# IO: LogicalRewriteWrite

> Waiting for a write of logical rewrite mappings
---

<div class="event-kicker">PostgreSQL wait event dossier</div>
<div class="event-identity">
  <span>Class<strong>IO</strong></span>
  <span>Event<strong><code>LogicalRewriteWrite</code></strong></span>
  <span>Versions<strong>PG 13-18</strong></span>
  <span>Evidence<strong>2 source location(s)</strong></span>
</div>

## Official description {#official-description}

Waiting for a write of logical rewrite mappings

| PG 13 | PG 14 | PG 15 | PG 16 | PG 17 | PG 18 |
| :---: | :---: | :---: | :---: | :---: | :---: |
| ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |

## Trigger mechanism {#mechanism}

`WAIT_EVENT_LOGICAL_REWRITE_WRITE` at `src/backend/access/heap/rewriteheap.c:881` is the grep-verified reporting path. The public identity/resource is mapped at `src/backend/utils/activity/wait_event_names.txt:235`. The instrumented operation is: Waiting for a write of logical rewrite mappings. PostgreSQL reports LogicalRewriteWrite 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-vs-trouble}

- <span class="severity-normal">**Normal:**</span> The wait is normal when the workload is expected to perform this read, write, sync, allocation, or completion operation at the observed rate.
- <span class="severity-watch">**Investigate:**</span> Investigate when it persists with foreground latency, many concurrent waiters, storage tail latency, throttling, or errors.

## Diagnostic SQL {#diagnostic-sql}

```sql {title="Sessions waiting on IO/LogicalRewriteWrite"}
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 = 'LogicalRewriteWrite'
ORDER BY query_age DESC NULLS LAST;
```

```sql {title="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;
```

```sql {title="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 = 'LogicalRewriteWrite'
ORDER BY a.pid, l.granted, l.locktype, l.mode;
```

## Response {#response}

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

## Source evidence {#source}

- [PostgreSQL 18.6 · src/backend/access/heap/rewriteheap.c:881](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/access/heap/rewriteheap.c#L881) — `WAIT_EVENT_LOGICAL_REWRITE_WRITE`
- [PostgreSQL 18.6 · src/backend/utils/activity/wait_event_names.txt:235](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/utils/activity/wait_event_names.txt#L235) — `LOGICAL_REWRITE_WRITE`

## Related controls and signals {#related}

- **GUCs:** None directly.
- **Metrics:** [[metric:waiting_sessions]](/metric/waiting-sessions/) · [[metric:wait_event_share]](/metric/wait-event-share/) · [[metric:io_read_time]](/metric/io-read-time/) · [[metric:io_write_time]](/metric/io-write-time/)