# fsync

> fsync — Forces synchronization of updates to disk. Observed in PG9.0–19 Beta 3; its last measured boot default is on 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:** “Forces synchronization of updates to disk.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `bool` | Upstream pg_settings type |
| Context | `sighup` | Takes effect after configuration reload |
| 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 | `on` | on |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG9.0 (research boundary) |
| Present in | PG9.0–19 Beta 3 |
| Removed in | No |
| Introduction commit | Not asserted: predates the PG9.0 research boundary |
| Commit date | — |
| Discussion | — |

## Default history {#default-history}

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

## How it works {#mechanism}

Forces synchronization of updates to disk. A configuration reload applies a new value; existing work already in flight is not retroactively changed.

When on, PostgreSQL issues durability barriers so WAL and data ordering survive an operating-system or power failure. Turning it off may improve write benchmarks, but a crash can leave corruption that crash recovery cannot repair; re-enabling it does not retroactively synchronize earlier unsafe writes.

Monitor and change fsync together with data_sync_retry, restart_after_crash, recovery_init_sync_method. 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 | Keep fsync=on for persistent production data. Address storage latency, checkpoints, WAL compression, and batching instead of exchanging crash safety for throughput. |
| OLAP | Bulk loads still need recoverability. Use explicitly rebuildable UNLOGGED/temporary staging to narrow the durability scope rather than disabling protection cluster-wide. |
| Small nodes | Keep it on for persistent small instances too. Only a clearly isolated, disposable cluster may make an exception after explicitly accepting rebuild risk. |
{.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: PG9.0–19 Beta 3 unmodified; OLAP: PG9.0–19 Beta 3 unmodified; CRIT: PG9.0–19 Beta 3 unmodified; TINY: PG9.0–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Disabling it on persistent data and assuming UPS or RAID cache alone is sufficient.
- Re-enabling it after unsafe operation and assuming earlier writes became durable.
- Benchmarking only clean shutdowns instead of power-loss recovery.
- Confusing pg_settings base units with human-readable configuration units.
- Benchmarking throughput without a crash-recovery and archive-restore test.

## Related parameters {#related-parameters}

[`data_sync_retry`](/parameters/data-sync-retry/) · [`restart_after_crash`](/parameters/restart-after-crash/) · [`recovery_init_sync_method`](/parameters/recovery-init-sync-method/) · [`full_page_writes`](/parameters/full-page-writes/) · [`wal_sync_method`](/parameters/wal-sync-method/) · [`synchronous_commit`](/parameters/synchronous-commit/)

## References {#references}

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