IO: AioIoCompletion
Waiting for another process to complete IO
PostgreSQL wait event dossier
ClassIO
Event
AioIoCompletion
VersionsPG 18
Evidence2 source location(s)
Official description
Waiting for another process to complete IO
| PG 13 | PG 14 | PG 15 | PG 16 | PG 17 | PG 18 |
|---|---|---|---|---|---|
| — | — | — | — | — | ✓ |
Trigger mechanism
WAIT_EVENT_AIO_IO_COMPLETION at src/backend/storage/aio/aio.c:643 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/utils/activity/wait_event_names.txt:196. The instrumented operation is: Waiting for another process to complete IO. PostgreSQL reports AioIoCompletion 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.
Normal or trouble?
- Normal: The wait is normal when the workload is expected to perform this read, write, sync, allocation, or completion operation at the observed rate.
- Investigate: Investigate when it persists with foreground latency, many concurrent waiters, storage tail latency, throttling, or errors.
Diagnostic SQL
Sessions waiting on IO/AioIoCompletion
Current IO cohort
Lock and relation context for these sessions
Response
- 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.
Source evidence
- PostgreSQL 18.6 · src/backend/storage/aio/aio.c:643 —
WAIT_EVENT_AIO_IO_COMPLETION - PostgreSQL 18.6 · src/backend/utils/activity/wait_event_names.txt:196 —
AIO_IO_COMPLETION
Related controls and signals
- GUCs: None directly.
- Metrics: [metric:waiting_sessions] · [metric:wait_event_share] · [metric:io_read_time] · [metric:io_write_time]