WALWrite

Mechanism

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.

Expected behavior

Brief waits occur when concurrent committers help write WAL.

When to investigate

Sustained WALWrite with commit latency usually means the WAL write path is slow or cannot absorb the generated WAL rate.

Diagnostic actions

• Compare WAL write and sync time with commit latency.
• Check the WAL filesystem/device, virtualization throttling, and checkpoint overlap.
• Reduce burstiness; tune wal_writer settings only after storage evidence confirms the path.

Incident patterns

A cloud volume hits its burst-credit ceiling; WAL writes lengthen, committers queue on WALWrite, and synchronous commits slow together.

Diagnostic queries

Sessions waiting on LWLock/WALWrite

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;

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;

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