Introduction

You go to install your SSD, and you find it’s easier to put the SSD in the Secondary M.2 socket.  Usually, because it is easily more accessible than the Primary M.2 SSD socket which is typically sandwiched between the video card and CPU area of the motherboard.  But you ask yourself, does it really matter?

Your Primary M.2 socket and your Secondary M.2 socket are not the same.  They may both claim to have PCI-Express 4.0 x4 performance support, but the path to get there, and the lanes they connect to, come from different places.  This potentially creates latency issues and potentially causes performance differences on high-performance SSDs. 

That is what today’s review is about, we are going to directly compare the performance of the Primary M.2 socket and Secondary M.2 socket in a typical AMD X570 chipset-based motherboard to find out once and for all if it really matters what slot you put your SSD in.

Fast NVMe 1.4 SSDs

There is a new wave of SSDs out on the market right now, let’s call these 2nd generation PCIe Gen 4 NVMe SSDs.  Just like this MSI SPATIUM M480 2TB HS PCIe 4.0 Gen4 NVMe SSD we recently reviewed.  These new SSDs employ the likes of NVMe 1.4 protocol version and controllers like Phison’s E18 using 96-Layer 3D TLC NAND flash memory. 

These new breeds of SSDs can hit peak sequential read throughputs of 7000MB/s+ (7GB/s) and sequential write throughputs of 6800MB/s+ (6.8GB/s) or better.  These new breeds of PCIe Gen 4×4 NVMe SSDs supersede the last generation of Gen4 SSDs by a great deal, which could only reach 5000MB/s (5GB/s) reads and 4000MB/s (4GB/s) writes at best.

This is due to the newer breeds of SSDs now using NVMe 1.4 protocol, and newer controllers able to push NAND flash faster.  So, you go out of your way to spend a lot of money on these fast SSDs, you don’t want to leave any performance on the floor. 

But did you know that what M.2 SSD slot you put your new shiny SSD in could hold back its performance on these newer SSDs?  This is all because SSD M.2 slots are not all made equal, even if they can electrically support PCI-Express 4.0 x4 interfaces.  Where they grab their PCIe lanes from, matters.

AMD X570 Chipset

AMD X570 Chipset Diagram Explaining PCI-Express Lanes

The reason why this is so is right here in this AMD X570 Chipset diagram.  Motherboard makers can make choices when designing motherboards and allocate PCI-Express lanes to various components.  Coming from the CPU motherboard manufacturers have a choice to pick one, either a PCIe 4.0 x4 NVMe socket, or two SATA plus an x2 NVMe socket, or two x2 NVMe sockets. 

Then, from the chipset, they have a choice to pick up a wider variety of SATA and NVMe configurations.  They can in fact pick up a couple more PCIe 4.0 x4 NVMe sockets, or divvy that up into different speeds and configurations.  However, the kicker is those extra NVMe sockets are connected directly through the chipset, not the CPU.  Therefore, if motherboard manufacturers want two Gen 4×4 sockets, one has to come from the CPU, and one has to come from the chipset.

ASUS TUF GAMING X570-PLUS WI-FI Storage Specifications

In this ASUS TUF GAMING X570-PLUS (WI-FI) motherboard, which represents a typical X570 motherboard, you will see the above configuration shown on the specifications page for the motherboard.  What you see here is that from the “3rd Gen AMD Ryzen Processor” you get the 1 M.2 socket that supports PCIe 4.0 x4.  Then from the “AMD X570 Chipset,” you get the second M.2 Socket that supports PCIe 4.0 x4.

Therefore, both M.2 sockets electrically and physically support PCIe 4.0 x4, but one pulls its lanes from the CPU, while the other pulls its lanes from the X570 Chipset.  Therefore, the question is, does that matter?  We aim to find out.

Installing the SSD for Testing

ASUS TUF GAMING X570-PLUS WI-FI Motherboard Demonstrating Primary M.2 socket PCI-Express CPU Lanes versus Secondary M.2 socket PCI-Express Chipset Lanes

The motherboard we are using for testing is an ASUS TUF GAMING X570-PLUS (WI-FI) motherboard, which is a very typical motherboard for SSD installation. It has a primary M.2_1 socket at the top that is sandwiched between the GPU and the CPU, making installation difficult if the motherboard is already installed in your computer. Alternatively, it also has a secondary M.2_2 socket at the bottom, under the second PCI-Express expansion slot, making its position easier to install, and it also has a motherboard heatsink for the SSD here.

Both sockets support PCI-Express 4.0 x4. Therefore your inclination may be to install the SSD into this slot, but whoa there nelly, this may not be the best thing to do. The SSD we are using is an MSI SPATIUM M480 2TB HS PCIe 4.0 Gen4 NVMe SSD which is an NVMe 1.4 PCIe Gen4x4 SSD that can run at 7000MB/s read and 6800MB/s write. It also comes with its own heatsink that we will use. Everything about this setup is exactly the same, for the testing, all we did is switch the SSD between the two sockets, that’s it.

ASUS TUF GAMING X570-PLUS WI-FI Motherboard M.2_1 socket
ASUS TUF GAMING X570-PLUS WI-FI Motherboard M.2_1 socket closeup
ASUS TUF GAMING X570-PLUS WI-FI Motherboard M.2_1 socket with MSI SPATIUM 2TB SSD installed

This is the SSD installed in the Primary M.2_1 socket with its PCIe lanes coming from the CPU.

ASUS TUF GAMING X570-PLUS WI-FI Motherboard M.2_2 socket
ASUS TUF GAMING X570-PLUS WI-FI Motherboard M.2_2 socket closeup
ASUS TUF GAMING X570-PLUS WI-FI Motherboard M.2_2 socket with MSI SPATIUM 2TB SSD installed

This is the same SSD but now installed in the M.2_2 socket which has its PCIe lanes coming from the chipset.

