Which slot to put my M.2

[Blackcloud]

I have a Strix Z690-E gaming wifi mobo, i9-12900k cpu, 4080 super. My OS is on a Samsung 980 pro in slot 2. I just bought a Samsung 990 pro. If I move my OS drive to slot 1 will it steal lanes from my GPU? And if so, should I keep it in slot 2 and put the new M.2 in slot 3? I don't understand the whole stealing lanes thing.

[powertoys]

I was going to write something, but I stumbled upon this Reddit post by u/Materidan that explains way better than I could:

 

Quote

12th gen CPUs have 16 x PCIe 5.0 lanes, and 4 x PCIe 4.0 lanes, the latter of which is intended for a NVMe SSD. Other PCIe lanes come from the Z690 chipset.

 

Most boards have a single PCIe 5.0 x16 slot, or for high end boards, dual PCIe 5.0 slots that will split to x8/x8. They will also have a single PCIe 4.0 m.2 connected directly to the CPU, and other m.2s connected to the chipset.

 

The vast majority of Z690 motherboards do NOT have an onboard PCIe 5.0 m.2 connector, however the ASUS ROG Strix Z690-E is one of the few that actually does. If you use this specific connector, regardless of the speed of your SSD (3.0, 4.0 or 5.0) it will drop the GPU to x8 speeds. This is because it's literally stealing the lanes.

 

So, if you own the Z690-E, you do NOT want to use the M.2_1 connector (at least not until you have actual PCIe 5.0 devices and it might be worthwhile). Instead, use M.2_2 which is directly connected to the CPU's 4 x 4.0 lanes (like most other boards). The M.2_3 slot is connected to the chipset and is also safe to use.

 

 

Full reply here: https://www.reddit.com/r/intel/comments/rdqe3a/comment/ho378kj/

 

In your situation, assuming that the 990 Pro is a faster SSD than the 980 Pro, the best case scenario is: OS installed and running on the 990 Pro that is placed on the M.2_2 slot; 980 Pro used for storage purposes on M.2_3 slot. Realistically though, the slower speeds of the 980 Pro running through chipset lanes should make little to no difference at all.

[Zando_]

1 hour ago, powertoys said:

OS installed and running on the 990 Pro

Do not waste space on the 990 Pro installing your OS. The big gains on NVMe drives are for large file transfers, the OS loads a ton of tiny files. The OS doesn't boot any faster off the latest NVMe drives than it does off a decent SATA SSD. Heck on my stuff with a bunch of PCIe lanes/devices (older HEDT), it's actually slower to boot off NVMe (as that uses PCIe) than a SATA SSD. I only have my OS on an NVMe drive for convenience sake.

[powertoys]

4 minutes ago, Zando_ said:

The big gains on NVMe drives are for large file transfers, the OS loads a ton of tiny files.

True, but running the 990 Pro through the chipset lanes will also reduce those speeds a little. But in either case, I highly doubt that OP could tell the difference in a blind test. I have one of the best SATA SSDs (Crucial MX500) and I couldn't tell the difference between that and a good NVMe drive for day to day use 

[Zando_]

20 minutes ago, powertoys said:

True, but running the 990 Pro through the chipset lanes will also reduce those speeds a little. But in either case, I highly doubt that OP could tell the difference in a blind test. I have one of the best SATA SSDs (Crucial MX500) and I couldn't tell the difference between that and a good NVMe drive for day to day use 

990 in the CPU lanes slot, 980 in the chipset lanes. So 990 in M.2_1 and the 980 in M.2_3. M.2_2 is PCIe 4.0 through the CPU as well, but the motherboard manual is not clear on how this is accomplished and whether it'll cause the GPU slot to drop to x8.

 

The chipset has an x8 DMI 4.0 (equivalent bandwidth to PCIe 4.0 IIRC) uplink to the CPU so with OP's setup there shouldn't be any noticeable speed drop with the 980 in chipset lanes anyways.

 

 

 

[StDragon]

34 minutes ago, Zando_ said:

990 in the CPU lanes slot, 980 in the chipset lanes. So 990 in M.2_1 and the 980 in M.2_3. M.2_2 is PCIe 4.0 through the CPU as well, but the motherboard manual is not clear on how this is accomplished and whether it'll cause the GPU slot to drop to x8.

**M.2_1 is PCIe 5.0 and 4.0 to the CPU
M.2_2 is PCIe 4.0 to the CPU.
M.2_3 is PCIe 4.0 through the Z690 chipset of which bandwidth is shared along the DMI link back to the CPU.

**When M.2_1 is occupied with SSD, PCIe X16 PCIe 5.0 will run in x8 mode only.

The Samsung 990 Pro is only PCIe 4.0, so that means you can place it into either M.2_1 or M.2_2 without losing lanes to the x16 PCIe 5.0 slot. However (if I'm reading this correctly), if you populate the M.2_1 slot with a future PCIe 5.0 NVMe SSD, then that x16 will drop down to a x8 PCIe 5.0 slot.

So if you have a PCIe 5.0 NVMe in the future, and you don't want to split lanes on the x16 slot, put it into an M.2_2 slot to force it into PCIe 4.0 x4 mode.

[powertoys]

1 hour ago, StDragon said:

However (if I'm reading this correctly), if you populate the M.2_1 slot with a future PCIe 5.0 NVMe SSD, then that x16 will drop down to a x8 PCIe 5.0 slot.

 

It seems to behave like this: if you populate M.2_1 with any SSD at all (regardless if it's PCIe 3.0, 4.0, 5.0, whatever), the main PCIe slot will get it's lanes halved

[StDragon]

1 hour ago, powertoys said:

 

It seems to behave like this: if you populate M.2_1 with any SSD at all (regardless if it's PCIe 3.0, 4.0, 5.0, whatever), the main PCIe slot will get it's lanes halved

Maybe, I'm not really sure other than going by what the manual states. Either way it's 20 lanes max to the CPU.

Below is the block diagram of the Z690 chipset to the CPU. In either configuration possible, it still adds up to 20 lanes regardless if the x16 slot splits into two x8 lanes.

Note: The path leading to the bottom was truncated from view, but it's the DMI link to the chipset not shown.

[kokosnh]

There is a PCIe switch, if you populate the first M.2 with anything, you get x8 to the first PCIe slot. 
 

„When M.2_1 is occupied with SSD device, PCIEX16(G5)_1 will run x8 only.”

they are waisting x4 PCIe 5.0 lanes in that config.