PCIe(Peripheral Component Interconnect Express)
The high-speed expansion bus that connects GPUs, NVMe drives and network cards to the CPU.
PCIe lanes are a limited resource on small boards. Adding a 10G NIC, an HBA or extra NVMe drives all compete for the available lanes.
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NVMe
A fast storage interface for SSDs that connects over PCIe, far quicker than SATA.
HBA(Host Bus Adapter)
A card that adds many drive ports, commonly flashed to pass disks straight through to ZFS.
M.2
A compact slot on a motherboard used mainly for NVMe or SATA SSDs.