# wal_log_hints

> wal_log_hints — Writes full pages to WAL when first modified after a checkpoint, even for a non-critical modification. Observed in PG9.4–19 Beta 3; its last measured boot default is off in PG19 Beta 3, with postmaster context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Writes full pages to WAL when first modified after a checkpoint, even for a non-critical modification.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `bool` | Upstream pg_settings type |
| Context | `postmaster` | Requires a server restart |
| Unit | — | Raw unit |
| Range | — | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Write-Ahead Log / Settings | Upstream classification |
| Latest boot value | `off` | off |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG9.4 |
| Present in | PG9.4–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`961bf59fb7a7`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=961bf59fb7a7e4fab751e20e9372de7ab37c5649) — Rename wal_log_hintbits to wal_log_hints, per discussion on pgsql-hackers. |
| Commit date | 2013-12-21 |
| Discussion | — |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG9.4–19 Beta 3 | `off` | — | off |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

Writes full pages to WAL when first modified after a checkpoint, even for a non-critical modification. The value is fixed when the server starts, so changing it requires a restart.

When data checksums are off, this forces full-page WAL for the first hint-bit change after a checkpoint, giving pg_rewind the block-change safety it needs. With checksums enabled, equivalent hint logging already occurs, so the setting adds no further effect.

Monitor and change wal_log_hints together with wal_compression, full_page_writes, wal_level. Validate on the relevant server role and real workload, then use its postmaster context to choose session change, reload, or restart; a historical boot default is not the current effective value.

## Tuning advice {#tuning-advice}

> [!TIP]
> **Advice.** These are workload-specific starting points and must be validated with measurements.

| Workload | Guidance |
| --- | --- |
| OLTP | Establish durability and recovery objectives first, then tune wal_log_hints from WAL rate, flush latency, checkpoints, and peak pg_wal usage. Validate every change with crash/recovery and archive monitoring. |
| OLAP | Provision WAL, archive bandwidth, and recovery I/O for bulk-load peaks. Coordinate load pacing and checkpoints when reducing spikes; never break the recovery chain for throughput. |
| Small nodes | Start from safe defaults or the measured Pigsty value and set explicit alerts for limited disk capacity. Do not disable durability or break the recovery chain merely to save modest I/O. |
{.full-width}

## Pigsty {#pigsty}

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG19 Beta 3; this does not assert current Pigsty support for that historical or beta release.

| Template | Effective value | Versus upstream boot | Source expression |
| --- | --- | --- | --- |
| OLTP | `on` | different | `'on'` |
| OLAP | `on` | different | `'on'` |
| CRIT | `on` | different | `'on'` |
| TINY | `on` | different | `'on'` |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG9.4–19 Beta 3 = on (dcs); OLAP: PG9.4–19 Beta 3 = on (dcs); CRIT: PG9.4–19 Beta 3 = on (dcs); TINY: PG9.4–19 Beta 3 = on (dcs). Advice, pending human review — Editorial interpretation, pending human maintainer review: the override appears intended to keep pg_rewind viable even when data checksums are not the mechanism forcing hint-bit WAL; confirm it against the current Pigsty templates, hardware fixture, and operational guarantees before publication.

## Common pitfalls {#common-pitfalls}

- Assuming it logs every hint update rather than the first page image after a checkpoint.
- Turning it on but not completing the restart before relying on pg_rewind.
- Forgetting that checksums already force the relevant hint WAL behavior.
- Confusing pg_settings base units with human-readable configuration units.
- Benchmarking throughput without a crash-recovery and archive-restore test.

## Related parameters {#related-parameters}

[`wal_compression`](/parameters/wal-compression/) · [`full_page_writes`](/parameters/full-page-writes/) · [`wal_level`](/parameters/wal-level/) · [`wal_buffers`](/parameters/wal-buffers/) · [`max_wal_senders`](/parameters/max-wal-senders/) · [`max_replication_slots`](/parameters/max-replication-slots/)

## References {#references}

- [PostgreSQL 19 Beta 3 — wal_log_hints](https://www.postgresql.org/docs/19/runtime-config-wal.html#GUC-WAL-LOG-HINTS)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)