# LWLock: WALInsert

> 等待将 WAL 数据插入内存 buffer。
---

<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>WALInsert</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}

等待将 WAL 数据插入内存 buffer。

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

## 触发机制 {#mechanism}

后端在共享 WAL 缓冲区中预留空间并复制 WAL 记录时，必须持有一个 WAL insertion lock。并发 WAL 生产者很多、插入临界区变长时，会在此排队。

## 正常还是麻烦？ {#normal-vs-trouble}

- <span class="severity-normal">**正常:**</span> 并发写入与提交突发期间的瞬时等待很正常。
- <span class="severity-watch">**需要调查:**</span> 前台会话反复堆积说明 WAL 插入已成为串行点，常伴随极高写并发、全页镜像或缓冲区换页过慢。

## 诊断 SQL {#diagnostic-sql}

```sql {title="正在等待 LWLock/WALInsert 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 = 'WALInsert'
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 = 'WALInsert'
ORDER BY a.pid, l.granted, l.locktype, l.mode;
```

## 处置建议 {#response}

1. 按 WAL 生成量与调用频率排序语句。
2. 检查检查点/全页镜像时机与并发写会话数量。
3. 批量合并微小写入、减少无效索引抖动，并处理 WAL 写延迟。

## 源码证据 {#source}

- [PostgreSQL 18.6 · src/backend/access/transam/xlog.c:1399](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/access/transam/xlog.c#L1399) — `WALInsert`
- [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:372](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/utils/activity/wait_event_names.txt#L372) — `WALInsert`

## 关联控制项与信号 {#related}

- **GUC:** [[guc:wal_buffers]](/zh/guc/wal-buffers/) · [[guc:full_page_writes]](/zh/guc/full-page-writes/) · [[guc:checkpoint_timeout]](/zh/guc/checkpoint-timeout/)
- **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_fpi]](/zh/metric/wal-fpi/) · [[metric:statement_wal_bytes]](/zh/metric/statement-wal-bytes/)

## 典型事故模式 {#incident}

扇出任务在检查点后立即发起大量单行提交，全页镜像与写并发共同让 WAL 插入成为瓶颈。

---

反链：

- [LWLock](/zh/lwlock/)