The FlashArray//XR4 architecture fundamentally changed how external capacity is attached compared to older SAS-based or early NVMe platforms. Because DirectFlash Shelves (DFS) utilize massive 100GbE RDMA over Converged Ethernet (RoCE) for their backend loops to maintain NVMe-level performance, specific port and slot assignments are heavily regulated.
On an //XR4 model (such as the //X70 R4 or //X90 R4), the controllers feature dedicated, integrated 100GbE ports on the motherboard (specifically eth18 and eth19) which are strictly reserved for the first DirectFlash Shelf loop.
However, when an Implementation Engineer needs to install a second DirectFlash Shelf, the integrated onboard ports are already fully saturated. Pure Storage does not support daisy-chaining NVMe-oF RoCE shelves in the same manner as legacy SAS enclosures. Instead, a new dedicated backend fabric loop must be established. To accomplish this, the engineer must install an additional 100GbE RoCE PCIe expansion card.
According to the official //XR4 PCIe Slot Population Matrix, the designated location for this secondary backend expansion card is PCIe Slot 4 . Installing the card into other frontend-designated slots (like Slot 0 or Slot 3) will trigger a topology mismatch during the array's pre-flight hardware checks and prevent the new shelf from initializing.