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Question Asus Rog Strix B550-F Gaming M.2 and Pcie slot usage.

MGallik

Golden Member
Oct 9, 1999
1,787
4
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Hi folks. I need to pick the brains of experts so I came here. Is a Pcie 3.0 Video card (GTX 1660 Super) in PCIe slot 1 faster than it would be in slot 2? Why? Same question with top M.2 slot occupied with a SAMSUNG 970 EVO PLUS M.2 2280 2TB PCIe Gen 3.0 x4, NVMe. Why? And again with the 970 in the bottom M.2 slot. Why? Would the SSD be faster or slower in slot 2 with the video card in the top slot? Would the SSD be faster or slower in the top slot with the video card in the bottom slot?

I can't find the answer to these questions other than in a few 1 or 2 word condescendingly unhelpful replies in a few threads at Toms and Reddit. Basically the result of all combinations of the above components populating in every configuration. I know the top slots are connected to the CPU and the bottom to the SATA bus but don't know how that affects my system's performance. I'm looking for the optimum combination. And, of course, it would help immensely to know the why's.

Thank you all for as much insight as you feel free to share.
 
Last edited:

ch33zw1z

Lifer
Nov 4, 2004
33,989
12,821
146
Hi folks. I need to pick the brains of experts so I came here. Is a Pcie 3.0 Video card (GTX 1660 Super) in PCIe slot 1 faster than it would be in slot 2? Why? Same question with top M.2 slot occupied with a SAMSUNG 970 EVO PLUS M.2 2280 2TB PCIe Gen 3.0 x4, NVMe. Why? And again with the 970 in the bottom M.2 slot. Why? Would the SSD be faster or slower in slot 2 with the video card in the top slot? Would the SSD be faster or slower in the top slot with the video card in the bottom slot?

I can't find the answer to these questions other than in a few 1 or 2 word condescendingly unhelpful replies in a few threads at Toms and Reddit. Basically the result of all combinations of the above components populating in every configuration. I know the top slots are connected to the CPU and the bottom to the SATA bus but don't know how that affects my system's performance. I'm looking for the optimum combination. And, of course, it would help immensely to know the why's.

Thank you all for as much insight as you feel free to share.
Without going into the manual myself, it's basically like this: (hint - PCIe is all about lanes to the CPU)

1. PCIe VGA: If both slot 1 and 2 are x16, then they will perform the same.

2. PCIe SSD (aka NVMe): if both m.2 ports can run at x4, then they'll perform the same, the limiting factor is the SSD's performance (when using two different SSD's)

Using a m.2 SSD that is SATA is not the same interface as PCIe and is going to perform with less speed than a x4 NVMe SSD.

You're finding condescending answers because people can be mean. The "Why's" to these questions are straightforward when you stick to the technical specifications of each technology. It gets murkier when you dive into what the companies design the boards are really doing.

Rest assured, if you follow the manual and plug an x16 VGA into an x16 slot, and x4 SSD's into m.2 NVMe ports, you won't be disappointed
 

Golgatha

Lifer
Jul 18, 2003
12,028
282
126
Without going into the manual myself, it's basically like this: (hint - PCIe is all about lanes to the CPU)

1. PCIe VGA: If both slot 1 and 2 are x16, then they will perform the same.

2. PCIe SSD (aka NVMe): if both m.2 ports can run at x4, then they'll perform the same, the limiting factor is the SSD's performance (when using two different SSD's)

Using a m.2 SSD that is SATA is not the same interface as PCIe and is going to perform with less speed than a x4 NVMe SSD.

You're finding condescending answers because people can be mean. The "Why's" to these questions are straightforward when you stick to the technical specifications of each technology. It gets murkier when you dive into what the companies design the boards are really doing.

Rest assured, if you follow the manual and plug an x16 VGA into an x16 slot, and x4 SSD's into m.2 NVMe ports, you won't be disappointed
1) Electrically VGA slot 2 is a 4x PCIe slot. It also goes down to 1x bandwidth when any of the 1x slots on the board are in use. Minimal to huge difference in performance for the VGA card.

2) Bottom M.2 port probably shares 4x PCIe lanes with the rest of the chipset (disables SATA 5&6 when in use). Top slot is 4x lanes directly connected to the CPU. This can make a huge difference in performance.
 
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ch33zw1z

Lifer
Nov 4, 2004
33,989
12,821
146
1) Electrically VGA slot 2 is a 4x PCIe slot. It also goes down to 1x bandwidth when any of the 1x slots on the board are in use. Minimal to huge difference in performance for the VGA card.

2) Bottom M.2 port probably shares 4x PCIe lanes with the rest of the chipset (disables SATA 5&6 when in use). Top slot is 4x lanes directly connected to the CPU. This can make a huge difference in performance.
Cool, so

1) dont use slot 2 for a vga

2) pretty sure my board is the same way
 

Muadib

Lifer
May 30, 2000
16,896
455
126
Cool, so

1) dont use slot 2 for a vga

2) pretty sure my board is the same way
Actually he could use slot 2 for the GTX 1660 Super, as it's only a pci 3.0 card. Slot 1 is pci 4.0 so he would get the same performance either way. It's the same for the SAMSUNG 970 EVO PLUS M.2 2280 2TB PCIe Gen 3.0 x4. He would get the same performance in either M.2 slot, because the 970 EVO Plus is a Gen 3.0 drive. He needs to upgrade to a Gen 4.0 drive to see an improvement in speed.
 

