# LWLock: WALBufMapping

> 等待替换 WAL buffer 中的页面。
---

<div class="event-kicker">PostgreSQL 等待事件档案</div>
<div class="event-identity">
  <span>类别<strong>LWLock</strong></span>
  <span>事件<strong><code>WALBufMapping</code></strong></span>
  <span>版本<strong>PG 13-18</strong></span>
  <span>证据<strong>3 个源码位置</strong></span>
</div>

## 官方描述译文 {#official-description}

等待替换 WAL buffer 中的页面。

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

## 触发机制 {#mechanism}

WALBufMappingLock 协调 WAL 页号与有限 wal_buffers 槽位之间的映射。后端推进到新 WAL 页或替换缓冲页时需要取得此锁。

## 正常还是麻烦？ {#normal-vs-trouble}

- <span class="severity-normal">**正常:**</span> WAL 生成推进并跨越缓冲页时会短暂出现。
- <span class="severity-watch">**需要调查:**</span> 持续竞争说明 WAL 生成换页速度超过写路径推进速度，常见于写突发或检查点期间。

## 诊断 SQL {#diagnostic-sql}

```sql {title="正在等待 LWLock/WALBufMapping 的会话"}
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="当前 LWLock 等待分布"}
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="这些会话的锁与关系上下文"}
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;
```

## 处置建议 {#response}

1. 关联 WAL 字节量、WAL 写入与检查点时间。
2. 检查写突发期间 wal_buffers 是否反复耗尽。
3. 先平滑写突发并解决 WAL 设备延迟，再调整缓冲区大小。

## 源码证据 {#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`

## 关联控制项与信号 {#related}

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

## 典型事故模式 {#incident}

批量导入把 WAL 生成推到极限，同时 WAL 设备发生停顿，导致频繁的 WAL 缓冲页重映射。

---

反链：

- [LWLock](/zh/lwlock/)