backend_flush_after

backend_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 0 B (0 × 8kB) in PG19 Beta 3, with user 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

Type , Valueinteger: Upstream pg_settings type
Context , Valueuser: Settable by an ordinary user
Unit , Value8kB: Raw unit
Range , Value0 – 256: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueResource Usage / I/O: Upstream classification
Latest boot value , Value0: 0 B (0 × 8kB)

Lifecycle

Fact	Value
First observed	PG9.6
Present in	PG9.6–19 Beta 3
Removed in	No
Introduction commit	`428b1d6b29ca` — Allow to trigger kernel writeback after a configurable number of writes.
Commit date	2016-02-19
Discussion	—

Default history

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

How it works

After one backend has written more than backend_flush_after bytes, PostgreSQL asks the operating system to begin writing those dirty page-cache pages toward storage. Zero disables these writeback hints.

This is not fsync and does not make a transaction durable earlier. Its aim is to limit large dirty-page bursts and later stalls; support and effect depend on the operating system.

The threshold applies independently to each backend, so concurrent bulk writers can each generate writeback. It complements bgwriter_flush_after and checkpoint_flush_after, which cover different writer processes. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value.

Tuning advice

Tip

Advice. These are workload-specific starting points and must be validated with measurements.

Workload	Guidance
OLTP	Change backend_flush_after only after identifying the corresponding resource bottleneck under concurrency. Budget total memory, I/O, disk, or kernel capacity rather than optimizing one process in isolation.
OLAP	Benchmark backend_flush_after with representative bulk and scan phases. Include sustained throughput, spill/writeback, and interference with other sessions, not only one operation’s elapsed time.
Small nodes	Keep backend_flush_after conservative on a small host and prefer the upstream default when evidence is weak. A setting copied from a large server can consume a disproportionate share of resources.

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	—	—

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

Changing backend_flush_after 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.

bgwriter_flush_after · checkpoint_flush_after · bgwriter_delay · shared_buffers · track_io_timing