PCIE Bifurcation and Why You Need It in Your Home Server

Building and running your own home server is one of the best ways to learn and allows ones to experiment and learn technologies that they might not get to learn in a production environment. When you are building your home server, there are various technologies that can give you options to run different types of storage and other technologies. One of those is PCIE bifurcation. What is PCIE bifurcation and why do you need it?

What is PCIE bifurcation?

Bifurcation allows you to split up the PCIE “lanes” of a PCIE slot so that you can use these for more than one type of device. For example, you could split these into 4 lanes for NVMe, another 4 lanes for NVMe, and then another 8 lanes for a different device. This is just one example

Most motherboards that support bifurcation on the PCIE slot allow you to mix and match the different types of lane configurations on the PCIE slot. What is the advantage of doing this or enabling bifurcation? Well as we have alluded to above, instead of burning all those lanes for a single PCIE card that may not use or need all of the lanes, you can split these up so that you can use them for different purposes, essentially giving you the option to run more hardware in your PCI-e slot.

How PCIe Bifurcation Works

When you enable bifurcation, it allows the motherboard’s chipset or CPU to allocate specific lanes to multiple devices and is generally configured in the BIOS

When enabled, PCIe bifurcation allows the motherboard’s chipset or CPU to allocate specific lanes to multiple devices. This is often configured in the system BIOS or UEFI firmware. As another example, PCIe x16 bifurcation splitting the lanes into x8/x8 mode, means two x8 devices can be installed in the same slot using a compatible riser or adapter. Also, an x16 slot can be split into four x4 connections for M.2 NVMe SSDs via a PCIe expansion card which is a common use case I see most often.

For years, I ran Xeon-D Supermicro servers with bifurcation turned on to allow running a Supermicro add-on card for running (2) NVMe cards on the onboard PCI-e slot.

Why PCIe Bifurcation Matters for Home Servers

In today’s mini PCs that are very popular or common for building home labs, or with motherboards from Minisforum and others that are used for building your own server, these may only have a single PCIe slot. But, if you want to use multiple high-performance add-in cards, bifurcation can definitely help and can be a game-changer in terms of the hardware we can use in our server builds.

Note the following use cases:

1. Adding JBOD storage

One of the most common use cases with bifurcation is using an add-in card that will allow you to add multiple M.2 drives to a PCI-e slot. If you are using a motherboard or mini PC that is constrained on the number of M.2 slots available, this can be a viable option if you have a PCI-e slot available.

Below is an example of a PCI-e add-in card for adding 2 NVMe drives to a single PCI-e slot: https://amzn.to/4iFVlPr (Amazon affiliate link that I earn a small commission from)

As an example, with PCIe bifurcation, you can install an NVMe expansion card that allows four M.2 NVMe drives to be added using a single PCIe x16 slot. In this way, you don’t have to have a RAID controller or something else to add the additional storage.

2. Multiple Network Adapters for a virtual host

Another use case that is probably less common is adding multiple network adapters to a single slot. If you have an add-on card that turns 1 PCIe slot into 2 slots or more, this can be an effective way to add more networking bandwidth to your server, such as multipel 10 GbE or 25 GbE cards.

Networking performance and bandwidth is super important, especially if you get into running HCI storage solutions that require high-bandwidth, low latency connections.

3. Cost Savings and motherboard efficiency

Instead of having to buy a motherboard that is more expensive with multiple PCI-e slots, you can often get away with a single PCI-e slot that supports bifurcation as many devices don’t require many PCI lanes for functionality. So, you can use multiple devices with one slot.

How to Enable PCIe Bifurcation

There are several things you need to do before counting on being able to use bifurcation with your server build or mini PC configuration.

1. Motherboard and CPU Support
   Not every motherboard or mini workstation/pc will support PCIe bifurcation. You’ll need to check with the vendor and spec information to determine if this is possible. Also, you can check UEFI BIOS settings to make sure you seee bifurcation modes.
2. Enable bifurcation
   • Boot into BIOS/UEFI.
   • Navigate to PCIe slot settings (this is different for every board, but is often found under Advanced or Chipset settings).
   • Look for an option like “PCIe Bifurcation Mode” or “PCIe Slot Configuration.”
   • Select the bifurcation mode you want to implement (e.g., x8/x8, x4/x4/x4/x4, etc.).
   • Save and exit.
3. Use a Compatible PCIe Riser or Adapter
   A PCIe expansion card or riser is required to physically separate the lanes between the individual connectors. For example:
   • NVMe SSD expansion cards
   • PCIe riser cables for GPUs
   • Multi-port NIC cards with bifurcation support
4. Verify in the Operating System
   Once installed, boot into your OS and use tools like lspci (Linux) or Device Manager (Windows) to make sure that all devices are recognized and operating as you would expect

As you can see below, the Minisforum BD795M motherboard has this setting found under Advanced > PCIe/GFX Lanes Configuration.

Limitations and Considerations

While PCIe bifurcation is very helpful and extremely cool, there are a few limitations that you need to keep in mind.

• Motherboard and CPU Support: Many consumer motherboards only support bifurcation on the first PCIe x16 slot and depending on the CPU, this may or may not be an option
• Physical limitations – You may run into physical limitations with your case or other factors when trying to plug multiple cards in a single slot. Sometimes this requires adapters and other physical components to make it work correctly.
• Power and cooling: More devices in one slot means higher power consumption and heat output potentially. Keep an eye on temps and power usage
• Firmware: Some expansion cards may need to have specific BIOS settings or firmware updates to operate using bifurcated slots

Wrapping up

PCIe bifurcation is a great feature for a home server that allows you to take full advantage of limited PCI-e slot configurations. In a small mini server that may only have 1 PCI-e slot, bifurcation support allows you to potentially use multiple devices on this single PCI-e slot where you would be limited to a single device otherwise.

I have used bifurcation many times in the past to add NVMe storage to home servers when limited on the number of M.2 slots built onto the motherboard. I am planning on testing a few other things out with my recent home server build as well. So stay tuned with that. How about you all? Do you currently take advantage of bifurcation with your home server?