Rigg

Senior member
May 6, 2020
218
354
96
Actually he could use slot 2 for the GTX 1660 Super, as it's only a pci 3.0 card. Slot 1 is pci 4.0 so he would get the same performance either way. It's the same for the SAMSUNG 970 EVO PLUS M.2 2280 2TB PCIe Gen 3.0 x4. He would get the same performance in either M.2 slot, because the 970 EVO Plus is a Gen 3.0 drive. He needs to upgrade to a Gen 4.0 drive to see an improvement in speed.
Uh no. Slot 2 is electronicly x4 and shares bandwidth with everything else connected to the chipset (2nd m.2 included) via its gen 3 x4 link.
 

Rigg

Senior member
May 6, 2020
218
354
96
Is a Pcie 3.0 Video card (GTX 1660 Super) in PCIe slot 1 faster than it would be in slot 2? Why? Same question with top M.2 slot occupied with a SAMSUNG 970 EVO PLUS M.2 2280 2TB PCIe Gen 3.0 x4, NVMe. Why? And again with the 970 in the bottom M.2 slot. Why? Would the SSD be faster or slower in slot 2 with the video card in the top slot? Would the SSD be faster or slower in the top slot with the video card in the bottom slot?
The top PCIE slot is gen 4.0 x16. The top M.2 is gen 4.0 x4. These connect directly to the CPU. The remaining PCIE slots and the second M.2 are connected to the CPU through the B550 chipset. The B550 chipset uplink is gen 3.0 x4 and anything connected to it will share that bandwidth. Gen 4.0 offers 2x the bandwidth of gen 3.0 although it can't be taken advantage of without a gen 4.0 device. Gen 3.0 devices will still take advantage of the available lanes in the top slots. Connecting multiple hi speed PCIE devices to the CPU via the slow Gen 3.0 x4 chipset uplink will likely create a completely unnecessary bottleneck.

 
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Muadib

Lifer
May 30, 2000
16,896
455
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Uh no. Slot 2 is electronicly x4 and shares bandwidth with everything else connected to the chipset (2nd m.2 included) via its gen 3 x4 link.
Yes, I see that slot 2 is a x4 slot. However if you use m.2, you lose Sata 5 & 6, so your NVMe drive should be fine.
 

Rigg

Senior member
May 6, 2020
218
354
96
Yes, I see that slot 2 is a x4 slot. However if you use m.2, you lose Sata 5 & 6, so your NVMe drive should be fine.
Yes it should be fine in most circumstances. It still has the potential to run slower if other devices connected to the chipset are simultaneously sucking up bandwidth. I wouldn't hesitate to use it for a second NVME drive in most systems but there is no reason to use it over the gen 4.0 M.2 slot for the primary drive. The second slot does have the potential to run slower in certain configurations/circumstances. Disabling sata ports doesn't make the chipset uplink any faster.
 

Muadib

Lifer
May 30, 2000
16,896
455
126
Yes it should be fine in most circumstances. It still has the potential to run slower if other devices connected to the chipset are simultaneously sucking up bandwidth. I wouldn't hesitate to use it for a second NVME drive in most systems but there is no reason to use it over the gen 4.0 M.2 slot for the primary drive. The second slot does have the potential to run slower in certain configurations/circumstances. Disabling sata ports doesn't make the chipset uplink any faster.
I was looking at the chart that you posted, and maybe I'm reading it wrong. I took the right side to mean that it would support either 2 Sata 6Gbps or 2 PCIe lanes make your choice. As they remove two Sata ports if you use the M.2_2 NVMe port, I took that to mean full speed ahead for the M.2_2port. The manual for this mb doesn't really say, so do you have a link?
 

Rigg

Senior member
May 6, 2020
218
354
96
I was looking at the chart that you posted, and maybe I'm reading it wrong. I took the right side to mean that it would support either 2 Sata 6Gbps or 2 PCIe lanes make your choice. As they remove two Sata ports if you use the M.2_2 NVMe port, I took that to mean full speed ahead for the M.2_2port. The manual for this mb doesn't really say, so do you have a link?
Look at the slide again. If a gen 4.0 x4 m.2 slot is implemented all the sata has to come through the chipset uplink. Everything on the right side half of the flow chart goes through the uplink and is potentially bottlenecked by it.
 
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Muadib

Lifer
May 30, 2000
16,896
455
126
Look at the slide again. If a gen 4.0 x4 m.2 slot is implemented all the sata has to come through the chipset uplink. Everything on the right side half of the flow chart goes through the uplink and is potentially bottlenecked by it.
Okay I get it now. Thanks for the explanation.
 
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