Does it matter which M.2 slot I use?

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Ricky T

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I tried googling this and actually the responses are kind of all over the place plus I'm pretty ignorant when it comes to hard drives. Right now I have two SATA Samsung 4 TB drives which will both be completely removed for the installation of a single Sabrent 8 TB NVMe drive. So really all I need to know is it better to use the top or the bottom M.2 slot or does it even matter? The bottom slot of course has the heat shield on this MSI Z390 Gaming Pro Carbon motherboard. The only other thing I will have installed that will use the PCH lanes will be a sound card in the PCI_E5 slot.
 
According to the latest manual it should not matter which of the two ports that you utilize. Unfortunately it does not state which of the ports share with which of the PCIe slots it only shows which of the ports share with the SATA ports.
 
I just want to make sure it gets the full x4 bandwidth and strangely I don't see anything in the bios either and I know some bios lets you choose between x2 and x4 for those slots. I would prefer to put it in the top slot since I think I don't have to remove anything to get to it although my CPU cooler is pretty freaking huge. Then again the second slot has the m.2 shield that would keep it cooler but it would require me removing the GPU and the sound card just to get to it.
 
Ricky T said:
I tried googling this and actually the responses are kind of all over the place plus I'm pretty ignorant when it comes to hard drives. Right now I have two SATA Samsung 4 TB drives which will both be completely removed for the installation of a single Sabrent 8 TB NVMe drive. So really all I need to know is it better to use the top or the bottom M.2 slot or does it even matter? The bottom slot of course has the heat shield on this MSI Z390 Gaming Pro Carbon motherboard. The only other thing I will have installed that will use the PCH lanes will be a sound card in the PCI_E5 slot.
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both slots can be used (all m.2 NVME 4x lines from the the chipset not the CPU) Note if bottom slot is used (with the heatsink) you lose 2 bottom Sata ports

https://www.msi.com/Motherboard/support/MPG-Z390-GAMING-PRO-CARBON-AC#down-manual << page 33 >>

the bottom slot is the most ideal for running a NVME SSD (performance wise) as it has the heatsink to keep it cool, but you lose 2 sata ports
 
There used to be a thought of the closer the m2 is to the cpu the faster it will perform. Use either slot. M2_2 on my board disables sata ports 5 and 6. I used M2_2 since I have 2 ssds in raid 0 on ports 1 and 2. So no worry there. Just use whatever you'd like. There is no difference except disabling sata ports.
 
jarablue said:
There used to be a thought of the closer the m2 is to the cpu the faster it will perform. Use either slot. M2_2 on my board disables sata ports 5 and 6. I used M2_2 since I have 2 ssds in raid 0 on ports 1 and 2. So no worry there. Just use whatever you'd like. There is no difference except disabling sata ports.
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Given that the M.2 slots on a Z390 board go through the PCH, there is no difference between them.
 
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apart from the bottom one has a heatsink which can prevent the NVME ssd from thermal throttling
 
On my B450 pro4 MB I have a M.2 Ultra and a M.2 but I dont know the features. I think I read I couldnt use the normal M.2 as a boot drive
 
NVMe, or at least the host interface, is a logical means to connect the storage to the system. SATA HBA has to follow good old DOS Laws, and are physical before they are logical. This is why whenever I connect SATA drives the importance of the drive, and such, are in a particular order from port 0 to the ending port for any system. This will always equal on the drive target number, and if not labeled I still have a sort of identification. I always go from SSD to HDD placing slower and less important towards the last ports. When installing OS's and when drivers are loaded to expose the hardware, so can the physical and logical partitions. If the order/target is not proper, which it should with my methods, then I know I need to do particular actions. Open Disk Management and you will see as to what I mean. Drive 0 will equal SATA port 0. Then with the physical drive, any logical partitions will have the letters assigned. For example, SATA port 0 with 3 logical partitions will be Drive 0 with letters C, D, E if no other partitions exist. If you have three physical SATA drives 0, 1, 2 with only one logical partition each and an NVMe drive as the OS drive you should have drive 0 D, drive 1 E, drive 2 F, and the NVMe as C

This is also as to why "position" only matters due to lane routing from the Root Complex (q.v. HSIO allocation, PCIe interface version, lane allocation). This is also why you will see an NVMe drive letter "lower" in the hierarchy of Task Manager, due to DOS Law even though it is logically the first drive higher in letter assignment. I wish it had the logics to sort proper, but it kind of informs you that it is logical interface in the assigned letter order. Always, read the manual for the HSIO lane subtraction and allocation of resources. Because, with some core logics you will not be able to use some interfaces while others are in use (q.v. Z390 if both M.2's are in use you will will SATA port 4 and 5)

The M.2 interfaces do have physical order and are after SATA. Sometimes this can be annoying because M.2A seems as if before M.2M, but it is not always the case to as which is the physical order. Literature in the manual may not say which one is first. So, your first physical M.2 will be first in letter, and as to why you may see a driver letter be ordered descending. For example, top M.2 can be the second physical and the bottom will be first physical. If you installed the OS drive on the top physical M.2, and the bottom is additional storage with one partition each then Task manager will show D above C instead of C and then D. Guess M is main? Just wish they stick with numerical 0-x. M.2.0 and M.2.1 is better no?

So, SATA and then physical M.2 placements is the physical order to drive numbers for Drive Managers' numbers. For example, three SATA drives will be drive 0, 1, 2 and two M.2's will be 3 and 4. The letters will be logically within the allocations and installtion.
 
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Closer to the CPU = shorter path for electrons to travel
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Depends on how the root complex is mapped to the end points. Generally speaking closer with PCIe, the modern middle and top of the line desktop processors you will have 20'ish direct from processor. Intel has 20. 16 are usually used for dGPU, and 4 DMI to the PCH. AMD has 24. 16 are usually used for dGPU, 4 usually mapped for NVMe, and 4 for southbridge connection. HEDT have a LOT more like the beastly Threadripper. So close to cache and memory-this is where you place bandwidth thirsty I/O such as 10GbE. I look forward to the day that most is integrated, or much closer with much more lanes to allocate for I/O.

Gigabyte posts a PCIe diagram of the root complex (used to be printed in the manuals) and I wish that all manufacturers did something of the sort to know where things are routed. This way you really know how things are routed.

https://en.wikipedia.org/wiki/Root_complex
 

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The Lurker said:
Closer to the CPU = shorter path for electrons to travel
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But no real world benchmarks would show any difference worth anything. Just use whatever slot. I used M2_2 and my nvme wd sn750 ssd ate up SATA port 5 and 6 when running gen3x4 speeds.
 
I tried both slots and the only real difference was temps since one slot has a heat shield. Scores dropped just a tiny bit on the slot with higher temps.
 
The Lurker said:
Closer to the CPU = shorter path for electrons to travel
Click to expand...

That doesn't matter on Intel since it goes thru the chipset anyways. And on platforms that go thru the cpu, it is partly limited by the pcie lanes available like on x570 you have to go to the top most nvme slot since that's the dedicated x4 link. On TRX40, it also doesn't matter as they use redrivers/retimers to keep signal integrity up, thus we can use massive 4x nvme bifurication setups w/o issue.
 
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