From 10bea3a8faefc7adf8771f5bfd57a37435ccede4 Mon Sep 17 00:00:00 2001
From: Jinwoo Hong <73622457+Jinwoo-H@users.noreply.github.com>
Date: Mon, 5 Oct 2026 17:55:52 -0400
Subject: [PATCH] feat(relay): report lane service times, 503 causes, per-site
 hold p99 and a director-vs-cell lock-wait split (#25645)

* feat(relay): report lane service times, 503 causes, per-site hold p99 and lock-wait split

Adds the step-1 observability: drain-return and sticky slot service times,
every /v1/assign 503 by cause with a non-drain total, a p99 per lock hold
site (drain-return regional rows get their own site), a director-side
pg_stat_activity sample that splits lock waiters by director vs cell, and
a log line for each reserved_requests drift reconciliation corrects.
Log-based metrics for the new fields are declared in Terraform, not applied.

* fix(relay): classify a lock waiter by the first relay table its statement names

* fix(relay): attribute lock waiters to the root holder; document the director hold alert change

Every waiter after the first in a row-lock convoy is blocked by the first
waiter, so the sample now walks pg_blocking_pids to the root, reading it once
per waiter. The sampler is single-flight. Drain-return regional holds feeding
the non-paging director hold policy is documented as expected.

* docs(relay): note the lock-wait sample undercounts director waiters when the pool is full
---
 cloud/apps/relay/src/app.ts                   | 63 ++++++++++----
 ...signment-isolated-cell-replacement.test.ts | 18 ++++
 cloud/apps/relay/src/assignment-store.test.ts | 33 ++++++++
 cloud/apps/relay/src/assignment-store.ts      | 26 ++++--
 .../src/cell-inventory-hold-samples.test.ts   | 20 +++++
 .../relay/src/cell-inventory-hold-samples.ts  | 26 +++++-
 .../relay/src/drain-return-admission.test.ts  | 26 +++++-
 .../apps/relay/src/drain-return-admission.ts  |  3 +
 cloud/apps/relay/src/index.ts                 | 17 ++++
 ...postgres-lock-wait-sample-postgres.test.ts | 76 +++++++++++++++++
 .../relay/src/postgres-lock-wait-sample.ts    | 57 +++++++++++++
 ...ublic-assignment-drain-return-lane.test.ts | 36 +++++++-
 .../relay/src/relay-observability.test.ts     | 78 +++++++++++++++++
 cloud/apps/relay/src/relay-observability.ts   | 83 +++++++++++++++++++
 cloud/apps/relay/src/relay-server.ts          |  3 +
 cloud/docs/orca-relay-operations.md           | 22 +++++
 cloud/docs/relay-incident-monitor.md          | 10 +++
 cloud/infra/terraform/relay-observability.tf  | 18 +++-
 18 files changed, 586 insertions(+), 29 deletions(-)
 create mode 100644 cloud/apps/relay/src/postgres-lock-wait-sample-postgres.test.ts
 create mode 100644 cloud/apps/relay/src/postgres-lock-wait-sample.ts

diff --git a/cloud/apps/relay/src/app.ts b/cloud/apps/relay/src/app.ts
index c3db6291287..e88162dfd88 100644
--- a/cloud/apps/relay/src/app.ts
+++ b/cloud/apps/relay/src/app.ts
@@ -54,6 +54,7 @@ import type { RelayReadinessDependency } from './relay-readiness.js'
 import type {
   AssignmentAdmissionLane,
   AssignmentAdmissionOutcome,
+  AssignmentUnavailableCause,
   RegionalRehomeSafetySnapshot,
   RelayRuntimeCounts
 } from './relay-observability.js'
@@ -121,6 +122,8 @@ export function createRelayApp(
       reason: AssignmentAdmissionRejection
     ) => void
     recordDrainReturnRetryAfter?: (seconds: number) => void
+    recordAdmissionServiceMs?: (lane: 'sticky' | 'drain-return', durationMs: number) => void
+    recordAssignmentUnavailable?: (cause: AssignmentUnavailableCause) => void
     recordRegionRequest?: (region: RelayRegion | undefined) => void
     recordRegionSelection?: (input: {
       targetRegion: RelayRegion
@@ -202,11 +205,25 @@ export function createRelayApp(
     context.header('Retry-After', String(stickyRetryAfterSeconds))
     return context.json({ error: 'assignments_temporarily_unavailable' }, 503)
   }
+  // The slot's hold, not the request's latency: herd recovery is slots / this.
+  const timeStickySlot = (lease: { release(): void }): { release(): void } => {
+    const startedAt = performance.now()
+    let released = false
+    return {
+      release: () => {
+        if (released) return
+        released = true
+        operations.recordAdmissionServiceMs?.('sticky', performance.now() - startedAt)
+        lease.release()
+      }
+    }
+  }
   // Classified server-side from the host's own assignment row, never from a
   // client claim: only a host whose home is roll-isolated reaches this lane.
   const drainReturnAdmission = new RelayDrainReturnAdmission(publicAssignmentAdmission, {
     maxConcurrent: Math.max(1, drainReturnConcurrency),
-    maxRetryAfterSeconds: config.drainReturnMaxRetryAfterSeconds ?? 300
+    maxRetryAfterSeconds: config.drainReturnMaxRetryAfterSeconds ?? 300,
+    onServiceMs: (durationMs) => operations.recordAdmissionServiceMs?.('drain-return', durationMs)
   })
   const deferDrainReturn = (context: Context, retryAfterSeconds: number): Response => {
     context.header('Retry-After', String(retryAfterSeconds))
@@ -287,7 +304,10 @@ export function createRelayApp(
   })
   app.post('/v1/assign', async (context) => {
     if (config.role === 'cell') return context.json({ error: 'director_only' }, 404)
-    if (!config.publicAssignmentsEnabled) return rejectPublicAssignment(context)
+    if (!config.publicAssignmentsEnabled) {
+      operations.recordAssignmentUnavailable?.('disabled')
+      return rejectPublicAssignment(context)
+    }
     const bearer = readBearer(context.req.header('authorization'))
     if (!bearer) return context.json({ error: 'invalid_token' }, 401)
     const claims = await verifyRelayToken(bearer)
@@ -312,11 +332,12 @@ export function createRelayApp(
     let lane: AssignmentAdmissionLane = 'placement'
     if (body.data.reconnect) {
       let rejection: AssignmentAdmissionRejection | undefined
-      const fastLane = await stickyAssignmentAdmission.acquire(claims.relayHostId, (reason) => {
+      const stickySlot = await stickyAssignmentAdmission.acquire(claims.relayHostId, (reason) => {
         rejection = reason
       })
-      if (!fastLane) {
+      if (!stickySlot) {
         operations.recordAssignmentAdmission?.('sticky-rejected')
+        operations.recordAssignmentUnavailable?.('sticky-lane')
         logAdmissionRejection({
           route: 'assign',
           lane: 'sticky',
@@ -326,6 +347,7 @@ export function createRelayApp(
         })
         return rejectStickyAssignment(context)
       }
+      const fastLane = timeStickySlot(stickySlot)
       let verified: ResolvedRelayAssignment | null = null
       try {
         verified = await operations.assignments.resolve(identity, {
@@ -342,6 +364,7 @@ export function createRelayApp(
             relayHostId: claims.relayHostId,
             reason: operationError(error)
           })
+          operations.recordAssignmentUnavailable?.('sticky-verify-database')
           return rejectStickyAssignment(context)
         }
         throw error
@@ -356,6 +379,7 @@ export function createRelayApp(
           operations.recordAssignmentAdmission?.('drain-return-deferred')
           operations.recordAssignmentRejectionReason?.('drain-return', drainReturn.reason)
           operations.recordDrainReturnRetryAfter?.(drainReturn.retryAfterSeconds)
+          operations.recordAssignmentUnavailable?.('drain-return-deferred')
           logAdmissionRejection({
             route: 'assign',
             lane,
@@ -391,6 +415,7 @@ export function createRelayApp(
           relayHostId: claims.relayHostId,
           reason: rejection
         })
+        operations.recordAssignmentUnavailable?.('placement-lane')
         return rejectPublicAssignment(context)
       }
     }
@@ -430,6 +455,7 @@ export function createRelayApp(
         })
         // The host's own release is settling; its next dial finds the row free.
         context.header('Retry-After', String(ASSIGNMENT_ROW_BUSY_RETRY_AFTER_SECONDS))
+        operations.recordAssignmentUnavailable?.('relay_assignment_row_busy')
         return context.json({ error: 'assignment_row_busy' }, 503)
       }
       if (isRelayAssignmentUnavailableError(error) || isRelayDatabaseTransientError(error)) {
@@ -442,13 +468,16 @@ export function createRelayApp(
           ...homeCellRejectionDetail(error)
         })
       }
-      if (isRelayAssignmentUnavailableError(error)) {
+      const unavailable = assignmentUnavailableCause(error)
+      if (unavailable) {
         if (lane === 'placement') {
           operations.recordRegionSelection?.({ targetRegion, fallback: false })
         }
+        operations.recordAssignmentUnavailable?.(unavailable)
         return context.json({ error: operationError(error) }, 503)
       }
       if (isRelayDatabaseTransientError(error)) {
+        operations.recordAssignmentUnavailable?.('database')
         return lane === 'placement'
           ? rejectPublicAssignment(context)
           : rejectStickyAssignment(context)
@@ -2054,16 +2083,22 @@ function logAssignmentRejection(input: {
 
