IoWorkerMain

Mechanism

WAIT_EVENT_IO_WORKER_MAIN at src/backend/storage/aio/method_worker.c:573 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/utils/activity/wait_event_names.txt:60. The instrumented operation is: Waiting in main loop of IO Worker process. The IoWorkerMain background path reports this event around its latch wait when its main loop has no immediate work. A signal, timeout, or new work clears the sleep and the process loops again.

Expected behavior

This is normally expected idleness for the matching background process.

When to investigate

Investigate only when queued work or a shutdown/failover deadline is not advancing while the process remains here, or when the event appears on an unexpected backend type.

Diagnostic actions

• Match the row to backend_type.
• Inspect the process-specific backlog and last-progress timestamp.
• Fix the missing work signal or stalled upstream phase; do not kill a normally idle worker.

Diagnostic queries

Sessions waiting on Activity/IoWorkerMain

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 = 'Activity'
  AND wait_event = 'IoWorkerMain'
ORDER BY query_age DESC NULLS LAST;

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