PCIE x16 wired as x4?

[yhwhshua.lamoore]

I'm getting an Optiplex 790 MT, I already have the build planned basically, but I noticed that I could plug in another extension in the x16 wired as x4 slot. I've heard that when you plug an x16 it runs at four times less than its normal bus, but I actually want to add a PCIE x4 card into the slot. I noticed that it won't fill the entire slot, so I was wondering if the x4 will run 4 times slower as well? At an x1 bus? It's an m.2 to x4 converter, I want my SSD to run as fast as it can at the x4 bus speed, but I want to be sure before I buy both items.

 

[bigpinkdragon286]

PCI-e slots are not wired randomly or different from one manufacturer to the next. There is a proper standard and they follow it.

If you have a x16 slot wired as x4, that means you have 4 lanes wired to a slot that can physically accommodate a card that has a 16 lane wide connector on it. It has nothing to do with the speed of the slot. The speed is a function of bandwidth per lane multiplied by how many lanes you are using. The lanes run at the same speed, whether you have 1, 4, 8, or 16. PCI-e devices will work with fewer lanes than their physical card-edge connector allows by only communicating using the lanes available in the add-in board slot on the motherboard, and this is by design.

Whether or not your device needs as much bandwidth as it's connector allows has to be taken on a per device basis, but usually, even with graphics cards, the bandwidth requirements are much lower than the interface supports.

A few reasons to have a x16 add-in board slot wired to only 4 PCI-e lanes come to mind.

If the motherboard had only a x4 physical slot, you couldn't use a card that uses a x16 connector, such as a graphics card. Since the vast majority of graphics cards fall short of needing a full 16 PCI-e lanes for bandwidth, plugging a mid-tier or lower GPU into a x4 slot isn't as unreasonable as it may seem on the surface, arguably opening up the opportunity for AMD's CrossFire.

You may have a x16 slot wired to only 4 lanes is because, often times there are only 4 lanes left to wire there, after all other devices on the motherboard have been wired up. Manufacturers don't have an unlimited amount of lanes to throw around, therefore you usually have at least one full sized slot that they don't have enough free PCI-e lanes left to fully wire up.

Routing PCI-e lanes on a motherboard adds to expense. The more fully wired slots the more expensive the motherboard. Having some slots not fully wired is a way to reduce cost, especially when most folks will never need a 2nd, fully wired x16 slot.

As for a x4 device going into a x16 slot wired to 4 lanes... Your slot has 4 lanes wired to it, therefore will give you a standard x4 connection. The only difference is the physical add-in board slot on the motherboard. It's big enough to accommodate up to a x16 card edge connector.

 

[yhwhshua.lamoore]

PCI-e slots are not wired randomly or different from one manufacturer to the next. There is a proper standard and they follow it.

If you have a x16 slot wired as x4, that means you have 4 lanes wired to a slot that can physically accommodate a card that has a 16 lane wide connector on it. It has nothing to do with the speed of the slot. The speed is a function of bandwidth per lane multiplied by how many lanes you are using. The lanes run at the same speed, whether you have 1, 4, 8, or 16. PCI-e devices will work with fewer lanes than their physical card-edge connector allows by only communicating using the lanes available in the add-in board slot on the motherboard, and this is by design.

Whether or not your device needs as much bandwidth as it's connector allows has to be taken on a per device basis, but usually, even with graphics cards, the bandwidth requirements are much lower than the interface supports.

A few reasons to have a x16 add-in board slot wired to only 4 PCI-e lanes come to mind.

If the motherboard had only a x4 physical slot, you couldn't use a card that uses a x16 connector, such as a graphics card. Since the vast majority of graphics cards fall short of needing a full 16 PCI-e lanes for bandwidth, plugging a mid-tier or lower GPU into a x4 slot isn't as unreasonable as it may seem on the surface, arguably opening up the opportunity for AMD's CrossFire.

You may have a x16 slot wired to only 4 lanes is because, often times there are only 4 lanes left to wire there, after all other devices on the motherboard have been wired up. Manufacturers don't have an unlimited amount of lanes to throw around, therefore you usually have at least one full sized slot that they don't have enough free PCI-e lanes left to fully wire up.

Routing PCI-e lanes on a motherboard adds to expense. The more fully wired slots the more expensive the motherboard. Having some slots not fully wired is a way to reduce cost, especially when most folks will never need a 2nd, fully wired x16 slot.

As for a x4 device going into a x16 slot wired to 4 lanes... Your slot has 4 lanes wired to it, therefore will give you a standard x4 connection. The only difference is the physical add-in board slot on the motherboard. It's big enough to accommodate up to a x16 card edge connector.

No I'm not talking about 4 pcie x16 lanes, I'm talking about one normal PCIE x16 and another x16 slot wired as a PCIE x4

 

[bigpinkdragon286]

I wasn't talking about 4 PCI-e lanes either. I was talking about PCI-e lanes, which are the electrical connections used to carry traffic between the devices on current generation motherboards. A single, fully wired x16 add-in board slot on a motherboard has 16 PCI-e lanes electrically wired to it. A single x16 add-in board slot on a motherboard that is wired as a x4 has 4 PCI-e lanes wired electrically to the slot.

It sounds like you misunderstand how PCI-e works.

Each slot is wired with a given number of lanes. A full x16 slot has 16 PCI-e lanes wired to the slot. A x16 slot wired as a PCI-e x4 has 4 PCI-e lanes wired to it and is electrically equivalent to a x4 slot, but can physically accept x16 cards.

A x16 card in a x16 slot that is wired as a x4 slot will operate in x4 mode.

A x4 card in a x16 slot that is wired as a x4 slot will operate in x4 mode.

If the SSD you mention in your original post says that it requires a x4 connection, that is because it can communicate using up to 4 PCI-e lanes. Since your PCI-e x16 slot is wired to 4 PCI-e lanes, your SSD will have all four lanes that it needs for full speed. Plugging your SSD into this slot WILL NOT cause performance issues of any kind.

 

[yhwhshua.lamoore]

Thanks, this information helps out a lot, another major reason why I'm doing this is because the sata ports in an optiplex 790 have a speed of 3gb/s instead of 6, on top of that I've seen that m.2 is actually faster than sata 3, which is awesome as well. I might just straight up get an x4 ssd, because those are apparently faster than m.2

 

[bigpinkdragon286]

While a good m.2 or PCI-e based SSD can run circles around an SSD that is running using the SATA interface, the first and most major performance benefit comes when you move from a spinning platter hard drive to an SSD. Beyond that, the gains are harder to notice, but they are there.

If you're using spinning platter hard drives on the 3G ports, I wouldn't worry too much, as the mechanical drives aren't going to exceed even the 3G speeds with their sustained transfer rates. Also, all though the theoretical throughput of an SSD on a 3G port would be lower, the big benefit of SSDs is their seek time, which wouldn't be affected by the older generation SATA port. It's pretty rare under normal conditions that you would ever saturate a 3G connection for the length of time necessary to be significantly hampered, unless you are routinely working with large files, and I suspect most folks wouldn't be keeping that many large files on an SSD anyway, unless they're in some sort of production environment. There's absolutely no good purpose to keep media that is only ever used for playback by a single user on such expensive storage media.