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0017 — Adopt Multus for the Longhorn storage network

Add Multus, chained with Cilium, so Longhorn can use the dedicated storage mesh.

Public edition condensed on 2026-10-21; original decision date and meaning retained.

Context​

ADR 0009 built a dedicated SFP+ mesh for storage replication. Longhorn needs a second pod interface to use it.

Decision​

Adopt Multus in thick-plugin mode as a meta-CNI chained with Cilium. Longhorn attaches replica traffic to the mesh through a NetworkAttachmentDefinition. Longhorn is the only planned consumer; any other consumer needs its own intake.

This decision selects the mechanism. The NetworkAttachmentDefinition and Longhorn's storage-network wiring were left to execution work, and the decision itself does not show that replica traffic uses the mesh.

Alternatives considered​

  • Do nothing: replication stays on the slower management network.
  • Replace the primary CNI: a much larger blast radius than adding Multus to a healthy Cilium.
  • Thin-plugin Multus: rejected in favor of the upstream-recommended thick-plugin DaemonSet.

Consequences​

Multus adds a cluster networking component and depends on the reference CNI plugins being present on every node. Validate secondary-interface addressing and pod-to-pod connectivity before sending storage traffic over it. Revisit if the mesh or the CNI architecture changes.