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]>

conformance/tests/l0-static/alert-rules-unit-tested/check.sh

@@ -173,8 +173,12 @@ UNTESTED_FILES = {
 # own rule nothing asserts on. This is the half that stops a new alert being slipped into an
 # already-"tested" file untested: anything not listed here must appear in a covering test.
 # The monitoring block is large because backups.test.yaml (the repo's first suite) deliberately
-# covers the Velero/backup rules only; pvc-capacity-alerts.yaml is flattened into the same
-# rules file by the suite convention but has no assertions of its own yet.
+# covers the Velero/backup rules only. It SHRANK by seven on 2026-09-13, when
+# platform/monitoring/tests/longhorn.test.yaml took on the whole platform-health.longhorn group
+# plus the three kubelet-based PVC-fill rules it has to be measured against (the per-volume
+# Longhorn "capacity" rule was deleted as a permanently-firing duplicate of them, so proving the
+# redundancy meant asserting on the rules that survive). Only
+# PersistentVolumeFillingUpPredicted is still unasserted in pvc-capacity-alerts.yaml.
 UNTESTED_ALERTS = {
     ("platform/cnpg/alerts.yaml", "CNPGWALArchiveBacklog"),
     ("platform/cnpg/alerts.yaml", "CNPGNoRecoverabilityPoint"),
@@ -188,10 +192,6 @@ UNTESTED_ALERTS = {
     ("platform/monitoring/alerts.yaml", "NodeCPULoadHigh"),
     ("platform/monitoring/alerts.yaml", "NodeIOPressureHigh"),
     ("platform/monitoring/alerts.yaml", "NodeIOPressureCritical"),
-    ("platform/monitoring/alerts.yaml", "LonghornVolumeDegraded"),
-    ("platform/monitoring/alerts.yaml", "LonghornVolumeFaulted"),
-    ("platform/monitoring/alerts.yaml", "LonghornVolumeCapacityHigh"),
-    ("platform/monitoring/alerts.yaml", "LonghornNodeStorageHigh"),
     ("platform/monitoring/alerts.yaml", "FluxReconciliationFailure"),
     ("platform/monitoring/alerts.yaml", "FluxResourceSuspended"),
     ("platform/monitoring/alerts.yaml", "CertificateExpiringSoon"),
@@ -203,13 +203,10 @@ UNTESTED_ALERTS = {
     ("platform/monitoring/alerts.yaml", "KeycloakDatabaseDown"),
     ("platform/monitoring/alerts.yaml", "KeycloakClientSecretReconcilerFailing"),
     ("platform/monitoring/alerts.yaml", "KeycloakClientSecretReconcilerStale"),
-    ("platform/monitoring/alerts.yaml", "MonitoringVolumeFillingUp"),
     ("platform/monitoring/alerts.yaml", "PrometheusTargetsMissing"),
     ("platform/monitoring/alerts.yaml", "ImageUpdateAutomationFailing"),
     ("platform/monitoring/alerts.yaml", "ImagePolicyStalled"),
     ("platform/monitoring/alerts.yaml", "UpdateHealthGateFailed"),
-    ("platform/monitoring/pvc-capacity-alerts.yaml", "PersistentVolumeFillingUp"),
-    ("platform/monitoring/pvc-capacity-alerts.yaml", "PersistentVolumeAlmostFull"),
     ("platform/monitoring/pvc-capacity-alerts.yaml", "PersistentVolumeFillingUpPredicted"),
 }