# checkpoint_flush_after

> checkpoint_flush_after — Number of pages after which previously performed writes are flushed to disk. Observed in PG9.6–19 Beta 3; its last measured boot default is 256 KiB (32 × 8kB) in PG19 Beta 3, with sighup context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Number of pages after which previously performed writes are flushed to disk.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `integer` | Upstream pg_settings type |
| Context | `sighup` | Takes effect after configuration reload |
| Unit | `8kB` | Raw unit |
| Range | `0` – `256` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Write-Ahead Log / Checkpoints | Upstream classification |
| Latest boot value | `32` | 256 KiB (32 × 8kB) |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG9.6 |
| Present in | PG9.6–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`428b1d6b29ca`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=428b1d6b29ca599c5700d4bc4f4ce4c5880369bf) — Allow to trigger kernel writeback after a configurable number of writes. |
| Commit date | 2016-02-19 |
| Discussion | — |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG9.6–19 Beta 3 | `32` | `8kB` | 256 KiB (32 × 8kB) |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

Number of pages after which previously performed writes are flushed to disk. A configuration reload applies a new value; existing work already in flight is not retroactively changed.

Checkpoint writes are periodically handed to the operating system for writeback after this much data, smoothing the final sync and reducing dirty-cache bursts. Zero disables these intermediate flush requests; the best value is operating-system and storage dependent.

Monitor and change checkpoint_flush_after together with checkpoint_warning, checkpoint_timeout, checkpoint_completion_target. Validate on the relevant server role and real workload, then use its sighup 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 checkpoint_flush_after 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 | Unmodified | — | — |
| OLAP | Unmodified | — | — |
| CRIT | Unmodified | — | — |
| TINY | Unmodified | — | — |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG9.6–19 Beta 3 unmodified; OLAP: PG9.6–19 Beta 3 unmodified; CRIT: PG9.6–19 Beta 3 unmodified; TINY: PG9.6–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Treating the raw value as bytes even though an unqualified value is measured in database blocks.
- Setting it too small and increasing writeback calls and I/O fragmentation.
- Setting it too large and losing the intended smoothing before the checkpoint sync phase.
- Confusing writeback hints with a durability guarantee; fsync and wal_sync_method still define persistence.
- Copying a value across operating systems or filesystems without measuring checkpoint latency and dirty-page behavior.

## Related parameters {#related-parameters}

[`checkpoint_warning`](/parameters/checkpoint-warning/) · [`checkpoint_timeout`](/parameters/checkpoint-timeout/) · [`checkpoint_completion_target`](/parameters/checkpoint-completion-target/) · [`max_wal_size`](/parameters/max-wal-size/) · [`min_wal_size`](/parameters/min-wal-size/) · [`archive_cleanup_command`](/parameters/archive-cleanup-command/)

## References {#references}

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