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fix: make small scratch emptyDirs RAM-backed instead of node-disk #530

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k.renner merged 1 commits from fix-emptydir-tmpfs into master 2026-08-20 04:38:22 +00:00
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claude-botandClaude Opus 5 b34450b22e fix: make small scratch emptyDirs RAM-backed instead of node-disk
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Follow-up to the trmnl renderer finding. An `emptyDir` without `medium: Memory`
is NODE STORAGE, which is easy to read as "ephemeral, therefore free" -- it is
ephemeral, but every byte still hits the node's disk.

Survey of the repo: 23 emptyDir declarations, only 2 RAM-backed. This converts
12 of the remaining ones.

HONEST SCOPE: this buys almost no write reduction. Ranking every pod by OS-disk
writes over 6h shows trmnl-renderer at 67 GB/day (fixed separately) and then
nothing emptyDir-driven -- the rest of the top of that list is Ceph mons
(hostPath, not emptyDir), kube-apiserver, and container logs from databases.
Every volume converted here measured 8.0K in the running pod:

  /var/lib/code-server   8.0K
  /mosquitto/auth        8.0K
  /state (z2m guards)    8.0K

So the value is correctness and, for mosquitto, keeping credential material off
the node's filesystem: that volume holds the generated passwd/acl files.

Each converted volume gets an explicit sizeLimit (16Mi where none existed), so
tmpfs cannot silently consume a container's memory limit. All are orders of
magnitude above measured usage.

Behaviour is unchanged otherwise: an emptyDir persists for the pod's lifetime on
either medium, so the mosquitto shim's assumption that the passwd file survives
a container restart still holds.

DELIBERATELY NOT CONVERTED:
  - kubernetes/talos/cilium.yaml and talos/patches/cilium-cni.yaml -- vendored
    and generated from the chart; a hand edit would be lost on upgrade.
  - trmnl/renderer.yaml -- converted separately, where the actual 34 GB/day was.

Cost: this rolls the affected pods once.

Refs #434

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
 2026-08-20 06:30:23 +02:00