 // The home-cell reason is not capacity, but it is the same answer to the client:
 // retry, the director cannot place you right now.
+const RELAY_ASSIGNMENT_UNAVAILABLE_ERRORS = [
+  'relay_capacity_exhausted',
+  'relay_connection_headroom_exhausted',
+  'relay_home_cell_unavailable',
+  'relay_assignment_row_busy'
+] as const satisfies readonly AssignmentUnavailableCause[]
+
+function assignmentUnavailableCause(
+  error: unknown
+): (typeof RELAY_ASSIGNMENT_UNAVAILABLE_ERRORS)[number] | undefined {
+  if (!(error instanceof Error)) return undefined
+  return RELAY_ASSIGNMENT_UNAVAILABLE_ERRORS.find((message) => message === error.message)
+}
+
 function isRelayAssignmentUnavailableError(error: unknown): boolean {
-  return (
-    error instanceof Error &&
-    [
-      'relay_capacity_exhausted',
-      'relay_connection_headroom_exhausted',
-      'relay_home_cell_unavailable',
-      'relay_assignment_row_busy'
-    ].includes(error.message)
-  )
+  return assignmentUnavailableCause(error) !== undefined
 }
 
 function homeCellRejectionDetail(
diff --git a/cloud/apps/relay/src/assignment-isolated-cell-replacement.test.ts b/cloud/apps/relay/src/assignment-isolated-cell-replacement.test.ts
index 21cfcc3cbe6..8c1e9088cf8 100644
--- a/cloud/apps/relay/src/assignment-isolated-cell-replacement.test.ts
+++ b/cloud/apps/relay/src/assignment-isolated-cell-replacement.test.ts
@@ -8,6 +8,7 @@ import {
 } from './cell-admission-selector.js'
 import type { RelayCellConfig } from './config.js'
 import {
+  consumeRelayCellInventoryHold,
   openInMemoryRelayDatabase,
   type RelayDatabase,
   type RelayLockOptions,
@@ -365,6 +366,23 @@ describe('re-placing a host off a cell isolated for a roll', () => {
     }
   })
 
+  // Why its own site: the drain-return lane's service time is either this lock or
+  // the inventory, and only a separate p99 can say which.
+  it('samples the regional target rows it locks under their own hold site', async () => {
+    const { store, database, isolateForRoll } = await setup()
+    const first = await store.assign(IDENTITY, 'us-central1')
+    await isolateForRoll(first.cellId)
+    consumeRelayCellInventoryHold(database)
+
+    await store.assign(IDENTITY, 'us-central1')
+
+    expect(consumeRelayCellInventoryHold(database)).toMatchObject({
+      cellInventoryHolds: 1,
+      cellInventoryHoldMaxSite: 'isolated-replacement',
+      isolatedReplacementHolds: 1
+    })
+  })
+
   it('stops re-placing once restore clears the stamp', async () => {
     const { store, isolateForRoll, restore, rollIsolatedAt } = await setup()
     const first = await store.assign(IDENTITY, 'us-central1')
diff --git a/cloud/apps/relay/src/assignment-store.test.ts b/cloud/apps/relay/src/assignment-store.test.ts
index 9b889a133a8..39eeaa0f924 100644
--- a/cloud/apps/relay/src/assignment-store.test.ts
+++ b/cloud/apps/relay/src/assignment-store.test.ts
@@ -2476,6 +2476,39 @@ describe('RelayAssignmentStore', () => {
     })
   })
 
+  it('logs the drift reconciliation corrects, once, after it commits', async () => {
+    const store = await setup(() => 100, [
+      { id: 'cell-a', url: 'https://relay-a.example.com', capacityRequests: 10 },
+      { id: 'cell-b', url: 'https://relay-b.example.com', capacityRequests: 10 }
+    ])
+    const grant = await store.assign({ userId: 'user-a', relayHostId: 'host000000000001' })
+    await database!.query(`UPDATE relay_cells SET reserved_requests = 5 WHERE cell_id = ?`, [
+      grant.cellId
+    ])
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {})
+
+    let lines: string[]
+    try {
+      await store.cellEvacuationStatus('cell-a', 'cell-b', true)
+      await store.cellEvacuationStatus('cell-a', 'cell-b', true)
+    } finally {
+      lines = warn.mock.calls.map((call) => String(call[0]))
+      warn.mockRestore()
+    }
+
+    const drift = lines
+      .filter((line) => line.includes('orca_relay_reservation_drift'))
+      .map((line) => JSON.parse(line) as unknown)
+    expect(drift).toEqual([
+      {
+        event: 'orca_relay_reservation_drift',
+        cellId: grant.cellId,
+        reservedRequests: 5,
+        leaseUnits: 1
+      }
+    ])
+  })
+
   it('blocks aggregate fenced completion on assignment accounting mismatch', async () => {
     let now = 100
     const cells = [
diff --git a/cloud/apps/relay/src/assignment-store.ts b/cloud/apps/relay/src/assignment-store.ts
index fac9c514916..2eaf143940b 100644
--- a/cloud/apps/relay/src/assignment-store.ts
+++ b/cloud/apps/relay/src/assignment-store.ts
@@ -69,6 +69,7 @@ import {
   REGIONAL_REHOME_DEFAULT_HOST_COOLDOWN_MS
 } from './database.js'
 import type { RelayCellConfig } from './config.js'
+import type { CellLockHoldSite } from './cell-inventory-hold-samples.js'
 import type {
   RelayDatabase,
   RelayLockOptions,
@@ -7374,7 +7375,7 @@ export class RelayAssignmentStore {
     targetCellId: string
   ): Promise<void> {
     const now = this.now()
-    await this.database.transaction(async (transaction) => {
+    const drift = await this.database.transaction(async (transaction) => {
       // Reconciliation takes the same assignment→activity→cell order as live
       // mutations so correcting drift never races a credential or socket lease.
       const assignments = await transaction.queryLocked(
@@ -7494,6 +7495,7 @@ export class RelayAssignmentStore {
         )
       }
 
