Secondary SATA Controllers, PCIE, and Intel Controllers

[mjperk]

I have an ASRock Z97 OC Formula, that has 6 Intel SATA 3 ports and 2 ASMedia SATA 3 ports. I have read that secondary, i.e. ASMedia controllers often use converted PCIE (x1) lanes which explains their supposed lack of performance. I have done tests on my system and can prove that sequential write speeds are in fact about 20% slower for a given drive. Anyway, that's not what I'm really curious about. What I am wondering, is if they all do use PCIE lanes, and what that impact might be to throughput on the PCH or PCIE system.

For example, I have GTX 780's running in SLI on two PCIE 3.0x16 slots (I realize they step down to x8 in this configuration). I am wondering if putting drives on the ASMedia controllers sucks graphics performance because of the bandwidth they might use on the from the PCIE system. So I guess I really have a few questions--please excuse my ignorance...
1.) I understand the PCIE slots are on different lanes. But, for example my PCIE1 slot and PCIE4 slot are the graphics card 3.0x16 connectors that are connected to some degree in that they step down to x8, x8 when you hook up SLI. But then you have other slots like PCIE6, which is a 2.0x16 slot that seemingly has no connection. So what I'm getting at here is, if I plug in more and more things to ANY PCIE slot, will I be at risk for robbing bandwidth for the system, or does it handle it on a per-lane basis?
2.) This sort of leads me into my original opening statement about secondary SATA controllers being on the PCIE system. If data is being transferred through the ASMedia SATA ports, does this in turn rob my PCIE system of bandwidth as well?

I guess overall, I do not understand the relationship between the PCIE system, the southbridge, and the PCH. I basically have a few options in which I hook up storage drives--Intel SATA conrollers, ASMedia SATA controllers, or PCIE converters (hooked up to a slot that is faster than x1). My main concern is getting maximum simultaneous throughput for information on drives, while not degrading my graphics card performance.

 

[popatim]

Ok,
With a z97 system you get 16 PCIe gen3 lanes from the motherbd and a 8 lanes of PCIe gen 2 from the PCH/northbridge. These PCH lanes are not shared with the CPU's GPU bus and using them will not directly degrade your GPU's performance.

 

[mjperk]

Ok,
With a z97 system you get 16 PCIe gen3 lanes from the motherbd and a 8 lanes of PCIe gen 2 from the PCH/northbridge. These PCH lanes are not shared with the CPU's GPU bus and using them will not directly degrade your GPU's performance.

Ok, so I guess that explains why 3.0x16 slots for graphics cards step down to x8, x8 in dual SLI and x4, x4, x4 in triple SLI.

So do the lanes between gens interact at all? I'm guessing not. And then the last question is regarding PCIE M.2 SSDs. My understanding is that they utilize some of these lanes as well--so the question is which one? Is an M.2 SSD like this going to rob bandwidth?

 

[popatim]

1:not at all
2: M.2 drives can be pcie based or sata based. the connector usually have 'wiring' for both. The PCIE versions take two to four lanes from whichever set you plug them into.

Something to be aware of is how lanes are assigned. The ones from the CPU are in one group and the PCH ones are in the other group. Often a motherbd employs lane switches to change between something like two pcie slots from running x16/x0 to x8/x8. When a card is inserted into the 2nd slot, all 8 lanes are switched even if the device can only use 2. so you won't find yourself running in x14/x2 if you do something like that.

 

[mjperk]

1:not at all
2: M.2 drives can be pcie based or sata based. the connector usually have 'wiring' for both. The PCIE versions take two to four lanes from whichever set you plug them into.

Something to be aware of is how lanes are assigned. The ones from the CPU are in one group and the PCH ones are in the other group. Often a motherbd employs lane switches to change between something like two pcie slots from running x16/x0 to x8/x8. When a card is inserted into the 2nd slot, all 8 lanes are switched even if the device can only use 2. so you won't find yourself running in x14/x2 if you do something like that.

I think I mostly understand your point about lane switching. When I asked about the M.2, I meant does that use gen 2 or 3 lanes? I have my two graphics cards on gen 3 (so x8, x8) and I understand that they might not use all those lanes. My gen 2 lanes are probably unused--at least from having cards plugged into the motherboard. So if I plug in the M.2 SSD is it going to pull one of the gen 3 lanes that the graphics are potentially on, gen 2 that nothing is on, or have I missed the point completely?

 

[popatim]

I think you are just a little overwhelmed is all. Keep in mind that there are two sets of pcie lens and that they are not allowed to mix. CPU and PCH.

Your motehrbd has 2 gen3 x16 slots. Those lane come from the cpu ant it will drop to x8/x8 if something is plugged into the 2nd slot.

It also has an gen2 x16 pcie slot that is only wired as x4.
These 4 lanes come from the PCH. If you plug a gen3 device in it it will still only work as gen2 and the device will have to switch modes to work. Just like putting a sata3 drive into a sata port- the drive has to drop to sata2 speed.

A NVME M.2 SSD can be either Gen2 or Gen3, check the drives specs to find out which, but if you plug it into a gen2 slot - its going to run at the gen2 speed no matter which one it is.

If you are willing to drop your GPU to x8 so you can get a gen3 M.2 drive, I urge you to contact your motherbd manufacturer to find out if it will even work. After all you are plugging in a storage device into a GPU slot...
I've heard it go both ways, Some work fine and other boards wont.

 

[mjperk]

I think you are just a little overwhelmed is all. Keep in mind that there are two sets of pcie lens and that they are not allowed to mix. CPU and PCH.

Your motehrbd has 2 gen3 x16 slots. Those lane come from the cpu ant it will drop to x8/x8 if something is plugged into the 2nd slot.

It also has an gen2 x16 pcie slot that is only wired as x4.
These 4 lanes come from the PCH. If you plug a gen3 device in it it will still only work as gen2 and the device will have to switch modes to work. Just like putting a sata3 drive into a sata port- the drive has to drop to sata2 speed.

A NVME M.2 SSD can be either Gen2 or Gen3, check the drives specs to find out which, but if you plug it into a gen2 slot - its going to run at the gen2 speed no matter which one it is.

If you are willing to drop your GPU to x8 so you can get a gen3 M.2 drive, I urge you to contact your motherbd manufacturer to find out if it will even work. After all you are plugging in a storage device into a GPU slot...
I've heard it go both ways, Some work fine and other boards wont.

Ok, thanks.

I don't think I need to drop the GPU lower than it is--I run SLI so they are at 3.0x8 and 3.0x8 by default. The gen 2.0x16 running with 4 lanes also makes sense. I do seem to recall reading that in the manual.

So the MB has an M.2 that is Gen 2.0x2, which as I understand is the 2nd to last release since new M.2 SSDs are using 4 lanes. I also know you can get PCIe M.2 SSD's that will plug into the GPU slots. So since I am not using the 2.0x16 (running with 4 lanes) wouldn't it make more sense to get an M.2 that can use 4 lanes and plug it into this 2.0x16 slot instead of the dedicated M.2 slot on the board? The only thing that I'm not sure about then is that the SSD's that use 4 lanes are PCIe 3.0x4, so even if I have one of these and plug it into the 2.0x"4" will it use all the lanes and see the benefit.