A Clean Raspberry Pi Cluster: Two Nodes Per 1U, Hot-Swappable
Every Pi cluster starts the same way: one Pi on a desk, then two, then a third "for Kubernetes," and suddenly there's a nest of USB cables and micro-HDMI adapters that you're embarrassed to photograph. Mine got to four nodes before I admitted it needed a rack.
This is the setup I landed on: two Raspberry Pi 5s per 1U, in hot-swap caddies, in a 10-inch rack. Here's the thinking, and the mistakes.
Why hot-swap matters more than you think
The dirty secret of Pi clusters is that you're always pulling one node out. A corrupted SD card. A HAT that needs reseating. That one Pi that drops off the network for reasons you'll never fully explain. If removing a node means unscrewing a tray while the rest of the stack wobbles, you'll dread maintenance — and skip it.
So the design rule for our dual Pi caddy was: each Pi rides its own slide-out tray. Grip, pull, done. The rack stays put, the other node stays running.
Two per U is the right density
You can find designs that cram four Pis into 1U, standing vertically. I tried the vertical thing. It looks fantastic in photos and it's miserable to live with — GPIO faces the wrong way, PoE HATs don't fit, and airflow between boards is basically theoretical.
Two Pi 5s side by side, flat, gives you full clearance for a PoE HAT or an NVMe HAT on each board, and the SoCs actually get air. A six-node cluster is 3U. That's still tiny.
Power and cabling, the tidy way
PoE HATs turned my cluster from a cable nightmare into a single switch uplink per node. If your switch does PoE (I feed mine from a UniFi Lite 8, racked right below), each Pi needs exactly one cable. Ethernet in, done. USB-C power bricks are the fallback — route them down one side and don't let them near the front face.
Print notes
PETG or PLA both work here — Pis run cooler than mini PCs. 0.2mm layers, 3 perimeters, no supports; the trays print flat and the fit is toleranced at ±0.2mm, so a well-calibrated printer gives you a satisfying click when the tray seats. If the slide feels tight, check your first-layer squish before blaming the file — elephant's foot is the usual culprit.
Worth it?
A month in: I've pulled nodes maybe a dozen times — SD card swaps, one HAT reseat — and never once thought about it. That's the whole point. The caddy is $14 here, or bundled with the mini PC, fan, switch and cable mounts in the Starter Kit.