wal_writer_flush_after

wal_writer_flush_after — Amount of WAL written out by WAL writer that triggers a flush. Observed in PG9.6–19 Beta 3; its last measured boot default is 1 MiB (128 × 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: “Amount of WAL written out by WAL writer that triggers a flush.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valuesighup: Takes effect after configuration reload
Unit , Value8kB: Raw unit
Range , Value0 – 2147483647: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueWrite-Ahead Log / Settings: Upstream classification
Latest boot value , Value128: 1 MiB (128 × 8kB)

Lifecycle

Fact	Value
First observed	PG9.6
Present in	PG9.6–19 Beta 3
Removed in	No
Introduction commit	`7975c5e0a992` — Allow the WAL writer to flush WAL at a reduced rate.
Commit date	2016-02-15
Discussion	—

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG9.6–19 Beta 3	`128`	`8kB`	1 MiB (128 × 8kB)

How it works

Amount of WAL written out by WAL writer that triggers a flush. A configuration reload applies a new value; existing work already in flight is not retroactively changed.

After this volume since its last flush, the WAL writer forces written WAL to durable storage; otherwise it may only write to the OS. Zero flushes immediately, while larger values trade background smoothing against a bigger dirty-WAL window for asynchronous commits.

Monitor and change wal_writer_flush_after together with wal_buffers, wal_writer_delay, wal_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

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_writer_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.

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	`1MB`	same as boot	`1MB`
OLAP	`1MB`	same as boot	`1MB`
CRIT	`0`	different	`0`
TINY	`1MB`	same as boot	`1MB`

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG9.6–19 Beta 3 = 1MB (dcs); OLAP: PG9.6–19 Beta 3 = 1MB (dcs); CRIT: PG9.6–19 Beta 3 = 0 (dcs); TINY: PG9.6–19 Beta 3 = 1MB (dcs). Advice, pending human review — Editorial interpretation, pending human maintainer review: the override appears intended to flush WAL writer output in 1 MiB batches, while CRIT requests immediate flushes; confirm it against the current Pigsty templates, hardware fixture, and operational guarantees before publication.

Common pitfalls

Reading an unqualified value as bytes rather than WAL blocks.
Forgetting that zero requests an immediate flush from the WAL writer.
Choosing a large batch without considering asynchronous-commit loss exposure and dirty WAL.
Treating the setting as a replacement for synchronous commit flushes.
Tuning it without wal_writer_delay and storage flush latency.

wal_buffers · wal_writer_delay · wal_sync_method · synchronous_commit · commit_delay · commit_siblings