# LWLock: BufferMapping

> Waiting to associate a data block with a buffer in the buffer pool
---

<div class="event-kicker">PostgreSQL wait event dossier</div>
<div class="event-identity">
  <span>Class<strong>LWLock</strong></span>
  <span>Event<strong><code>BufferMapping</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 associate a data block with a buffer in the buffer pool

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

## Trigger mechanism {#mechanism}

The buffer manager hashes a relation/fork/block tag into a partition protected by BufferMappingLock. Lookup, insertion, eviction, and tag reassignment briefly take that partition lock; broad concurrent misses or buffer churn increase collisions.

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

- <span class="severity-normal">**Normal:**</span> Brief waits occur when pages enter or leave shared buffers.
- <span class="severity-watch">**Investigate:**</span> Sustained recurrence suggests a working set that churns through shared buffers, many parallel scans, or concentrated access mapping into a few partitions.

## Diagnostic SQL {#diagnostic-sql}

```sql {title="Sessions waiting on LWLock/BufferMapping"}
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 = 'BufferMapping'
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 = 'BufferMapping'
ORDER BY a.pid, l.granted, l.locktype, l.mode;
```

## Response {#response}

1. Correlate with buffer hit ratio and read volume by database and statement.
2. Look for a new scan, undersized cache, or concurrency jump.
3. Reduce concurrent scan fan-out or fix the access path before simply enlarging shared_buffers.

## Source evidence {#source}

- [PostgreSQL 18.6 · src/backend/storage/lmgr/lwlock.c:152](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/storage/lmgr/lwlock.c#L152) — `BufferMapping`
- [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:377](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/utils/activity/wait_event_names.txt#L377) — `BufferMapping`

## Related controls and signals {#related}

- **GUCs:** [[guc:shared_buffers]](/guc/shared-buffers/) · [[guc:max_parallel_workers_per_gather]](/guc/max-parallel-workers-per-gather/)
- **Metrics:** [[metric:waiting_sessions]](/metric/waiting-sessions/) · [[metric:wait_event_share]](/metric/wait-event-share/) · [[metric:buffer_hit_ratio]](/metric/buffer-hit-ratio/) · [[metric:blocks_read]](/metric/blocks-read/)

## Typical incident pattern {#incident}

A plan regression launches many concurrent large scans; buffer-table partitions become hot while useful pages churn out of cache.

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Backlinks:

- [LWLock](/lwlock/)