# file_extend_method

> file_extend_method — Selects the method used for extending data files. Observed in PG16–19 Beta 3; its last measured boot default is posix_fallocate 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:** “Selects the method used for extending data files.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `enum` | Upstream pg_settings type |
| Context | `sighup` | Takes effect after configuration reload |
| Unit | — | Raw unit |
| Range | — | Raw limits in the last observed version |
| Enum values | `posix_fallocate`, `write_zeros` | — for non-enum types |
| Category | Resource Usage / Disk | Upstream classification |
| Latest boot value | `posix_fallocate` | posix_fallocate |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG16 |
| Present in | PG16–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`e37b59802846`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=e37b598028469d260650e5e1671c8275d30e22b6) — Add file_extend_method=posix_fallocate,write_zeros. |
| Commit date | 2025-05-31 |
| Discussion | [thread 1](https://postgr.es/m/b1843124-fd22-e279-a31f-252dffb6fbf2%40gmx.net) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG16–19 Beta 3 | `posix_fallocate` | — | posix_fallocate |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

file_extend_method chooses how PostgreSQL grows relation files during bulk extension: posix_fallocate when available or explicit zero writes. Extensions of eight blocks or fewer still use zero writes.

The first supported method is platform dependent. posix_fallocate reserves space without writing every block, but unsupported file systems silently fall back; on current BTRFS it can disable compression for the file.

This affects allocation behavior and bulk-write latency, not WAL durability. Storage reservations, sparse-file behavior, compression, and copy-on-write semantics vary by file system. Its SIGHUP context allows configuration reload without a server restart.

## Tuning advice {#tuning-advice}

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

| Workload | Guidance |
| --- | --- |
| OLTP | Keep file_extend_method at the detected/default value unless operating-system support and a controlled benchmark justify a change. Validate startup and recovery on the exact kernel and file system. |
| OLAP | Large allocations or bulk I/O can make file_extend_method visible, but platform capability is the first gate. Benchmark on production-equivalent storage and include failure/fallback behavior. |
| Small nodes | Avoid nondefault file_extend_method on a small or heterogeneous fleet unless it solves a verified platform issue. Portability and reliable startup usually outweigh a speculative gain. |
{.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: PG16–19 Beta 3 unmodified; OLAP: PG16–19 Beta 3 unmodified; CRIT: PG16–19 Beta 3 unmodified; TINY: PG16–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Changing file_extend_method without applying its documented unit and configuration context.
- Optimizing an isolated benchmark while ignoring concurrent aggregate resource use.
- Assuming a configured value guarantees operating-system or storage behavior.
- Failing to retest startup, failover, and workload latency after the change.

## Related parameters {#related-parameters}

[`file_copy_method`](/parameters/file-copy-method/) · [`backend_flush_after`](/parameters/backend-flush-after/) · [`checkpoint_flush_after`](/parameters/checkpoint-flush-after/) · [`wal_sync_method`](/parameters/wal-sync-method/)

## References {#references}

- [PostgreSQL 19 Beta 3 — file_extend_method](https://www.postgresql.org/docs/19/runtime-config-resource.html#GUC-FILE-EXTEND-METHOD)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)