VacuumTruncate

Mechanism

WAIT_EVENT_VACUUM_TRUNCATE at src/backend/access/heap/vacuumlazy.c:3280 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/utils/activity/wait_event_names.txt:183. The instrumented operation is: Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed. The VacuumTruncate path deliberately waits on a timer, retry interval, throttle, or safety backoff. The latch timeout expires before work is reconsidered.

Expected behavior

The wait is normal when the configured policy is intentional and useful work advances at the expected rate.

When to investigate

Investigate when the policy causes missed maintenance, recovery, backup, or latency objectives, or when a spin delay persists.

Diagnostic actions

• Identify the GUC or function that owns the timer.
• Compare productive work with time spent delayed.
• Adjust the policy only after checking the CPU, I/O, and durability pressure it was designed to control.

Diagnostic queries

Sessions waiting on Timeout/VacuumTruncate

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 = 'Timeout'
  AND wait_event = 'VacuumTruncate'
ORDER BY query_age DESC NULLS LAST;

Current Timeout 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 = 'Timeout'
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 = 'Timeout'
  AND a.wait_event = 'VacuumTruncate'
ORDER BY a.pid, l.granted, l.locktype, l.mode;