# longhorn-degraded — A Longhorn volume has been degraded for 10 minutes **Alert:** `LonghornVolumeDegraded` (warning, 10m) **File:** `platform/monitoring/alerts.yaml`, group `platform-health.longhorn` **Expression:** `longhorn_volume_robustness{state="degraded"} == 1` (one-hot over `state`, NOT a 0-3 enum) ## What this means here A volume has fewer healthy replicas than it should. It is **still serving I/O** — that is the difference between degraded and faulted (`/runbooks/longhorn-faulted`). What you have lost is redundancy, and on a 3-node cluster with hard node anti-affinity (`replicaSoftAntiAffinity: false`, `numberOfReplicas: 3`) one degraded volume means one more node loss away from data loss. **Degraded is the NORMAL state after any node reboot on this cluster, and it is expensive.** Recorded plainly in the stabilization notes: *"Longhorn on the HDD: every node reboot degrades all 18 volumes → full rebuild (slow; 30%+ IO-delay). Roll reboots one node at a time; wait `robustness=healthy` before the next."* The rebuild is the thing that hurts, not the degradation: rebuild writes land on the same partition as the embedded etcd WAL (`/data/k3s` and `/data/longhorn` share one device), and that is the mechanism behind the 2026-09-07 `k3s-prod-1` NotReady outage. So the question to answer is never just "why is it degraded" but "is the rebuild about to take a node out". Cross-read `/runbooks/node-io`. ### Verify the alert can fire (measured 2026-09-13, and it was inert TWICE) This rule was inert for two independent reasons, both fixed on 2026-09-13, and either one alone was enough to make a faulted or degraded volume alert nobody: 1. **No scrape.** `count(longhorn_volume_robustness)` returned 0 series — the longhorn_* family was absent from the TSDB because the chart's `metrics.serviceMonitor.enabled` defaulted false. Fixed in `platform/longhorn/helmrelease.yaml`. 2. **Wrong metric shape.** With the series present the rule STILL could not fire. It read `longhorn_volume_robustness == 2` on the assumption of a 0–3 enum; on longhorn 1.12.1 the metric is one-hot over a `state` label with values 0/1 only. Measured on prod the same day: `count(longhorn_volume_robustness)` = 96 (24 volumes × 4 states), `count by (state) (longhorn_volume_robustness == 1)` = `{state="healthy"} 24`, and `count(longhorn_volume_robustness == 2)` = **empty**. Fixed by keying on the label. Both directions are now pinned offline by `platform/monitoring/tests/longhorn.test.yaml`, which keeps a regression case asserting the old `== 2` form matches nothing. Check the live series before trusting this alert's silence: ``` infra/devcontainer/dev.sh run kubectl --context k3s-prod -n monitoring exec \ prometheus-kube-prometheus-stack-prometheus-0 -c prometheus -- \ /bin/promtool query instant http://localhost:9090 \ 'count by (state) (longhorn_volume_robustness)' ``` Four states, 24 each, is healthy wiring. Empty means the scrape has broken and this alert is inert again — do not infer "no alert, so volumes are fine". ## Running the commands in this runbook `kubectl` does not work on the operator workstation. The toolchain lives only in the pinned dev container, and the host's `k3s-oidc` context hangs on a browser prompt once its token expires — that hang is not the cluster being down. Every command below runs as: infra/devcontainer/dev.sh run bash -c 'kubectl --context k3s-prod get nodes' Always pass `--context k3s-prod` explicitly. `dev.sh` merges BOTH clusters into one kubeconfig and picks a current-context per invocation, and **that default has already changed once** — it was k3s-dev until dev was decommissioned on 2026-09-13, and is k3s-prod now. A command that names its context means the same thing next month as it does today; one that relies on the default does not, and the failure is silent (it answers confidently about the other cluster). `CLUSTER=prod infra/devcontainer/dev.sh run kubectl ...` selects it explicitly, and every invocation prints a banner naming the cluster it is about to talk to — read it. Do not run two `dev.sh` invocations at once; they share one container and interfere. USE FULLY-QUALIFIED RESOURCE NAMES, ALWAYS. Three CRDs on this cluster register the kind `Backup`: `longhorn.io/v1beta2`, `postgresql.cnpg.io/v1` and `velero.io/v1`. The bare short name resolves to Longhorn's, which holds zero objects — so `kubectl get backup -A` prints **"No resources found"** on a cluster holding 6 Velero backups and 27 CNPG backups. Verified on prod 2026-09-13. Reading that as "the backups are gone" and acting on it turns a false negative into real data loss. Write `backups.velero.io`, `backups.postgresql.cnpg.io`, `clusters.postgresql.cnpg.io`, `settings.longhorn.io`, `volumes.longhorn.io`, `nodes.longhorn.io`. Mutating kubectl (`apply`, `patch`, `annotate`, `rollout`) and `flux reconcile` are blocked for agents here: every durable change goes through git. **`main` is a protected branch — a direct push is rejected.