+      const corrected: { cellId: string; reservedRequests: number; leaseUnits: number }[] = []
       for (const row of cells) {
         const cellId = text(row, 'cell_id')
         const expected = cellUnits.get(cellId) ?? 0
@@ -7505,8 +7507,18 @@ export class RelayAssignmentStore {
           `UPDATE relay_cells SET reserved_requests = ?, updated_at = ? WHERE cell_id = ?`,
           [expected, now, cellId]
         )
+        corrected.push({
+          cellId,
+          reservedRequests: integer(row, 'reserved_requests'),
+          leaseUnits: expected
+        })
       }
+      return corrected
     })
+    // Logged after COMMIT so a retried transaction reports each correction once.
+    for (const sample of drift) {
+      console.warn(JSON.stringify({ event: 'orca_relay_reservation_drift', ...sample }))
+    }
   }
 
   private async lockCellInventory(
@@ -7528,12 +7540,13 @@ export class RelayAssignmentStore {
   // row-lock order), keeps them off the fleet-wide lock without a cycle.
   // The wait policy follows the caller for the same reason the inventory lock's
   // does: a sweep must not fail terminally on ordinary contention. Hold time is
-  // deliberately not sampled here — the metric tracks the fleet-wide lock these
-  // rows replace, and mixing in short single-row holds would flatter it.
+  // sampled only for a caller that names its site: short single-row holds under
+  // the inventory label would flatter the fleet-wide lock they replace.
   private async lockCellRows(
     database: RelayDatabase,
     cellIds: string[],
-    mode: CellInventoryLockMode = 'request'
+    mode: CellInventoryLockMode = 'request',
+    holdSite?: CellLockHoldSite
   ): Promise<SqlRow[]> {
     const distinct = [...new Set(cellIds)]
     const { measureHoldMs: _sampled, ...wait } = cellInventoryLockOptions(mode)
@@ -7541,7 +7554,7 @@ export class RelayAssignmentStore {
       `SELECT * FROM relay_cells WHERE cell_id IN (${distinct.map(() => '?').join(', ')})
        ORDER BY cell_id ASC`,
       distinct,
-      wait
+      holdSite ? { ...wait, measureHoldMs: true, holdSite } : wait
     )
   }
 
@@ -7618,7 +7631,8 @@ export class RelayAssignmentStore {
     return await this.lockCellRows(
       database,
       candidates.map((row) => text(row, 'cell_id')),
-      mode
+      mode,
+      'isolated-replacement'
     )
   }
 
diff --git a/cloud/apps/relay/src/cell-inventory-hold-samples.test.ts b/cloud/apps/relay/src/cell-inventory-hold-samples.test.ts
index 61c7ef1ba9d..1e6fd86d2a9 100644
--- a/cloud/apps/relay/src/cell-inventory-hold-samples.test.ts
+++ b/cloud/apps/relay/src/cell-inventory-hold-samples.test.ts
@@ -102,6 +102,26 @@ describe('cell inventory hold samples', () => {
     expect(samples.consumeCounts()).toEqual(emptyCellInventoryHoldCounts())
   })
 
+  // Why: the shared max and p95 mix every lock, so only the per-site p99 says
+  // whether a drain return waits on its own regional rows or on the inventory.
+  it('reports a p99 per hold site', () => {
+    const samples = new CellInventoryHoldSamples()
+    for (let ms = 1; ms <= 100; ms++) {
+      samples.record(ms)
+      samples.record(ms * 2, 'isolated-replacement')
+    }
+    samples.record(7, 'rehome-target-row')
+
+    expect(samples.readCounts()).toMatchObject({
+      cellInventoryHolds: 201,
+      cellInventoryHoldMaxSite: 'isolated-replacement',
+      inventoryHoldMsP99: 99,
+      isolatedReplacementHoldMsP99: 198,
+      isolatedReplacementHolds: 100,
+      rehomeTargetRowHoldMsP99: 7
+    })
+  })
+
   // Why: the alert reads one max across every lock, so the label is the only
   // thing that says whether a long hold was the inventory or a rehome target row.
   it('names the site of the longest hold and reports rehome target rows apart', () => {
diff --git a/cloud/apps/relay/src/cell-inventory-hold-samples.ts b/cloud/apps/relay/src/cell-inventory-hold-samples.ts
index 3454c97b55a..f8fed1bb349 100644
--- a/cloud/apps/relay/src/cell-inventory-hold-samples.ts
+++ b/cloud/apps/relay/src/cell-inventory-hold-samples.ts
@@ -3,7 +3,8 @@
 // hold distribution, and no runtime metric carried it before this change.
 // Which lock a hold sample came from. Every site feeds the same max, so one
 // alert on cellInventoryHoldMsMax covers them all; the label names the holder.
-export type CellLockHoldSite = 'inventory' | 'rehome-target-row'
+// 'isolated-replacement' is a drain return's regional target rows.
+export type CellLockHoldSite = 'inventory' | 'rehome-target-row' | 'isolated-replacement'
 
 export type CellInventoryHoldCounts = {
   cellInventoryHoldMsMax: number
@@ -14,6 +15,11 @@ export type CellInventoryHoldCounts = {
   // reads its bound and its presence here.
   rehomeTargetRowHoldMsMax: number
   rehomeTargetRowHolds: number
+  // Per-site p99: the shared max and p95 cannot say which lock a drain waits on.
+  inventoryHoldMsP99: number
+  rehomeTargetRowHoldMsP99: number
+  isolatedReplacementHoldMsP99: number
+  isolatedReplacementHolds: number
   // Why: a failed acquisition produces no hold sample, so the hold fields alone
   // read healthy while the lock is saturated. Split by wait policy, not by
   // caller: fail-fast covers background sweeps that step aside by design AND
@@ -36,6 +42,10 @@ export function emptyCellInventoryHoldCounts(): CellInventoryHoldCounts {
     cellInventoryHoldMaxSite: 'none',
     rehomeTargetRowHoldMsMax: 0,
     rehomeTargetRowHolds: 0,
+    inventoryHoldMsP99: 0,
+    rehomeTargetRowHoldMsP99: 0,
+    isolatedReplacementHoldMsP99: 0,
+    isolatedReplacementHolds: 0,
     cellInventoryLockUnavailable: 0,
     cellInventoryLockTimeouts: 0
   }
@@ -79,19 +89,29 @@ export class CellInventoryHoldSamples {
     if (this.samples.length === 0) return { ...emptyCellInventoryHoldCounts(), ...failures }
     const sorted = [...this.samples].sort((left, right) => left.holdMs - right.holdMs)
     const max = sorted[sorted.length - 1]!
-    const rehome = sorted.filter((sample) => sample.site === 'rehome-target-row')
+    const site = (name: CellLockHoldSite) => sorted.filter((sample) => sample.site === name)
+    const rehome = site('rehome-target-row')
+    const isolated = site('isolated-replacement')
     return {
       cellInventoryHoldMsMax: round(max.holdMs),
-      cellInventoryHoldMsP95: round(sorted[Math.ceil(0.95 * sorted.length) - 1]?.holdMs ?? 0),
+      cellInventoryHoldMsP95: nearestRank(sorted, 0.95),
       cellInventoryHolds: sorted.length,
       cellInventoryHoldMaxSite: max.site,
       rehomeTargetRowHoldMsMax: round(rehome[rehome.length - 1]?.holdMs ?? 0),
       rehomeTargetRowHolds: rehome.length,
+      inventoryHoldMsP99: nearestRank(site('inventory'), 0.99),
+      rehomeTargetRowHoldMsP99: nearestRank(rehome, 0.99),
+      isolatedReplacementHoldMsP99: nearestRank(isolated, 0.99),
+      isolatedReplacementHolds: isolated.length,
       ...failures
     }
   }
 }
 
