Question about the M.2 slot on my ASUS Sabertooth Z170 S

[ern88]

I have the ASUS Sabertooth Z170 S mobo. I was thinking on getting a M.2 SSD. But in the manual. It says it shares bandwidth with the No. 1 PCI-E slot. Which my GTX 1080 is plugged into. My question is. Can I still use the slot without performance hit?

 

[raisonjohn]

I have the ASUS Sabertooth Z170 S mobo. I was thinking on getting a M.2 SSD. But in the manual. It says it shares bandwidth with the No. 1 PCI-E slot. Which my GTX 1080 is plugged into. My question is. Can I still use the slot without performance hit?

The M.2 slot shares bandwidth with the SATA Express_1 port (not "PCI Express_1" port as you mentioned). SATA Express_1 port is the gray SATA ports. (See page 1-27 of your manual)

If you are going to use an M.2 SSD in PCIe-mode, you must select the SATA Mode Configuration in BIOS to [SATA Express] where in the SATA Express ports can only support SATA while the M.2 socket can only support PCIE devices. (See Page 3-34)

If you are going to use an M.2 SSD in SATA-mode, you must select the SATA Mode Configuration in BIOS to [M.2] where in the M.2 socket will now be able to support SATA, while the SATA Express ports can only support PCIE devices. (See Page 3-34 as well)

Your GTX 1080 is plugged into the PCIEX16_1 slot (running at x16 speeds) and is no way affected by the M.2 slot <---> SATA Express port bandwidth sharing. The only instance that your PCIEX16_1 slot will have a "performance hit" (but not noticeable) is when you plug another card at the PCIEX16_2 slot, making the two slots run at x8/x8 speeds.

 

[ern88]

I have the ASUS Sabertooth Z170 S mobo. I was thinking on getting a M.2 SSD. But in the manual. It says it shares bandwidth with the No. 1 PCI-E slot. Which my GTX 1080 is plugged into. My question is. Can I still use the slot without performance hit?

The M.2 slot shares bandwidth with the SATA Express_1 port (not "PCI Express_1" port as you mentioned). SATA Express_1 port is the gray SATA ports. (See page 1-27 of your manual)

If you are going to use an M.2 SSD in PCIe-mode, you must select the SATA Mode Configuration in BIOS to [SATA Express] where in the SATA Express ports can only support SATA while the M.2 socket can only support PCIE devices. (See Page 3-34)

If you are going to use an M.2 SSD in SATA-mode, you must select the SATA Mode Configuration in BIOS to [M.2] where in the M.2 socket will now be able to support SATA, while the SATA Express ports can only support PCIE devices. (See Page 3-34 as well)

Your GTX 1080 is plugged into the PCIEX16_1 slot (running at x16 speeds) and is no way affected by the M.2 slot <---> SATA Express port bandwidth sharing. The only instance that your PCIEX16_1 slot will have a "performance hit" (but not noticeable) is when you plug another card at the PCIEX16_2 slot, making the two slots run at x8/x8 speeds.

So, im trying to understand this. Bare with me. So, the 6 grey sata ports on my Mobo. Is it saying that I shouldn't connect a another peripheral to that port then? But I can use the 5 remaining grey ports? or not use any of the grey connectors. I am sorry. Just trying to understand it.

 

[raisonjohn]

So, im trying to understand this. Bare with me. So, the 6 grey sata ports on my Mobo. Is it saying that I shouldn't connect a another peripheral to that port then? But I can use the 5 remaining grey ports? or not use any of the grey connectors. I am sorry. Just trying to understand it.

The photo below shows your 6x total SATA ports in your motherboard:

The 2x black SATA ports at the left (one above the other), labelled as "SATA6G_5" and "SATA6G_6" in your manual, are stand-alone/not connected to the SATA Express ports.

The 4x grey SATA ports at the right (two ports above the other two ports), labelled as "SATA6G_1", "SATA6G_2", and "SATA6G_3", "SATA6G_4", respectively, in your manual, are all part of the SATA Express ports, labelled as "SATAEXPRESS_12".

The SATAEXPRESS_12 is composed of 2x SATA Express host-plugs (one above the other), and each SATA Express host-plug have 2x grey SATA ports (one beside the other). To illustrate, a single SATA Express host-plug looks like this:

So, in your motherboard,
SATAEXPRESS_1 ---> top row of grey ports (SATA6G_1 and SATA6G_2)
SATAEXPRESS_2 ---> bottom row of grey ports (SATA6G_3 and SATA6G_4)

Now, your motherboard manual/specification states, "The M.2 socket shares SATA ports with SATA Express_1 port. Adjust BIOS settings to use an M.2 SATA device."

This means that, when you plug an SSD in the M.2 socket and run it in SATA-mode, the two grey SATA ports in the SATAEXPRESS_1 host-plug ("SATA6G_1" and "SATA6G_2") can only be run using a PCIe device (so you can't run a SATA device, like a 3.5" SATA HDD or a 2.5" SATA SSD on those two grey ports, as the bandwidth is now being used by the M.2 socket).

Consequently, when you plug an SSD in the M.2 socket and run it in PCIe-mode, the same two grey SATA ports in the SATAEXPRESS_1 host-plug can only be run using a SATA device (you can now plug your regular 3.5" SATA HDD or 2.5" SATA SSD in those two ports).

In short:
If the M.2 is in PCIe-mode --> all 6x SATA ports can be used for SATA devices
If the M.2 is in SATA-mode --> only 4x SATA ports can be used for SATA devices; SATA6G_1 and SATA6G_2 can be used for PCIe devices only.

 

[ern88]

Thank you @raisonjohn that makes sense to me. Really appreciate the time to explain it to me.