HSIO Lanes
To complicate things further, Intel has High Speed I/O lanes (HSIO). These appeared on its 1xx and 2xx series motherboard chipsets. They’re a development of the Flexible I/O system on the X99, which enabled selected PCIe lanes to be used to run SATA or USB ports instead. As the name says, they are high-speed internal lanes that are used to connect all the PCIe lanes; one HSIO lane per PCIe lane. But they’re also used for SATA, USB, network, and other gear. Every non- PCIe device is going to eat into your HSIO lanes. Some rough math tells us you can’t connect nearly as much gear to a Z170 board as you might think.
The Z170 chipset has 26 HSIO lanes and 20 PCIe lanes. The first six HSIO are fixed to running USB 3.0 ports. This leaves everything else to share the 20 of each left. If you fill your SATA, USB, and GbE ports with hardware, you have nine motherboard chipset PCIe left. It’s not very flexible either; specific HSIO lanes are tied to specific ports. If you have a storage device that needs four PCIe lanes, you have three possible blocks of HSIO lanes it can use. These are also assigned to your SATA ports. A second M.2 board will cost you two SATA ports, a third will cost you all six.
The Z270 chipset has 30 HSIO lanes, similarly configured. Your board may also sport companion controllers that add more ports or variations. While there are some decent technical arguments behind using HSIO—it brings all your high-speed ports together, and means board manufacturers have lots of options—it does undercut some of the flexibility.