+function nearestRank(sorted: { holdMs: number }[], rank: number): number {
+  return round(sorted[Math.ceil(rank * sorted.length) - 1]?.holdMs ?? 0)
+}
+
 function round(value: number): number {
   return Number(value.toFixed(3))
 }
diff --git a/cloud/apps/relay/src/drain-return-admission.test.ts b/cloud/apps/relay/src/drain-return-admission.test.ts
index 33ee05eace1..560d4515ca0 100644
--- a/cloud/apps/relay/src/drain-return-admission.test.ts
+++ b/cloud/apps/relay/src/drain-return-admission.test.ts
@@ -9,7 +9,13 @@ import { RelayPublicAssignmentAdmission } from './public-assignment-admission.js
 
 type Timer = { at: number; callback: () => void; cancelled: boolean }
 
-function harness(overrides: { maxQueued?: number; maxRetryAfterSeconds?: number } = {}) {
+function harness(
+  overrides: {
+    maxQueued?: number
+    maxRetryAfterSeconds?: number
+    onServiceMs?: (durationMs: number) => void
+  } = {}
+) {
   let now = 0
   const timers: Timer[] = []
   const placement = new RelayPublicAssignmentAdmission({
@@ -33,7 +39,8 @@ function harness(overrides: { maxQueued?: number; maxRetryAfterSeconds?: number
   const lane = new RelayDrainReturnAdmission(placement, {
     maxConcurrent: 1,
     maxRetryAfterSeconds: overrides.maxRetryAfterSeconds ?? 300,
-    now: () => now
+    now: () => now,
+    onServiceMs: overrides.onServiceMs
   })
   return {
     lane,
@@ -133,6 +140,21 @@ describe('drain-return admission', () => {
     expect(Math.max(...retries)).toBe(3)
   })
 
+  // The unclamped sample: the EWMA's floor and ceiling would hide the real tail.
+  it('reports each slot hold once, as measured', async () => {
+    const samples: number[] = []
+    const { lane, advance } = harness({ onServiceMs: (ms) => samples.push(ms) })
+    const fast = admitted(await lane.acquire(host(1)))
+    advance(5)
+    fast.lease.release()
+    fast.lease.release()
+    const slow = admitted(await lane.acquire(host(2)))
+    advance(20_000)
+    slow.lease.release()
+
+    expect(samples).toEqual([5, 20_000])
+  })
+
   it('answers a host’s own early retry with its interval, not a place behind the cohort', async () => {
     const { lane } = harness()
     admitted(await lane.acquire(host(0))).lease.release()
diff --git a/cloud/apps/relay/src/drain-return-admission.ts b/cloud/apps/relay/src/drain-return-admission.ts
index 955b7a3b640..5b9989b6b5f 100644
--- a/cloud/apps/relay/src/drain-return-admission.ts
+++ b/cloud/apps/relay/src/drain-return-admission.ts
@@ -44,6 +44,8 @@ export class RelayDrainReturnAdmission {
       maxConcurrent: number
       maxRetryAfterSeconds: number
       now?: () => number
+      // The raw sample behind the EWMA, so the lane's service time is reported.
+      onServiceMs?: (durationMs: number) => void
     }
   ) {}
 