Brent Justice

Brent Justice has been reviewing computer components for 20+ years, educated in the art and method of the computer hardware review he brings experience, knowledge, and hands-on testing with a gamer oriented...

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9 Comments

  1. Great article!

    Presuming a 2 drive approach where one drive is OS and misc programs and the other drive is a steam drive, which should be in the primary slot? The game load times would lead me to believe that the steam drive should be in the secondary slot, but would love to get thoughts on this.

  2. Thanks @Brent_Justice for the great write up!

    Presuming a 2 drive approach where one drive is OS and misc programs and the other drive is a steam drive, which should be in the primary slot? The game load times would lead me to believe that the steam drive should be in the secondary slot, but would love to get thoughts on this.

    Obviously I defer to anything Brent says but in my X570 build I went so far as to put my OS stuff on a SATA SSD and then my Steam drive is in the primary. Now this was in part because I was on a budget when I did my build and could only afford the one PCIe 4.0 m.2 drive and I really wanted to see what it could do. It has been faster, but not by much, for gaming, than my previous SATA or PCIe 3.0 m.2 drives. I’ve done the crystal mark tests and everything checks out nicely but it’s still not that much better for gaming.

    I am considering getting a 2nd M.2 drive around BF and per this article I’ll probably just drop a 3.0 2 TB drive in the secondary slot since it obviously won’t really be able to utilize max speeds anyway. I think a lot of us are hoping that sometime in the future game devs are able to better optimize 4.0 speeds but so far it hasn’t been that encouraging on the PC front.

    In other news, 5.0 is only about a year off for the larger mainstream but who knows about prices for that stuff and if games will be able to utilize those fantastical looking speeds.

  3. I guess another related question.

    970 pro for lower latency MLC, or 980 prob for higher bandwidth? Which slot for which use?

    970 pro on primary slot would yield the lowest latency, but would not take advantage of the bandwidth available.

    does anyone make a 512gb SLC NVME drive?

  4. I guess another related question.

    970 pro for lower latency MLC, or 980 prob for higher bandwidth? Which slot for which use?

    970 pro on primary slot would yield the lowest latency, but would not take advantage of the bandwidth available.

    does anyone make a 512gb SLC NVME drive?

    I’ll have to leave that one for Brent. I barely keep track of such things in recent years since I’m mostly budgeting for my GPUs these days. Personally, I’d go for bandwidth but that can really depend on the application. Latency for these things could be tough to notice when comparing on the main slot, but again I admit it’s not my forte so I could be really wrong there.

    edit: Here you go-

    XPG GAMMIX S50 Lite Series: 512GB M.2 2280 NVMe 3D NAND Gen4 Gaming Internal SSD

  5. I’ll have to leave that one for Brent. I barely keep track of such things in recent years since I’m mostly budgeting for my GPUs these days. Personally, I’d go for bandwidth but that can really depend on the application. Latency for these things could be tough to notice when comparing on the main slot, but again I admit it’s not my forte so I could be really wrong there.

    edit: Here you go-

    XPG GAMMIX S50 Lite Series: 512GB M.2 2280 NVMe 3D NAND Gen4 Gaming Internal SSD

    That took me a bit to get specs on. It has a SLC cache, so the drive itself is probably TLC or QLC. I’m looking for something more at the optane end of the spectrum, but not a DC drive and not a 3+ year old consumer option. I don’t think a drive exists that matches what I want.

  6. That took me a bit to get specs on. It has a SLC cache, so the drive itself is probably TLC or QLC. I’m looking for something more at the optane end of the spectrum, but not a DC drive and not a 3+ year old consumer option. I don’t think a drive exists that matches what I want.

    Yeah, it looks like SLC-cache is the catch. I saw a few others with a similar SLC-TLC combo on Amazon.

  7. This is interesting. I hadn’t thought of testing this.

    My initial guess was that it would make a measurable difference, but a practically insignificant one.

    It looks like I was close, but the difference was a little bit more significant than I was expecting.

  8. This is interesting. I hadn’t thought of testing this.

    My initial guess was that it would make a measurable difference, but a practically insignificant one.

    It looks like I was close, but the difference was a little bit more significant than I was expecting.

    I’d argue that this would be the minimum amount of difference that you could see. Get a few more things running off the chipset (NIC, USB3 devices, etc.) and that’ll take away from the chipset <> CPU bandwidth even more.

  9. I’d argue that this would be the minimum amount of difference that you could see. Get a few more things running off the chipset (NIC, USB3 devices, etc.) and that’ll take away from the chipset <> CPU bandwidth even more.

    True.

    I was thinking it had more to do with latency and switching capacity issues than raw bandwidth though.

    I’m trying to remember the distribution of lanes on these things. They have 4x Gen4 lanes dedicated for the chipset, right?

    In a perfect world, that should give the chipset almost 8GB/s of bandwidth.

    I wonder how much overhead is involved in NVMe traffic on the PCIe bus…

    If there isn’t much overhead, you have almost 1GB/s of buffer for other devices on top of what the SSD is using at full load.

    Things like the NIC and Audio are probably fine with that amount of bandwidth. USB3 devices can pose more of a problem.

    You certainly CAN have them competing with eachother, but I’m thinking in most cases unless you are sequentially copying a large file from the SSD to a fast device on USB3 this probably won’t be the case most of the time. Things usually don’t peak at the same time in most cases outside of testing.

    I’m definitely leaning towards chipset switching efficiency being a bigger contributor here than absolute bandwidth constriction.

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