LWLock/WALWrite

各版本描述

相邻版本里类型、名称与英文描述都相同的合并成一段，旧的在前。中文取自图谱或本站手册译文，没有译文的只给英文原文。

1. 9.6

   LWLockNamed/WALWriteLock曾用名

   等待WAL缓冲区写入磁盘。

   Waiting for WAL buffers to be written to disk.

2. 10–12

   LWLock/WALWriteLock曾用名

   等待WAL缓冲区写入磁盘。

   Waiting for WAL buffers to be written to disk.

3. 13–20

   LWLock/WALWrite

   等待WAL缓冲区写入磁盘。

   Waiting for WAL buffers to be written to disk.

触发机制

WALWriteLock 串行化共享 WAL 缓冲区写入，以及 written/flushed 位置的推进。等待者正在当前执行或协调这项工作的后端之后排队。

WALWriteLock serializes progress that writes shared WAL buffers and advances the written/flushed WAL positions. A waiter is queued behind the backend currently performing or coordinating that work.

诊断 SQL

直接在 psql 里跑；标注的是语句里用到的视图与列所要求的最低版本。

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 = 'WALWrite'
ORDER BY query_age DESC NULLS LAST;

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;

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 = 'WALWrite'
ORDER BY a.pid, l.granted, l.locktype, l.mode;

处置步骤

1. 把 WAL write/sync 时间与提交延迟对照。
2. 检查 WAL 文件系统/设备、虚拟化限速与检查点重叠。
3. 降低突发性；只有存储证据确认后才调整 wal_writer 参数。

相关参数与指标

wal_writer_delaywal_writer_flush_aftersynchronous_commit waiting_sessionswait_event_sharewal_write_timewal_sync_timecommit_latency

源码证据

图谱在各版本源码树里核对到的位置，新的在前。「触发点」是真正上报等待事件的调用处，「目录定义」只说明这个名字在枚举里存在。

演化历史

相邻两个大版本之间的差异，新的在前。版本号链到该版的变动清单。

1. PostgreSQL 13 ← 12 更名

   更名 WALWriteLock→WALWrite

   等待WAL缓冲区写入磁盘。

2. PostgreSQL 10 ← 9.6 类型变动

   类型 LWLockNamed→LWLock

   等待WAL缓冲区写入磁盘。

3. PostgreSQL 9.6 新增此事件

   9.6 引入等待事件机制，此为本事件的首个版本

   等待WAL缓冲区写入磁盘。

同类事件