@@ -108,6 +110,7 @@ export class RelayDrainReturnAdmission {
   }
 
   private recordService(durationMs: number): void {
+    this.options.onServiceMs?.(durationMs)
     const sample = Math.min(SERVICE_MS_CEILING, Math.max(SERVICE_MS_FLOOR, durationMs))
     this.serviceMs += SERVICE_EWMA_WEIGHT * (sample - this.serviceMs)
   }
diff --git a/cloud/apps/relay/src/index.ts b/cloud/apps/relay/src/index.ts
index d7d37ce209b..9c34dfb7673 100644
--- a/cloud/apps/relay/src/index.ts
+++ b/cloud/apps/relay/src/index.ts
@@ -18,6 +18,7 @@ import {
 } from './database.js'
 import { runAssignmentCleanup } from './assignment-cleanup-steps.js'
 import { runRelayBackgroundOperation } from './relay-background-operation.js'
+import { readPostgresLockWaitSample } from './postgres-lock-wait-sample.js'
 import { jitteredSweepIntervalMs } from './relay-sweep-schedule.js'
 import { observedRelayRequests } from './relay-observability.js'
 import { startRegionalRehomeWorker } from './regional-rehome-worker.js'
@@ -87,8 +88,23 @@ const migrationInventoryTimer = roleOwnsAssignmentMaintenance(config.role)
       }, '[orca-relay] migration inventory failed')
     }, 5 * 60_000)
   : null
+// Directors only: one role's view covers every backend, and cells roll separately.
+// Single-flight, so a slow database never stacks samples on the 3-slot pool.
+let lockWaitSampling = false
+const lockWaitSampleTimer = roleOwnsAssignmentMaintenance(config.role)
+  ? setInterval(() => {
+      if (database.dialect !== 'postgres' || lockWaitSampling) return
+      lockWaitSampling = true
+      void runRelayBackgroundOperation(async () => {
+        observability.recordDatabaseLockWaitSample(await readPostgresLockWaitSample(database))
+      }, '[orca-relay] lock wait sample failed').finally(() => {
+        lockWaitSampling = false
+      })
+    }, 5_000)
+  : null
 cleanupTimer?.unref()
 assignmentCleanupTimer?.unref()
+lockWaitSampleTimer?.unref()
 inventorySnapshotTimer?.unref()
 migrationInventoryTimer?.unref()
 observability.start(() => ({
@@ -137,6 +153,7 @@ const shutdown = (): void => {
   if (assignmentCleanupTimer) clearInterval(assignmentCleanupTimer)
   if (inventorySnapshotTimer) clearInterval(inventorySnapshotTimer)
   if (migrationInventoryTimer) clearInterval(migrationInventoryTimer)
+  if (lockWaitSampleTimer) clearInterval(lockWaitSampleTimer)
   observability.stop()
   heartbeat?.stop()
   regionalRehomeWorker?.stop()
diff --git a/cloud/apps/relay/src/postgres-lock-wait-sample-postgres.test.ts b/cloud/apps/relay/src/postgres-lock-wait-sample-postgres.test.ts
new file mode 100644
index 00000000000..072dfcf1988
--- /dev/null
+++ b/cloud/apps/relay/src/postgres-lock-wait-sample-postgres.test.ts
@@ -0,0 +1,76 @@
+import { afterAll, beforeAll, describe, expect, it } from 'vitest'
+import { RelayAssignmentStore } from './assignment-store.js'
+import { openRelayDatabase, type RelayDatabase } from './database.js'
+import { readPostgresLockWaitSample } from './postgres-lock-wait-sample.js'
+
+const databaseUrl = process.env.ORCA_RELAY_TEST_POSTGRES_URL
+const describePostgres = databaseUrl ? describe : describe.skip
+
+const cell = {
+  id: 'lock-wait-sample-cell',
+  url: 'https://lock-wait-sample.example.com',
+  capacityRequests: 10
+}
+
+// The split rests on application_name surviving into pg_stat_activity for both
+// the waiter and its blocker, which only a real server shows.
+describePostgres('PostgreSQL lock-wait sample', () => {
+  let cellDatabase: RelayDatabase
+  let directorDatabase: RelayDatabase
+
+  beforeAll(async () => {
+    cellDatabase = await openRelayDatabase({
+      databaseUrl,
+      dataDir: '',
+      applicationName: `orca-relay/cell/${cell.id}`
+    })
+    directorDatabase = await openRelayDatabase({
+      databaseUrl,
+      dataDir: '',
+      applicationName: 'orca-relay/director/director'
+    })
+    await new RelayAssignmentStore(directorDatabase).reconcileCells([cell])
+  })
+
+  afterAll(async () => {
+    await directorDatabase?.query(`DELETE FROM relay_cells WHERE cell_id = ?`, [cell.id])
+    await cellDatabase?.close()
+    await directorDatabase?.close()
+  })
+
+  // Every waiter after the first is blocked by the first waiter's tuple lock, so
+  // only the root of the chain names the cell transaction that holds the row.
+  it('attributes a convoy of directors to the cell holding the relay_cells row', async () => {
+    expect(await readPostgresLockWaitSample(directorDatabase)).toEqual([])
+
+    let release!: () => void
+    const released = new Promise<void>((resolve) => (release = resolve))
+    let held!: () => void
+    const holding = new Promise<void>((resolve) => (held = resolve))
+    const holder = cellDatabase.transaction(async (transaction) => {
+      await transaction.query(
+        `UPDATE relay_cells SET reserved_requests = reserved_requests WHERE cell_id = ?`,
+        [cell.id]
+      )
+      held()
+      await released
+    })
+    await holding
+    const waiters = Array.from({ length: 3 }, () =>
+      directorDatabase.transaction(async (transaction) => {
+        await transaction.queryLocked(`SELECT * FROM relay_cells WHERE cell_id = ?`, [cell.id])
+      })
+    )
+
+    try {
+      await expect
+        .poll(async () => await readPostgresLockWaitSample(directorDatabase), { timeout: 900 })
+        .toEqual([
+          { waiterRole: 'director', table: 'relay_cells', holderRole: 'cell', waiters: 3 }
+        ])
+    } finally {
+      release()
+      await Promise.all([holder, ...waiters])
+    }
+  })
+})
diff --git a/cloud/apps/relay/src/postgres-lock-wait-sample.ts b/cloud/apps/relay/src/postgres-lock-wait-sample.ts
new file mode 100644
index 00000000000..0d9a46d69af
--- /dev/null
+++ b/cloud/apps/relay/src/postgres-lock-wait-sample.ts
@@ -0,0 +1,57 @@
+import type { RelayDatabase } from './database.js'
+import type { DatabaseLockWaitSample } from './relay-observability.js'
+
+// Every relay process connects as the same user through a socket, so neither
+// pg_stat_statements nor Query Insights can tell a director's lock wait from a
+// cell's. application_name (`orca-relay/<role>/<cell>`) can, so this samples it.
+// The holder is the root of the wait chain: in a row-lock convoy every later
+// waiter is blocked by the first waiter, not by the transaction holding the row.
+// Blockers are read once per waiter; non-relay waiters stay in so chains through
+// them resolve, and the depth cap bounds a cycle. The table is the first relay
+// table the waiting statement names, which may be one it only references.
+// It shares the director's 3-slot pool, so when every slot is a lock waiter the
+// sample queues and undercounts director waiters; a dedicated connection fixes that.
+const LOCK_WAIT_SAMPLE_SQL = `
+WITH RECURSIVE waiting AS MATERIALIZED (
+  SELECT pid, application_name, query, (pg_blocking_pids(pid))[1] AS blocker
+  FROM pg_stat_activity
+  WHERE datname = current_database() AND wait_event_type = 'Lock'
+), chain AS (
+  SELECT pid AS waiter, blocker AS pid, 1 AS depth FROM waiting
+  UNION ALL
+  SELECT chain.waiter, waiting.blocker, chain.depth + 1
+  FROM chain JOIN waiting ON waiting.pid = chain.pid
+  WHERE chain.depth < 8
+), root AS (
+  SELECT DISTINCT ON (waiter) waiter, pid FROM chain ORDER BY waiter, depth DESC
+)
+SELECT split_part(w.application_name, '/', 2) AS waiter_role,
+       COALESCE(
+         substring(w.query FROM '\\m(relay_cells|relay_assignments)\\M'),
+         'other'
+       ) AS waited_table,
+       split_part(holder.application_name, '/', 2) AS holder_role,
+       COUNT(*) AS waiters
+FROM waiting w JOIN root ON root.waiter = w.pid
+LEFT JOIN pg_stat_activity holder ON holder.pid = root.pid
+WHERE w.application_name LIKE 'orca-relay/%'
+GROUP BY 1, 2, 3`
+
+const RELAY_ROLES = new Set(['director', 'cell'])
+
+export async function readPostgresLockWaitSample(
+  database: RelayDatabase
+): Promise<DatabaseLockWaitSample> {
+  const rows = await database.query(LOCK_WAIT_SAMPLE_SQL)
+  return rows.map((row) => ({
+    waiterRole: relayRole(row['waiter_role']),
+    table: String(row['waited_table']),
+    holderRole: relayRole(row['holder_role']),
+    waiters: Number(row['waiters'])
+  }))
+}
+
+// Anything else (an operator session, a finished holder) stays one bounded key.
+function relayRole(value: unknown): string {
+  return typeof value === 'string' && RELAY_ROLES.has(value) ? value : 'other'
+}
diff --git a/cloud/apps/relay/src/public-assignment-drain-return-lane.test.ts b/cloud/apps/relay/src/public-assignment-drain-return-lane.test.ts
index 11ded0fa0e8..cd1344dc5b6 100644
--- a/cloud/apps/relay/src/public-assignment-drain-return-lane.test.ts
+++ b/cloud/apps/relay/src/public-assignment-drain-return-lane.test.ts
@@ -37,12 +37,14 @@ describe('drain-return lane', () => {
       relayHostId === drained ? isolatedHome(relayHostId) : assignment('cell-o', relayHostId)
     )
     const outcomes: string[] = []
+    const servedLanes: string[] = []
     const app = createRelayApp(config(), {
       store: {} as never,
       assignments: { assign, resolve } as never,
       drain: vi.fn(),
       ready: vi.fn(async () => true),
-      recordAssignmentAdmission: (outcome) => outcomes.push(outcome)
+      recordAssignmentAdmission: (outcome) => outcomes.push(outcome),
+      recordAdmissionServiceMs: (lane) => servedLanes.push(lane)
     })
 
     // The re-placement holds the drain lane; the sticky lane's only slot is free.
@@ -61,6 +63,8 @@ describe('drain-return lane', () => {
     replacement.resolve(assignment('cell-new', drained))
     expect((await moving).status).toBe(200)
     expect(outcomes).toEqual(['drain-return', 'sticky'])
+    // The drained host held a sticky slot for its verification read too.
+    expect(servedLanes).toEqual(['sticky', 'sticky', 'drain-return'])
   })
 
   it('defers an overflowing drain return with a paced Retry-After and says why', async () => {
@@ -73,6 +77,7 @@ describe('drain-return lane', () => {
     const outcomes: string[] = []
     const reasons: string[] = []
     const retryAfters: number[] = []
+    const causes: string[] = []
     const app = createRelayApp(config({ drainReturnQueueMax: 1, drainReturnWaitMs: 50 }), {
       store: {} as never,
       assignments: { assign, resolve } as never,
@@ -80,7 +85,8 @@ describe('drain-return lane', () => {
       ready: vi.fn(async () => true),
       recordAssignmentAdmission: (outcome) => outcomes.push(outcome),
       recordAssignmentRejectionReason: (lane, reason) => reasons.push(`${lane}:${reason}`),
-      recordDrainReturnRetryAfter: (seconds) => retryAfters.push(seconds)
+      recordDrainReturnRetryAfter: (seconds) => retryAfters.push(seconds),
+      recordAssignmentUnavailable: (cause) => causes.push(cause)
     })
     const warn = vi.spyOn(console, 'warn').mockImplementation(() => {})
 