** A change lands by pushing a branch, opening a PR, and merging it once the gate is green; that merge is the deploy. Where this runbook gives a mutating command it is for an operator, and it says so. ## First commands The truth about robustness, straight from the CRs (this works whether or not the metric does): ``` infra/devcontainer/dev.sh run bash -c \ 'kubectl --context k3s-prod -n longhorn-system get volumes.longhorn.io \ -o custom-columns=NAME:.metadata.name,ROBUST:.status.robustness,STATE:.status.state,NODE:.status.currentNodeID,SIZE:.spec.size' ``` Which replicas are missing, and where: ``` infra/devcontainer/dev.sh run bash -c \ 'kubectl --context k3s-prod -n longhorn-system get replicas.longhorn.io \ -o custom-columns=NAME:.metadata.name,VOL:.spec.volumeName,NODE:.spec.nodeID,RUNNING:.status.currentState | sort -k2' ``` Which PVC/workload is affected: ``` infra/devcontainer/dev.sh run bash -c \ 'kubectl --context k3s-prod get pv -o custom-columns=PV:.metadata.name,CLAIM:.spec.claimRef.namespace/.spec.claimRef.name,SC:.spec.storageClassName' ``` Are the nodes and their disks schedulable, and is a rebuild in flight? ``` infra/devcontainer/dev.sh run bash -c \ 'kubectl --context k3s-prod -n longhorn-system get nodes.longhorn.io -o wide; echo; \ kubectl --context k3s-prod -n longhorn-system get settings.longhorn.io concurrent-replica-rebuild-per-node-limit -o custom-columns=NAME:.metadata.name,VALUE:.value' ``` And the pressure that rebuild is causing — the reason this alert matters: ``` ssh k3s-prod-1 'cat /proc/pressure/io' ``` ## Likely causes, most likely first ### 1. A node rebooted or was briefly NotReady (overwhelmingly the most common) Expected, and it self-heals by rebuilding. Your job is to keep the rebuild from taking a node down. Remediation (operator, mutating) — freeze rebuilds while replicas will **reattach** (an in-place bounce): ``` infra/devcontainer/dev.sh run bash -c \ 'kubectl --context k3s-prod -n longhorn-system patch settings.longhorn.io \ concurrent-replica-rebuild-per-node-limit --type=merge -p "{\"value\":\"0\"}"' ``` Thaw to `"1"` — never higher — once every volume reads `healthy`. Use `1` rather than `0` when the replicas are genuinely gone (an OS re-provision), so replenishment can actually proceed. `concurrentReplicaRebuildPerNodeLimit: 1` was **already in force during the 2026-09-07 outage and held**; do not "fix" this by setting a knob that is already set. `replicaRebuildingBandwidthLimit` is **v2/SPDK-only** and does nothing here, and there is no `rebuild-concurrent-sync-limit` Setting on 1.12.1 at all. What the incident actually called for was tightening every **other** Longhorn I/O source — and as of 2026-09-13 that is declared in `platform/longhorn/helmrelease.yaml` (`defaultSettings`), not just intended: | setting | declared | was, before | |---|---|---| | `concurrent-replica-rebuild-per-node-limit` | 1 | 1 (already declared) | | `concurrent-volume-backup-restore-per-node-limit` | **1** | 5 (Longhorn default, undeclared) | | `snapshot-heavy-task-concurrent-limit` | **1** | 5 (Longhorn default, undeclared) | | `replica-replenishment-wait-interval` | 600 | 600 (pinned, not changed) | | `backup-concurrent-limit` / `restore-concurrent-limit` | 2 / 2 | 2 / 2 (pinned) | | `concurrent-automatic-engine-upgrade-per-node-limit` | 0 | 0 (pinned) | | `node-drain-policy` | `block-if-contains-last-replica` | same, but as Longhorn's default only | > **WHAT CHANGED, AND WHAT TO CHECK BEFORE TRUSTING IT.** Until 2026-09-13 this page asserted a cap > that did not exist: `snapshot-heavy-task-concurrent-limit` and > `concurrent-volume-backup-restore-per-node-limit` both measured **5** on prod, neither name > appeared anywhere in the repository, and the work sat on an unmerged branch. They are now declared > in the HelmRelease and reconciled by Flux, which is what makes them survive a rebuild — a > `kubectl patch` is how the gap arose in the first place. > > **Verify, do not assume** (the whole point of this note): > `kubectl -n longhorn-system get setting -o jsonpath='{.value}'`. If a value reads 5, the > HelmRelease has not reconciled — check `kubectl -n longhorn-system get helmrelease longhorn` and > that `longhorn-default-setting` carries the key > (`kubectl -n longhorn-system get cm longhorn-default-setting -o jsonpath='{.data}'`). The chart > **drops unknown `defaultSettings` keys with no error**, so "the key is in git" is not the same as > "the key is in effect" — `orphanAutoDeletion` was inert here for two minor versions for exactly > that reason, and has now been removed. > > **These caps are a bound, not a fix.