DataFileTruncate

Mechanism

WAIT_EVENT_DATA_FILE_TRUNCATE at src/backend/storage/smgr/md.c:1329 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/utils/activity/wait_event_names.txt:219. The instrumented operation is: Waiting for a relation data file to be truncated. PostgreSQL reports DataFileTruncate around the instrumented file or asynchronous-I/O operation named by this event. The backend resumes after the kernel, storage stack, or I/O worker completes that step.

Expected behavior

The wait is normal when the workload is expected to perform this read, write, sync, allocation, or completion operation at the observed rate.

When to investigate

Investigate when it persists with foreground latency, many concurrent waiters, storage tail latency, throttling, or errors.

Diagnostic actions

• Confirm the workload phase should touch this file class.
• Correlate with pg_stat_io where available and per-device latency.
• Fix the access path, burst shape, or affected storage tier before tuning unrelated memory settings.

Diagnostic queries

Sessions waiting on IO/DataFileTruncate

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 = 'IO'
  AND wait_event = 'DataFileTruncate'
ORDER BY query_age DESC NULLS LAST;

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