fix(monitoring): delete the Longhorn rule that measured allocation and called it capacity

LonghornVolumeCapacityHigh went pending within minutes of the Longhorn scrape
landing today. Re-measured on prod: it read 0.9731 on the Prometheus TSDB volume
(actual_size 52243681280 / capacity 53687091200) while the kubelet reported the
SAME filesystem 20.8% used. actual_size is head + pinned snapshots, and one
user_created=false system snapshot from prod-1's replica rebuild holds 40510484480
bytes (37.7 GiB) of it; Longhorn returns no freed blocks without discard/TRIM, and
retentionSize 45GiB on a 50Gi volume targets 90% by design. The ratio is pinned
near 1.0 and cannot fall back under 0.85 — a permanently-firing alert, the same
shape as CNPGNoRecoverabilityPoint.

DELETED, not re-thresholded. Raising the threshold would tune away a true
statement about allocation. Redundancy proven, not asserted: of 24 Longhorn
volumes on prod, 23 appear in kubelet_volume_stats_inodes > 0 and so fall inside
MonitoringVolumeFillingUp (namespace="monitoring") or PersistentVolumeFillingUp /
PersistentVolumeAlmostFull. The 24th is unifi/unifi-rootdisk, volumeMode Block —
no filesystem for anyone to measure, and the deleted rule was not covering it
either.

AND A SECOND INERTNESS the scrape did not fix. longhorn_volume_robustness on
longhorn 1.12.1 is one-hot over a `state` label with values 0/1, not the 0-3 enum
the rules assumed:
  count(longhorn_volume_robustness)                  => 96  (24 volumes x 4 states)
  count by (state) (longhorn_volume_robustness == 1) => {state="healthy"} 24
  count(longhorn_volume_robustness == 2)             => (empty)
  count(longhorn_volume_robustness == 3)             => (empty)
So a FAULTED volume would still have alerted nobody. Re-keyed to the label; the
"44 series / 72 unknown(0, detached)" note in platform/longhorn/helmrelease.yaml
was the same misreading and is corrected in place.

The allocation question moved to where allocation is a budget. LonghornNodeStorageHigh
fired at 0.85, but storage-minimal-available-percentage is 25 on this cluster, so a
disk leaves the scheduling pool at 75% used — the old threshold could only fire
after scheduling had already been refused. Replaced by:
  LonghornDiskAllocationHigh  (longhorn_disk_usage_bytes / longhorn_disk_capacity_bytes) > 0.70, 30m
  LonghornDiskUnschedulable   longhorn_disk_status{condition="schedulable"} == 0, 15m
Per DISK, not per node: longhorn_node_storage_usage_bytes sums a node's disks, so
one full disk could hide behind a half-empty sibling. Measured prod usage
0.2254 / 0.2306 / 0.2369 on prod-1/2/3.

PROVEN NEITHER PERMANENTLY FIRING NOR INERT. platform/monitoring/tests/longhorn.test.yaml
adds 6 cases pinning every rule quiet on today's measured prod state and firing on
a constructed bad state, with every `for:` asserted, plus regression assertions that
the old `== 2`/`== 3` forms match nothing and that per-disk 0.72 is visible where the
node-summed 0.475 is not. Mutation-checked 11 ways; each turns the suite RED. Live on
prod: all four new expressions return empty, while the sum of their selector
cardinalities is 54. The L0 UNTESTED_ALERTS register shrinks by seven (7/56 -> 14/58
alert rules unit-tested), which also brings MonitoringVolumeFillingUp,
PersistentVolumeFillingUp and PersistentVolumeAlmostFull under test since the
redundancy claim rests on them.

REPORTED, NOT CHANGED (DECISIONS-LOG, owner decisions): retentionSize 45GiB on a
50Gi volume is too tight — recommend 36GiB, with reasoning; and discard/TRIM on the
longhorn StorageClass is feasible but not advisable now — remove-snapshots-during-
filesystem-trim reads false on prod, so trim would not have reclaimed the 37.7 GiB
anyway, and continuous discard on nodes where the etcd WAL shares the spindle points
at this cluster's known failure mode.

Co-Authored-By: Claude Opus 5 <[email protected]>

runbooks/README.md

@@ -37,12 +37,15 @@ negative is how a CLI quirk becomes real data loss. Write `backups.velero.io`,
 
 ## Alerts you should not trust the silence of (measured 2026-09-13)
 
-Prometheus scrapes 19 jobs on prod. There is **no** Longhorn, cert-manager, Flux-controller or
-Hubble/Cilium job, so these rule groups currently evaluate against nothing and **cannot fire**:
+The Longhorn and cert-manager scrapes were ENABLED on 2026-09-13, so the Longhorn row below is no
+longer inert — and re-keying the two robustness rules the same day fixed a SECOND inertness the
+scrape had left in place (the metric is one-hot over a `state` label, not a 0–3 enum). Take the
+Longhorn row as history; the rest of this table still holds, and Flux-controller and Hubble/Cilium
+rule groups still evaluate against nothing and **cannot fire**:
 
 | Missing scrape job | Inert alerts |
 |---|---|
-| Longhorn | `LonghornVolumeDegraded`, `LonghornVolumeFaulted`, `LonghornVolumeCapacityHigh`, `LonghornNodeStorageHigh` |
+| Longhorn | `LonghornVolumeDegraded`, `LonghornVolumeFaulted`, `LonghornDiskAllocationHigh`, `LonghornDiskUnschedulable` |
 | cert-manager | `CertificateExpiringSoon`, `CertificateExpiryImminent`, `CertificateNotReady` |
 | Flux (`gotk_*`) | `FluxReconciliationFailure`, `FluxResourceSuspended`, `ImageUpdateAutomationFailing`, `ImagePolicyStalled`, `UpdateHealthGateFailed` |
 | Hubble (`hubble_drop_total`) | `HubbleDeniedFlowSpike` |
@@ -65,7 +68,7 @@ Each affected runbook says so on its own page and gives the by-hand equivalent.
 - **`longhorn-degraded`** — a volume has lost redundancy. Normal after any reboot, and the rebuild is the
   expensive part. Freeze/thaw commands here.
 - **`longhorn-faulted`** — no usable replica. Data at risk; stop before anything destructive.
-- **`longhorn-capacity`** — a volume or a node's Longhorn disk near capacity.
+- **`longhorn-capacity`** — a node's Longhorn disk running out of room for replicas (allocation, not PVC fill; for PVC fill see `pvc-capacity` / `monitoring-capacity`).
 - **`pvc-capacity`** — PVC filling up / almost full / predicted to fill. These rules work.
 - **`monitoring-capacity`** — Prometheus or Loki running out of disk; don't let the monitor blind itself.