@@ -103,6 +109,7 @@ describe('drain-return lane', () => {
     expect(reasons).toEqual(['drain-return:queue-full', 'drain-return:wait-timeout'])
     expect(retryAfters).toEqual([2, Number(timedOut.headers.get('retry-after'))])
     expect(outcomes.filter((outcome) => outcome === 'drain-return-deferred')).toHaveLength(2)
+    expect(causes).toEqual(['drain-return-deferred', 'drain-return-deferred'])
     expect(warn.mock.calls.flat().join('\n')).toMatch(
       /lane=drain-return hinted=true reason=queue-full .*retryAfter=2/
     )
@@ -115,12 +122,14 @@ describe('drain-return lane', () => {
     const assign = vi.fn(async () => assignment('cell-s', host))
     const resolve = vi.fn(async () => assignment('cell-s', host))
     const outcomes: string[] = []
+    const causes: string[] = []
     const app = createRelayApp(config(), {
       store: {} as never,
       assignments: { assign, resolve } as never,
       drain: vi.fn(),
       ready: vi.fn(async () => true),
-      recordAssignmentAdmission: (outcome) => outcomes.push(outcome)
+      recordAssignmentAdmission: (outcome) => outcomes.push(outcome),
+      recordAssignmentUnavailable: (cause) => causes.push(cause)
     })
 
     expect(
@@ -131,6 +140,27 @@ describe('drain-return lane', () => {
     expect(repeat.status).toBe(503)
     expect(repeat.headers.get('retry-after')).toBe('2')
     expect(outcomes).toEqual(['sticky', 'sticky-rejected'])
+    expect(causes).toEqual(['sticky-lane'])
+  })
+
+  it('names the cause of a placement that found no capacity', async () => {
+    vi.spyOn(console, 'warn').mockImplementation(() => {})
+    const assign = vi.fn(async () => {
+      throw new Error('relay_capacity_exhausted')
+    })
+    const causes: string[] = []
+    const app = createRelayApp(config(), {
+      store: {} as never,
+      assignments: { assign, resolve: vi.fn() } as never,
+      drain: vi.fn(),
+      ready: vi.fn(async () => true),
+      recordAssignmentUnavailable: (cause) => causes.push(cause)
+    })
+
+    const response = await app.request('/v1/assign', assignmentRequest('cccccccccccccccc'))
+
+    expect(response.status).toBe(503)
+    expect(causes).toEqual(['relay_capacity_exhausted'])
   })
 
   it('never classifies an unhinted request, which keeps the placement lane', async () => {
diff --git a/cloud/apps/relay/src/relay-observability.test.ts b/cloud/apps/relay/src/relay-observability.test.ts
index 9881c54808d..04051519074 100644
--- a/cloud/apps/relay/src/relay-observability.test.ts
+++ b/cloud/apps/relay/src/relay-observability.test.ts
@@ -200,6 +200,84 @@ describe('relay observability', () => {
     })
   })
 
+  it('splits non-drain 503s from scheduled drain-return deferrals', () => {
+    const entries: Array<Record<string, unknown>> = []
+    const observability = new RelayObservability(
+      { role: 'director', cellId: 'director', region: 'us-central1' },
+      (entry) => entries.push(entry)
+    )
+    for (let index = 0; index < 40; index++) {
+      observability.recordAssignmentUnavailable('drain-return-deferred')
+    }
+    observability.recordAssignmentUnavailable('sticky-lane')
+    observability.recordAssignmentUnavailable('relay_capacity_exhausted')
+    observability.recordAssignmentUnavailable('relay_capacity_exhausted')
+    observability.flush(counts)
+    observability.flush(counts)
+
+    expect(entries[0]).toMatchObject({
+      assign503sByCauseDelta: {
+        'drain-return-deferred': 40,
+        'sticky-lane': 1,
+        relay_capacity_exhausted: 2
+      },
+      assignNonDrain503sDelta: 3
+    })
+    expect(entries[1]).toMatchObject({ assign503sByCauseDelta: {}, assignNonDrain503sDelta: 0 })
+  })
+
+  it('reports lane slot service times only for windows that served the lane', () => {
+    const entries: Array<Record<string, unknown>> = []
+    const observability = new RelayObservability(
+      { role: 'director', cellId: 'director', region: 'us-central1' },
+      (entry) => entries.push(entry)
+    )
+    for (let ms = 1; ms <= 100; ms++) {
+      observability.recordAdmissionServiceMs('sticky', ms)
+      observability.recordAdmissionServiceMs('drain-return', ms * 10)
+    }
+    observability.flush(counts)
+    observability.flush(counts)
+
+    expect(entries[0]).toMatchObject({
+      stickyServiceMsP50: 50,
+      stickyServiceMsP99: 99,
+      drainReturnServiceMsP50: 500,
+      drainReturnServiceMsP95: 950
+    })
+    expect(entries[1]).not.toHaveProperty('stickyServiceMsP50')
+    expect(entries[1]).not.toHaveProperty('drainReturnServiceMsP50')
+  })
+
+  it('sums lock waiters per role and table over the samples taken', () => {
+    const entries: Array<Record<string, unknown>> = []
+    const observability = new RelayObservability(
+      { role: 'director', cellId: 'director', region: 'us-central1' },
+      (entry) => entries.push(entry)
+    )
+    observability.recordDatabaseLockWaitSample([
+      { waiterRole: 'cell', table: 'relay_cells', holderRole: 'cell', waiters: 3 },
+      { waiterRole: 'director', table: 'relay_cells', holderRole: 'cell', waiters: 1 },
+      { waiterRole: 'cell', table: 'relay_assignments', holderRole: 'director', waiters: 2 }
+    ])
+    observability.recordDatabaseLockWaitSample([])
+    observability.recordDatabaseLockWaitSample([
+      { waiterRole: 'cell', table: 'relay_cells', holderRole: 'cell', waiters: 1 }
+    ])
+    observability.flush(counts)
+
+    expect(entries[0]).toMatchObject({
+      dbLockWaitSamplesDelta: 3,
+      dbLockWaitersByKeyDelta: {
+        'cell:relay_cells:cell': 4,
+        'director:relay_cells:cell': 1,
+        'cell:relay_assignments:director': 2
+      },
+      dbCellRowLockWaitersDirectorDelta: 1,
+      dbCellRowLockWaitersCellDelta: 4
+    })
+  })
+
   it('aggregates coarse region requests, selections, fallbacks, and outages', () => {
     const entries: Array<Record<string, unknown>> = []
     const observability = new RelayObservability(
diff --git a/cloud/apps/relay/src/relay-observability.ts b/cloud/apps/relay/src/relay-observability.ts
index 16ee785a3f6..bc7f03cc1d5 100644
--- a/cloud/apps/relay/src/relay-observability.ts
+++ b/cloud/apps/relay/src/relay-observability.ts
@@ -49,6 +49,29 @@ export type AssignmentAdmissionOutcome =
 
 export type AssignmentAdmissionLane = 'sticky' | 'placement' | 'drain-return'
 
