The Z270X-Gaming 8 offers a wealth of expansion for a mainstream Intel motherboard. Gigabyte outfitted the board with four PCIe x16 slots and two PCIe x1 slots, all of which deliver PCIe 3.0 throughput. Three of the PCIe x16 slots feed from the CPU's 16 lanes, but only the first slot is wired for x16 connectivity. The second slot is wired for x8 throughput, and the third is provisioned as an x4 slot.
As we've come to expect from modern Intel platforms, the Gaming 8 has to split up CPU PCIe connectivity once a bunch of expansion cards enter the picture. The first PCIe x16 slot gets the full 16 CPU lanes with one graphics card installed. Add a card into the second PCIe slot, and the Gaming 8 gives it eight lanes of PCIe 3.0 from the first slot. Fire up the third PCIe x16 slot on top of the first two, and the board further splits the eight lanes of PCIe 3.0 it peeled off the first PCIe x16 slot into two x4 channels.
The lane-splitting fun doesn't stop with chipset-connected devices. The Gaming 8 has two M.2 connectors and two U.2 connectors. The M2M_32G slot sits directly beneath the CPU socket, and it's connected to four PCIe 3.0 lanes. Plug any SATA or NVMe device into this slot, and the SATA3 4 and SATA3 5 ports will go dark.
The M2P_32G connector shares its four chipset lanes with the U2_32G_1 connector (the topmost of the two U.2 connectors). Plug a PCIe x4 storage device into the M2P_32G connector, and U2_32G_1 will go dark (and vice versa). Using a SATA device in this slot will cause the SATA3 0 port to become unusable.
The U2_32G_2 connector (the bottom U.2 port on this board) shares its lanes with the last PCIe 3.0 x16 slot (PCIEX4_2). Plug a PCIe x2 device into PCIEX4_2, and the U2_32G_2 will also operate in x2 mode. Plug a PCIe x4 card into the PCIEX4_2 slot or a PCIe x4 NVMe device into U2_32G_2 and the unoccupied port or slot will stop working.
To close out the Gaming 8's lane-splitting mojo, we turn our attention to the board's PCIe x1 slots. Plug a card into the PCIEX1_1 slot, and the SATA3 1 port goes dark. Light up PCIEX1_2, and the SATA3 2 port becomes unusable. If you're as addled as I am at this point, perhaps the following diagram will help:
Thanks to the paltry 16 PCIe 3.0 lanes available from Skylake and Kaby Lake chips, the Alpine Ridge controller that drives the Gaming 8's Thunderbolt 3 port doesn't get a direct connection to the CPU. Instead, it hangs off four lanes of PCIe 3.0 from the Z270 chipset, behind the DMI 3.0 bus shared with all of the Gaming 8's storage devices and some of its PCIe slots. The Killer 1535 wireless adapter relies on a PCIe 3.0 x1 connection from Z270, as well.
For traditional storage, the Gaming 8 offers up to eight SATA ports, although only six of these are drawn from the Z270 chipset. The SATA3 6 and SATA3 7 ports (the leftmost in the shot above) run from an ASMedia ASM1061 controller connected to the Z270 chipset.
While we've gotten used to the what-you-see-is-not-what-you-get reality of the flex I/O lanes of Intel chipsets, it's still a bit disappointing that two of the Gaming 8's six chipset SATA ports will be disabled from the get-go if a builder installs an increasingly-popular NVMe SSD as their boot drive. Fill up both of the PCIe x1 slots for any reason, and you're down to just two usable SATA ports from the Z270 chipset. The ports from the ASMedia controller are always available, but picky builders will take umbrage at running their SATA devices off a third-party controller. Storage-hungry builders will have to carefully balance the complement of expansion cards, SATA devices, and NVMe drives they can install all at once with the Gaming 8.
Until just recently, we would have accepted this head-spinning lane-sharing arrangement as the way things had to be, but AMD's competing X370 chipset has fewer caveats for storage devices (at least as it's implemented on Gigabyte's own AX370-Gaming 5). The only hitch with X370 storage devices comes when one installs a SATA device in the AX370-Gaming 5's M.2 slot, at which point one of the board's SATA ports becomes unavailable. Builders still get seven SATA ports from X370 with such a drive installed, even if they have fewer potential M.2 slots and PCIe x16 configurations to play with. The expansion story for X370 still isn't quite as straightforward for AMD as it is with X399's expansion capabilities, but it's far less tiresome to discuss than Z270's flex I/O lanes.