# LWLock: WALBufMapping

> Waiting to replace a page in WAL buffers
---

<div class="event-kicker">PostgreSQL wait event dossier</div>
<div class="event-identity">
  <span>Class<strong>LWLock</strong></span>
  <span>Event<strong><code>WALBufMapping</code></strong></span>
  <span>Versions<strong>PG 13-18</strong></span>
  <span>Evidence<strong>3 source location(s)</strong></span>
</div>

## Official description {#official-description}

Waiting to replace a page in WAL buffers

| PG 13 | PG 14 | PG 15 | PG 16 | PG 17 | PG 18 |
| :---: | :---: | :---: | :---: | :---: | :---: |
| ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |

## Trigger mechanism {#mechanism}

WALBufMappingLock coordinates the mapping between WAL page numbers and the finite wal_buffers slots. A backend needs it when advancing to or replacing a WAL buffer page.

## Normal or trouble? {#normal-vs-trouble}

- <span class="severity-normal">**Normal:**</span> Short waits appear as WAL generation advances through buffer pages.
- <span class="severity-watch">**Investigate:**</span> Sustained contention suggests WAL generation is turning over buffers faster than the write path advances, often during write bursts or checkpoints.

## Diagnostic SQL {#diagnostic-sql}

```sql {title="Sessions waiting on LWLock/WALBufMapping"}
SELECT pid, backend_type, usename, datname, application_name,
       state, now() - query_start AS query_age,
       now() - xact_start AS xact_age,
       wait_event_type, wait_event,
       pg_blocking_pids(pid) AS blocking_pids,
       left(query, 160) AS query
FROM pg_stat_activity
WHERE wait_event_type = 'LWLock'
  AND wait_event = 'WALBufMapping'
ORDER BY query_age DESC NULLS LAST;
```

```sql {title="Current LWLock cohort"}
SELECT wait_event, count(*) AS waiting_sessions,
       count(*) FILTER (WHERE state = 'active') AS active_waiters,
       max(now() - query_start) AS oldest_query
FROM pg_stat_activity
WHERE wait_event_type = 'LWLock'
GROUP BY wait_event
ORDER BY waiting_sessions DESC, wait_event;
```

```sql {title="Lock and relation context for these sessions"}
SELECT a.pid, l.locktype, l.mode, l.granted, l.fastpath,
       d.datname, n.nspname, c.relname,
       l.page, l.tuple, l.virtualxid, l.transactionid,
       l.classid, l.objid, l.objsubid
FROM pg_stat_activity AS a
LEFT JOIN pg_locks AS l ON l.pid = a.pid
LEFT JOIN pg_database AS d ON d.oid = l.database
LEFT JOIN pg_class AS c
  ON c.oid = l.relation
 AND l.database = (
       SELECT oid FROM pg_database WHERE datname = current_database()
     )
LEFT JOIN pg_namespace AS n ON n.oid = c.relnamespace
WHERE a.wait_event_type = 'LWLock'
  AND a.wait_event = 'WALBufMapping'
ORDER BY a.pid, l.granted, l.locktype, l.mode;
```

## Response {#response}

1. Correlate with WAL bytes, WAL writes, and checkpoint timing.
2. Check whether wal_buffers is repeatedly exhausted during bursts.
3. Smooth write bursts and fix WAL device latency before tuning buffer size.

## Source evidence {#source}

- [PostgreSQL 18.6 · src/backend/access/transam/xlog.c:1999](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/access/transam/xlog.c#L1999) — `WALBufMapping`
- [PostgreSQL 18.6 · src/backend/storage/lmgr/lwlock.c:739](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/storage/lmgr/lwlock.c#L739) — `pgstat_report_wait_start`
- [PostgreSQL 18.6 · src/backend/utils/activity/wait_event_names.txt:319](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/utils/activity/wait_event_names.txt#L319) — `WALBufMapping`

## Related controls and signals {#related}

- **GUCs:** [[guc:wal_buffers]](/guc/wal-buffers/) · [[guc:checkpoint_timeout]](/guc/checkpoint-timeout/) · [[guc:max_wal_size]](/guc/max-wal-size/)
- **Metrics:** [[metric:waiting_sessions]](/metric/waiting-sessions/) · [[metric:wait_event_share]](/metric/wait-event-share/) · [[metric:wal_bytes]](/metric/wal-bytes/) · [[metric:wal_write_time]](/metric/wal-write-time/)

## Typical incident pattern {#incident}

A bulk load saturates WAL generation while the WAL device stalls, forcing frequent WAL buffer remapping.

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Backlinks:

- [LWLock](/lwlock/)