+// Why every /v1/assign 503 has a cause: drain-return deferrals are scheduled
+// 503s, so a gate on all 503s trips on every fast drain.
+export type AssignmentUnavailableCause =
+  | 'disabled'
+  | 'sticky-lane'
+  | 'sticky-verify-database'
+  | 'placement-lane'
+  | 'drain-return-deferred'
+  | 'database'
+  | 'relay_capacity_exhausted'
+  | 'relay_connection_headroom_exhausted'
+  | 'relay_home_cell_unavailable'
+  | 'relay_assignment_row_busy'
+
+// Row-lock waiters seen in one pg_stat_activity sample, by who waits, on which
+// table, behind whom. Roles come from each pool's application_name.
+export type DatabaseLockWaitSample = {
+  waiterRole: string
+  table: string
+  holderRole: string
+  waiters: number
+}[]
+
 export interface RelayRuntimeObserver {
   recordAuth(success: boolean): void
   recordForwardedBytes(bytes: number): void
@@ -61,6 +84,9 @@ export interface RelayRuntimeObserver {
   recordAssignmentAdmission?(outcome: AssignmentAdmissionOutcome): void
   recordAssignmentRejectionReason?(lane: AssignmentAdmissionLane, reason: string): void
   recordDrainReturnRetryAfter?(seconds: number): void
+  recordAdmissionServiceMs?(lane: 'sticky' | 'drain-return', durationMs: number): void
+  recordAssignmentUnavailable?(cause: AssignmentUnavailableCause): void
+  recordDatabaseLockWaitSample?(sample: DatabaseLockWaitSample): void
   recordRegionRequest?(region: RelayRegion | undefined): void
   recordRegionSelection?(input: {
     targetRegion: RelayRegion
@@ -115,6 +141,11 @@ type RelayMetricDeltas = {
   drainReturnRejectionsByReason: Record<string, number>
   drainReturnRetryAfterSecondsMax: number
   drainReturnRetryAfterSecondsSum: number
+  drainReturnServiceMs: number[]
+  stickyServiceMs: number[]
+  assign503sByCause: Record<string, number>
+  dbLockWaitSamples: number
+  dbLockWaitersByKey: Record<string, number>
   requestedRegions: Record<string, number>
   selectedRegions: Record<string, number>
   regionFallbacks: Record<string, number>
@@ -161,6 +192,11 @@ const emptyDeltas = (): RelayMetricDeltas => ({
   drainReturnRejectionsByReason: {},
   drainReturnRetryAfterSecondsMax: 0,
   drainReturnRetryAfterSecondsSum: 0,
+  drainReturnServiceMs: [],
+  stickyServiceMs: [],
+  assign503sByCause: {},
+  dbLockWaitSamples: 0,
+  dbLockWaitersByKey: {},
   requestedRegions: {},
   selectedRegions: {},
   regionFallbacks: {},
@@ -279,6 +315,23 @@ export class RelayObservability implements RelayRuntimeObserver {
     this.deltas.drainReturnRetryAfterSecondsSum += seconds
   }
 
+  recordAdmissionServiceMs(lane: 'sticky' | 'drain-return', durationMs: number): void {
+    if (lane === 'sticky') this.deltas.stickyServiceMs.push(durationMs)
+    else this.deltas.drainReturnServiceMs.push(durationMs)
+  }
+
+  recordAssignmentUnavailable(cause: AssignmentUnavailableCause): void {
+    increment(this.deltas.assign503sByCause, cause)
+  }
+
+  recordDatabaseLockWaitSample(sample: DatabaseLockWaitSample): void {
+    this.deltas.dbLockWaitSamples++
+    for (const row of sample) {
+      const key = `${row.waiterRole}:${row.table}:${row.holderRole}`
+      this.deltas.dbLockWaitersByKey[key] = (this.deltas.dbLockWaitersByKey[key] ?? 0) + row.waiters
+    }
+  }
+
   recordRegionRequest(region: RelayRegion | undefined): void {
     increment(this.deltas.requestedRegions, region ?? 'unhinted')
   }
@@ -446,6 +499,30 @@ export class RelayObservability implements RelayRuntimeObserver {
       drainReturnRejectionsByReasonDelta: deltas.drainReturnRejectionsByReason,
       drainReturnRetryAfterSecondsMax: deltas.drainReturnRetryAfterSecondsMax,
       drainReturnRetryAfterSecondsSum: deltas.drainReturnRetryAfterSecondsSum,
+      // Slot hold times; a lane serves concurrency / service time per second.
+      // Omitted when empty for the same reason as the accept percentiles below.
+      ...(deltas.drainReturnServiceMs.length === 0
+        ? {}
+        : {
+            drainReturnServiceMsP50: roundMs(percentile(deltas.drainReturnServiceMs, 0.5)),
+            drainReturnServiceMsP95: roundMs(percentile(deltas.drainReturnServiceMs, 0.95))
+          }),
+      ...(deltas.stickyServiceMs.length === 0
+        ? {}
+        : {
+            stickyServiceMsP50: roundMs(percentile(deltas.stickyServiceMs, 0.5)),
+            stickyServiceMsP99: roundMs(percentile(deltas.stickyServiceMs, 0.99))
+          }),
+      assign503sByCauseDelta: deltas.assign503sByCause,
+      assignNonDrain503sDelta: Object.entries(deltas.assign503sByCause)
+        .filter(([cause]) => cause !== 'drain-return-deferred')
+        .reduce((sum, [, count]) => sum + count, 0),
+      // Waiters summed over samples; divided by samples it is the mean number of
+      // backends waiting, i.e. lock-wait seconds per second.
+      dbLockWaitSamplesDelta: deltas.dbLockWaitSamples,
+      dbLockWaitersByKeyDelta: deltas.dbLockWaitersByKey,
+      dbCellRowLockWaitersDirectorDelta: lockWaiters(deltas.dbLockWaitersByKey, 'director'),
+      dbCellRowLockWaitersCellDelta: lockWaiters(deltas.dbLockWaitersByKey, 'cell'),
       requestedRegionsDelta: deltas.requestedRegions,
       selectedRegionsDelta: deltas.selectedRegions,
       ...regionCounterFields('requestedRegion', deltas.requestedRegions),
@@ -526,6 +603,12 @@ function regionCounterFields(
   )
 }
 
+function lockWaiters(byKey: Record<string, number>, waiterRole: string): number {
+  return Object.entries(byKey)
+    .filter(([key]) => key.startsWith(`${waiterRole}:relay_cells:`))
+    .reduce((sum, [, waiters]) => sum + waiters, 0)
+}
+
 function increment(counts: Record<string, number>, key: string): void {
   counts[key] = (counts[key] ?? 0) + 1
 }
diff --git a/cloud/apps/relay/src/relay-server.ts b/cloud/apps/relay/src/relay-server.ts
index 57e4eac2c99..b957443a35f 100644
--- a/cloud/apps/relay/src/relay-server.ts
+++ b/cloud/apps/relay/src/relay-server.ts
@@ -168,6 +168,9 @@ export function createRelayServer(
     recordAssignmentRejectionReason: (lane, reason) =>
       observability.recordAssignmentRejectionReason?.(lane, reason),
     recordDrainReturnRetryAfter: (seconds) => observability.recordDrainReturnRetryAfter?.(seconds),
+    recordAdmissionServiceMs: (lane, durationMs) =>
+      observability.recordAdmissionServiceMs?.(lane, durationMs),
+    recordAssignmentUnavailable: (cause) => observability.recordAssignmentUnavailable?.(cause),
     recordRegionRequest: (region) => observability.recordRegionRequest?.(region),
     recordRegionSelection: (input) => observability.recordRegionSelection?.(input)
   })
diff --git a/cloud/docs/orca-relay-operations.md b/cloud/docs/orca-relay-operations.md
index fc56af8656e..bab64731cee 100644
--- a/cloud/docs/orca-relay-operations.md
+++ b/cloud/docs/orca-relay-operations.md
@@ -513,6 +513,28 @@ pause through `applyRegionalRehomeControl` waits 1 s for that row, 3 attempts,
 so it can fail during one commit: retry a pause that fails once, and do not
 treat that as a fault.
 
+Each hold site also reports its own p99: `inventoryHoldMsP99`,
+`rehomeTargetRowHoldMsP99` and `isolatedReplacementHoldMsP99`, the last being
+the regional target rows a drain return locks; a site with no holds in the
+interval reports 0, so read these filtered to values above 0. Directors also
+report lane slot times (`stickyServiceMsP50/P99`, `drainReturnServiceMsP50/P95`,
+omitted when the lane served nothing; a sticky slot held only for a drain-return
+or unverified host's verification read counts too, so its p50 moves with that
+share), every `/v1/assign` 503 by cause in
+`assign503sByCauseDelta`, and `assignNonDrain503sDelta`, which leaves out the
+scheduled `drain-return-deferred` answers a fast drain produces by design.
+Every 5 s each director samples `pg_stat_activity` for relay backends waiting
+on a lock, keyed by waiter role, table and holder role
+(`dbLockWaitersByKeyDelta`). The holder is the root of the wait chain, and the
+table is the first relay table the waiting statement names. The sample runs
+on the director's own 3-connection pool, so when every connection is waiting on
+a lock it queues behind them and undercounts director waiters; a dedicated
+sampler connection would fix that. Summed waiters divided by `dbLockWaitSamplesDelta`
+is the mean number waiting, i.e. lock-wait seconds per second; directors and
+cells share one database user, so Query Insights cannot make this split.
+Reconciliation logs `orca_relay_reservation_drift` for each cell whose
+`reserved_requests` it corrected.
+
 `host-cooldown-ms` is the minimum gap between two rehomes of one host. It bounds the damage from
 a desktop whose region probe flips: without it the host would be dragged back across the ocean on
 every flip, since the preference age never expires while the host keeps reconnecting.
diff --git a/cloud/docs/relay-incident-monitor.md b/cloud/docs/relay-incident-monitor.md
index f193991e3ac..699835b5d0e 100644
--- a/cloud/docs/relay-incident-monitor.md
+++ b/cloud/docs/relay-incident-monitor.md
@@ -252,6 +252,16 @@ paused. Do not drain or restart cells for a director hold. A cell that has
 stopped reporting emits no hold sample, so these policies catch lock convoys,
 not outages.
 
