Will 28 pcie lanes be enough

tical2399

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I'm looking to build with a 5820k and dual 980s but I really want the speed of an m.2 ssd. At first I really wanted to make sure the board I buy supported gen 3 4x m.2, but gen 2 4x is fine as its still faster than sata 6.

The question I have is will the 5820k be able to SLI the 980s AND have four lanes available for my m.2 ssd? If so, what board can handle this? BTW, the extra $200 for a 5930k is ridiculous and is totally out of the question.
 

VincentP

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You don't have to force anything in the BIOS.
You should see a note on the specifications for any motherboard that X16/X16 mode is used with CPUs that support 40 lanes and X16/X8 mode is used with CPUs the support 28 lanes.
This is what they are designed to do.
If you use another add-in card like a Xonar, it will depend on the motherboard you choose.
Most X99 boards have at least three X16 slots and using another one of these will cause the slots to operate in X8/X8/X8 mode.
If the board happens to have any X4 or smaller slots, these are likely just run off the the X99 chipset PCI-E 2.0 lanes and won't affect your PCI-E 3.0 bandwidth.

In any case, plug your two video cards in the slots recommended in the motherboard manual, plug your M.2 SSD into the M.2 slot and plug your Xonar card in another available slot and it will just work. Motherboard manufacturers know what they are doing.
 

VincentP

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The PCI-E lanes come from the processor rather than the chipset like in some older architectures.
The Core i7 5820K supports 28 lanes PCI-E 3.0.
These lanes are then used by the motherboard for connectivity to expansion slots and M.2 storage.
The X99 chipset has an additional 8 lanes PCI-E 2.0 which can be used for additional expansion slots, add-in storage chipsets, etc.
I'm sure tical2399 understands this from the way the question is worded.

PCI-E 3.0 slots are run in X16, X8 or X4 mode.
For this reason, 28 lanes are not just split between the two slots in a dual card configuration.
Boards will run the first slot at X16 speed and the second at X8 speed.
This leaves 4 lanes available for the M.2 device.
 

tical2399

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I'm not concerned about performance as pcie 3.0 x8 is not bottle necking anything out today. What i'm concerned about is if the cards are forced to work in 16x/16x (which exceeds available lanes) or 16x/8x will there be enough lanes left to use the m.2 at all.

Edit: So what i'm hearing is that the 5820k has the 28 lanes which is enough for the 16x/8x combo for a total of 24 lanes. On top of that the x99 chipset (all x99 boards) have their own 4 lanes that can be used for things like m.2 regardless of what is happening on the cpu. Is this correct?
 

VincentP

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X16/X16 is with a CPU supporting 40 lanes PCI-E 3.0.
X16/X8 is with a CPU supporting 28 lanes PCI-E 3.0.
X16/X8 uses 24 lanes.
This leaves 4 lanes for M.2.

Even with three graphics cards you would have enough as the board would run the PCI-E X16 slots in X8/X8/X8 mode, still only using 24 lanes and leaving 4 lanes for M.2.
 
G

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it will work. ^ this guy^ just did the math for you. i think that pretty much answers it?
 

tical2399

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I have no problem with the math, its just that I wasnt going to assume that the cards would drop down to 8x if needed. Reading the specs of some boards, they specifically tell you "if you have this many cards they run in this speed" some of the listings dont tell you that. I assume they would just drop down to the next highest speed, but with a couple hundred dollar investment, im not gonna assume anything before i buy.

But since people are saying that you can force 8x in bios I will probably do that. Never had to before, as I always do single card setups. I will probably do that as dual x8 cards will be fine, and I can have m.2 and have a few lanes left for something else like a xonar or something.
 

VincentP

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You don't have to force anything in the BIOS.
You should see a note on the specifications for any motherboard that X16/X16 mode is used with CPUs that support 40 lanes and X16/X8 mode is used with CPUs the support 28 lanes.
This is what they are designed to do.
If you use another add-in card like a Xonar, it will depend on the motherboard you choose.
Most X99 boards have at least three X16 slots and using another one of these will cause the slots to operate in X8/X8/X8 mode.
If the board happens to have any X4 or smaller slots, these are likely just run off the the X99 chipset PCI-E 2.0 lanes and won't affect your PCI-E 3.0 bandwidth.

In any case, plug your two video cards in the slots recommended in the motherboard manual, plug your M.2 SSD into the M.2 slot and plug your Xonar card in another available slot and it will just work. Motherboard manufacturers know what they are doing.
 

tical2399

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Fair enough, that's exactly what I was looking for. Dual gpus, in whatever config is optimal, full speed m.2 and some bandwidth left over for a sound or wifi card.
 

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