take for example the ASUS STRIX X470-F Gaming. This motherboard offers 2 x M.2 slot but with different lane counts.
The first M.2 slot on this motherboard uses 4 lanes whereas the second uses 2 lanes. While both would be able to operate an NVMe SSD, the second will have half the speed and will thus bottleneck your installed SSD significantly.
The configuration of the M.2 Slot’s PCIe lane count depends upon the manufacturer and plenty more factors like the motherboard chipset as well as the PCIe generation.
Let us briefly look at how differently the M.2 slot can be configured.
Motherboard Chipset and Impact on the M.2 Slot
Depending upon the motherboard chipset and the manufacturer’s discretion, the configuration of the M.2 slot can differ drastically.
For instance on some motherboards, the M.2 slot could be using the PCIe lanes of the CPU while on others the M.2 slot would be connected to the motherboard chipset lanes.
If there are two M.2 slots, then there could be a combination of one M.2 slot using CPU lanes and the second one using the motherboard chipset lanes.
The following image outlines the AMD B550 motherboard chipset’s lane configuration.
On the left hand side of the graphic you can see how the CPU lanes would be configured on this chipset for the M.2 slot(s).
A B550 motherboard can choose to have:
- One M.2 slot with 4 PCIe lanes
- 2 x M.2 SATA slots along with 1x NVMe SSD Slot with 2 PCIe lanes
- 2x NVMe SSD slots with 2 PCIe lanes
In most cases with motherboards using this chipset, they tend to go with with first option i.e One M.2 slot with 4 PCIe lanes coming from the CPU.
On the right hand side of the same graphics, you can see the PCIe lanes provided by the B550 Chipset itself (Not the CPU). Now these lanes could be configured into an M.2 slot or could be used for x1, or x4 PCIe slots.
Lets us look at an example of how M.2 slots can be configured on a motherboard:
Take the specifications of ASUS B550M-A Prime for instance (shown above).
This motherboard features 2 x M.2 slot. One of the M.2 Slots connects to the processor PCIe lanes and the other utilizes the Chipset lanes.
Therefore, to reiterate the point, the PCIe lanes used by the M.2 slot on your motherboard depends upon how it is configured.
Also Read: What are Motherboard Standoffs?
M.2 Slot MAY Share Lanes with Other Slots as Well!
It is also important to note that sometimes PCIe lanes on M.2 slots are shared.
For instance, check out the spec notes/caveats of the the ASUS Strix X470-F motherboard:
It states that if either of the PCIex1_1 or the PCIeX1_x3 slot are occupied, then the second M.2 slot (M.2_2) will dial down its performance to only SATA 3 mode i.e 560 MB/s at max instead of its original x2 PCIe V3.0 speeds of 1.969 GB/s.
Here is an another example of M.2 slot on a motherboard with shared lanes:
The specifications above show the M.2 slot configuration for ASUS ROG MAXIMUS XII HERO.
This motherboard features 3 x M.2 slots. However, due to the limited PCIe lanes, there are caveats attached to using the M.2 slots here.
For instance one of the caveats is that the PCIEX16_3 shares bandwidth with the M.2_2 slot. Thus when using the M.2_2 slot, the PCIEX16_3 will be disabled.
Hence, when figuring out how many PCIe lanes a particular M.2 slot actually uses on your motherboard and if there are caveats/notes attached to its usage, you must refer to the manufacturer information and board specifications.
PCIe Version and the Impact on SSD Performance
Not all PCIe lanes are equal as we saw earlier in the table above.
Depending upon what PCIe generation your system conforms to, the speed of the SSD will differ.
For instance the Fourth Gen Samsung 980 Pro NVMe SSD which has the potential to reach a whopping read speeds of about 7000 MB/s, requires a version 4.0 PCIe interface and an M.2 slot with four V4.0 lanes.
On the other hand, the third Gen Samsung 970 EVO NVMe SSD has a max read speed of 3500 MB/s and requires 4 x V3.0 PCIe lanes.
A few important observations can be made from here.
- If you install a Gen 4 M.2 NVMe SSD like the Samsung 980 Pro on an M.2 Slot conforming to the older PCIe v3.0, it will dial down its performance by a half!
- If you install a Gen 3 M.2 NVMe SSD like the Samsung 970 EVO on an M.2 slot conforming to the newer PCIe v4.0, it will NOT gain any type of performance boost.
- Similarly, if the M.2 slot operates at 2 lanes only, then the speed of the SSDs in all instances will be halved.
The PCIe generation you have depends upon the CPU and the Motherboard Chipset you have.
CPU and PCIe Version
Different CPUs support different PCIe version and can thus reflect differently on the SSD performance even if it utilizes the same amount of lanes.
The following example clarifies this:
MSI B550M PRO-VDH can support 2 x M.2 slots as shown above. However, depending upon which processor you install, the PCIe version the M.2 slot conforms to will differ.
In other words, for the motherboard above, if you want the M.2 slot conforming to v4.0 speeds, you will need an AMD Ryzen 3000 series processor or better.
If you install any older processor, say an AMD Ryzen processor from 2000 series or AMD Ahtlon processor, the speeds will be dialed down PCIe v3.0.
This means that if you were to install the latest 4th Gen PCIe NVMe SSDs but you have on older PCIe V3 processor, then you would have bottlenecked your SSD significantly.
Chipset and PCIe Version
The chipset of the motherboard can also determine the PCIe version of a certain M.2 Slot.
Take for instance the B550 chipset.
On this chipset, the CPU lanes conform to PCIe 4.0, but the motherboard chipset lanes conform only to v3.0.
So here a CPU connected M.2 slot would be able to support Gen 4 NVMe SSDs. However, the motherboard chipset connected M.2 slot would only be suitable for Gen 3 NVMe SSDs.
Now take the X570 Chipset for instance.
On a motherboard with the x570 chipset, all of the M.2 slots (both connected to CPU and motherboard chipset) would conform to the PCIe v4.0 and thus would support the 4th Gen NVMe SSDs to their max potential.
Here is an example of an actual X570 Motherboard specifications:
You can see above in the specifications for Gigabyte X570 Aorus Elite that both the chipset and the processor connected to M.2 slot can operate at PCIe 4.0 speeds.
Of course you do need to have a processor that can support PCIe V4 as mentioned earlier on this motherboard.
So in short when determining how many PCIe lanes does M.2 slot use and which PCIe version it conforms to, both the chipset and the processor matter.
Can PCIe Version Affect the Amount of Lanes for the M.2 slot?
Technically yes, if a certain 3rd Gen NVMe SSD has a max speed of 3500 MB/s and uses four PCIe v3.0 lanes , then it can theoretically have the same performance on only two PCIe V4.0 lanes.
The only last comment I want to make here is that reading the motherboard specifications is almost a must when figuring out the PCIe lanes for the M.2 slot, the physical size of SSD it can fit, the PCIe version it uses, and the Chipset it features.
All these factors are highly related to the choice of SSD you will make when you go buy one.
But if you want the short answer to how many PCIe lanes does M.2 slot use, it is 4!