** They limit how much competing I/O Longhorn is *allowed* to > generate. etcd's WAL and every replica still share one partition on every prod node, and no > scheduler or cgroup control arbitrates the ext4 journal that `fdatasync` waits on. The measurable > improvement came from hardware: node 1's 2011 SATA SSD was replaced, and under the same load class > (an 18-replica rebuild with node 1 as the write target) I/O PSI full fell **11.36 -> 3.79** and the > node stayed Ready. 3.79 against the 5% `NodeIOPressureHigh` threshold is the headroom you have. > > **Cost of the two caps, so nobody is surprised during an incident:** snapshot purge and clone run > one at a time per node, so space is reclaimed serially after a rebuild storm (watch > `LonghornDiskAllocationHigh`); and a Longhorn *volume* restore runs one at a time per node. The > latter costs nothing today — no Longhorn backup target is configured, and velero's nightly run is > kopia file-system backup (`defaultVolumesToFsBackup: true`, `snapshotVolumes: false`), which this > setting does not gate. During a real DR restore an operator may raise > `concurrent-volume-backup-restore-per-node-limit` for the duration and watch > `NodeIOPressureHigh` — that is a deliberate, temporary patch, like the rebuild freeze above. > > **Not ported, deliberately:** the same unmerged branch also carried node-level I/O arbitration > (BFQ on the shared device, `nr_requests` 64 -> 32, systemd `IOWeight` drop-ins; > `infra/ansible/roles/etcd_io_priority`). It was never run against any cluster, its own author > recorded it as unproven and node-3-first, and the drive it was written for has since been replaced. > It remains an operator option if I/O PSI climbs back toward 5%, not a merged default. ### 2. A node is still down Then the rebuild cannot complete and this will not clear. Go to `/runbooks/node-down` first; there is nothing to fix in Longhorn. ### 3. A disk was replaced and Longhorn refuses it Signature: `DiskFilesystemChanged` — *"record diskUUID doesn't match the one on the disk"*. Remediation, in exactly this order (the validating webhook enforces it): **disable the disk → remove the disk entry → re-add it**. Seen during the 2026-09-07 drive swap. ### 4. No node can hold the replica (anti-affinity or space) Hard anti-affinity means a 3-replica volume needs three distinct schedulable nodes. With one node cordoned or its disk over `storageMinimalAvailablePercentage: 25`, the third replica cannot be placed and the volume stays degraded indefinitely. Check `nodes.longhorn.io` scheduling and `/runbooks/longhorn-capacity`. ### 5. `replicaAutoBalance: best-effort` is shuffling replicas Normal background behaviour, transient. If degradation follows a pattern of replicas moving and never settling, that is worth an operator's attention. ## When to escalate or stop - **If more than one volume is degraded and I/O-PSI on any node is climbing, freeze rebuilds first and then get the operator.** That combination is the 2026-09-07 failure mode in progress. - **Never reboot, cordon or re-provision a second node while any volume is degraded.** This is the single most important rule in this runbook. - **Do not `kubectl drain`.** This cluster deliberately does not drain. `nodeDrainPolicy: block-if-contains-last-replica` is now declared (2026-09-13; before that it was in force only as Longhorn's own default, declared nowhere), so Longhorn will BLOCK a drain that would remove a volume's last healthy replica — that is a backstop against the worst case, not permission to drain. The upgrade plan still rejects system-upgrade-controller because "its auto-drain fights this cluster's slow-HDD Longhorn", and the node-loss conformance test evicts only its own writer pod. - **Do not delete a degraded volume's replicas to "force a clean rebuild"** unless the operator agrees — with 3 replicas and one already missing you are one mistake from `/runbooks/longhorn-faulted`. - Escalate for: a node that will not come back, a disk replacement, or a volume degraded for more than a few hours with all three nodes Ready (that is a Longhorn bug, not an operational state). --- Source of truth: `runbooks/longhorn-degraded.md`. Index: `/runbooks/` Records: `DECISIONS-LOG.md` "2026-08-31 — Post-upgrade stabilization" (the reboot tax) and "2026-09-07 — k3s-prod-1 NotReady"; `docs/plans/cluster-upgrade-plan.md` §6.2/§7 (the freeze/thaw).