+Since the step-1 observability deploy, director holds also include the
+regional target rows a drain return locks (`cellInventoryHoldMaxSite:
+isolated-replacement`), so the director policy can fire during a roll's drain.
+That is expected and needs no action; `isolatedReplacementHoldMsP99` is the
+per-site view. The same deploy adds that lock's NOWAIT refusals and bounded-wait
+timeouts to the director's `cellInventoryLockUnavailable` and
+`cellInventoryLockTimeouts`, so compare those fields across the deploy only with
+that site subtracted; `orca_relay_cloud_sql_lock_timeouts` is unaffected. Drain
+returns run only on directors, so the paging cell policy is unchanged.
+
 ## Implementation log
 
 - Gave `collector_failed` the same two-consecutive-sample tolerance as an unread
diff --git a/cloud/infra/terraform/relay-observability.tf b/cloud/infra/terraform/relay-observability.tf
index ba124407564..91601de9805 100644
--- a/cloud/infra/terraform/relay-observability.tf
+++ b/cloud/infra/terraform/relay-observability.tf
@@ -53,6 +53,10 @@ locals {
       description = "Relay statements Postgres cancelled after waiting out their lock timeout; a burst means one transaction is holding rows every other relay process needs."
       filter      = "resource.type=\"cloudsql_database\" AND resource.labels.database_id=\"${var.project_id}:${local.relay_database_instance_name}\" AND textPayload:\"db=orca_relay,\" AND textPayload:\"canceling statement due to lock timeout\""
     }
+    reservation_drift = {
+      description = "Cells whose reserved_requests disagreed with their lease units when reconciliation corrected them; should trend to zero."
+      filter      = "((resource.type=\"cloud_run_revision\" AND (${local.relay_service_log_filter})) OR resource.type=\"gce_instance\") AND jsonPayload.event=\"orca_relay_reservation_drift\""
+    }
   }
 
   relay_runtime_metrics = {
@@ -108,6 +112,18 @@ locals {
     cell_inventory_holds               = { field = "cellInventoryHolds", description = "Cell-inventory locks acquired in the interval; the percentiles above summarise these." }
     cell_inventory_lock_unavailable    = { field = "cellInventoryLockUnavailable", description = "Fail-fast cell-inventory acquisitions that found the lock held. Includes background sweeps, which step aside by design, so this is contention pressure rather than user-visible failure." }
     cell_inventory_lock_timeouts       = { field = "cellInventoryLockTimeouts", description = "Bounded cell-inventory waits that expired, counted per attempt rather than per request. This is the user-visible lane." }
+    inventory_hold_ms_p99              = { field = "inventoryHoldMsP99", description = "Cell-inventory lock (every row, or every general row) hold p99 in the interval, this site only." }
+    rehome_target_row_hold_ms_p99      = { field = "rehomeTargetRowHoldMsP99", description = "Rehome target-row lock hold p99 in the interval, this site only." }
+    isolated_replacement_hold_ms_p99   = { field = "isolatedReplacementHoldMsP99", description = "Drain-return regional target-row lock hold p99 in the interval, this site only." }
+    isolated_replacement_holds         = { field = "isolatedReplacementHolds", description = "Drain-return regional target-row locks acquired in the interval." }
+    drain_return_service_ms_p50        = { field = "drainReturnServiceMsP50", description = "Drain-return lane slot hold p50; lane capacity is concurrency / this. Omitted when no drain return ran." }
+    drain_return_service_ms_p95        = { field = "drainReturnServiceMsP95", description = "Drain-return lane slot hold p95 in the interval." }
+    sticky_service_ms_p50              = { field = "stickyServiceMsP50", description = "Sticky lane slot hold p50, verification included; herd recovery is concurrency / this." }
+    sticky_service_ms_p99              = { field = "stickyServiceMsP99", description = "Sticky lane slot hold p99 in the interval." }
+    assign_non_drain_503s              = { field = "assignNonDrain503sDelta", description = "Director /v1/assign 503s other than scheduled drain-return deferrals; the per-cause split is assign503sByCauseDelta." }
+    db_lock_wait_samples               = { field = "dbLockWaitSamplesDelta", description = "Relay-database lock-wait samples taken by this director in the interval; divide the waiter sums below by this." }
+    db_cell_row_lock_waiters_director  = { field = "dbCellRowLockWaitersDirectorDelta", description = "Director backends waiting on a relay_cells row lock, summed over samples." }
+    db_cell_row_lock_waiters_cell      = { field = "dbCellRowLockWaitersCellDelta", description = "Cell backends waiting on a relay_cells row lock, summed over samples." }
   }
 
   # Regions the director can hint or select. Pinned to relay-contract's RELAY_REGIONS by
@@ -323,7 +339,7 @@ resource "google_logging_metric" "relay_snapshot" {
   metric_descriptor {
     metric_kind = "DELTA"
     value_type  = "DISTRIBUTION"
-    unit        = contains(["sql_latency_ms", "control_rtt_ms_p50", "control_rtt_ms_p95", "control_rtt_ms_max", "client_accept_total_ms_p50", "client_accept_total_ms_p95", "client_accept_total_ms_max", "client_accept_assignment_ms_p95", "client_accept_credential_ms_p95", "client_accept_activity_ms_p95", "client_accept_attach_ms_p95", "client_accept_basis_ms_p95", "control_renewal_latency_ms_p50", "control_renewal_latency_ms_p95", "control_renewal_latency_ms_max", "http_latency_ms", "event_loop_ms_p99", "db_oldest_wait_ms", "db_wait_ms_max"], each.key) ? "ms" : each.key == "queued_bytes" || each.key == "heap_used_bytes" || each.key == "forwarded_bytes" ? "By" : "1"
+    unit        = contains(["sql_latency_ms", "control_rtt_ms_p50", "control_rtt_ms_p95", "control_rtt_ms_max", "client_accept_total_ms_p50", "client_accept_total_ms_p95", "client_accept_total_ms_max", "client_accept_assignment_ms_p95", "client_accept_credential_ms_p95", "client_accept_activity_ms_p95", "client_accept_attach_ms_p95", "client_accept_basis_ms_p95", "control_renewal_latency_ms_p50", "control_renewal_latency_ms_p95", "control_renewal_latency_ms_max", "http_latency_ms", "event_loop_ms_p99", "db_oldest_wait_ms", "db_wait_ms_max", "inventory_hold_ms_p99", "rehome_target_row_hold_ms_p99", "isolated_replacement_hold_ms_p99", "drain_return_service_ms_p50", "drain_return_service_ms_p95", "sticky_service_ms_p50", "sticky_service_ms_p99"], each.key) ? "ms" : each.key == "queued_bytes" || each.key == "heap_used_bytes" || each.key == "forwarded_bytes" ? "By" : "1"
 
     labels {
       key         = "role"