bgwriter_lru_multiplier

bgwriter_lru_multiplier — Multiple of the average buffer usage to free per round. Observed in PG9.0–19 Beta 3; its last measured boot default is 2 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: “Multiple of the average buffer usage to free per round.”

Identity

Type , Valuereal: Upstream pg_settings type
Context , Valuesighup: Takes effect after configuration reload
Unit , Value—: Raw unit
Range , Value0 – 10: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueResource Usage / Background Writer: Upstream classification
Latest boot value , Value2: 2

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

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG9.0–19 Beta 3	`2`	—	2

How it works

The background writer averages recent buffer allocations and multiplies that demand by bgwriter_lru_multiplier to choose a target number of clean reusable buffers. A value above 1 adds cushion for bursts.

This is a forecast multiplier, not a direct number of pages. Actual writes remain capped by bgwriter_lru_maxpages and occur once per bgwriter_delay round.

More cushion can reduce foreground backend writes and latency spikes, but pages dirtied repeatedly between checkpoints may be written more times, increasing total I/O. Its SIGHUP context allows configuration reload without a server restart.

Tuning advice

Tip

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

Workload	Guidance
OLTP	Tune bgwriter_lru_multiplier only with bgwriter and checkpoint metrics. The goal is fewer backend writes and smoother latency without excessive repeated writes; change one dimension at a time.
OLAP	Bulk writes can reach bgwriter_lru_multiplier limits continuously. Measure total bytes written, checkpoints, and storage queueing, not just foreground query latency.
Small nodes	A small host usually needs conservative write smoothing. Aggressive bgwriter_lru_multiplier can consume I/O needed by foreground work, so retain the default unless backend writes are a measured problem.

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	`5.0`	different	`5.0`
OLAP	`5.0`	different	`5.0`
CRIT	`5.0`	different	`5.0`
TINY	`5.0`	different	`5.0`

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG9.0–19 Beta 3 = 5.0 (dcs); OLAP: PG9.0–19 Beta 3 = 5.0 (dcs); CRIT: PG9.0–19 Beta 3 = 5.0 (dcs); TINY: PG9.0–19 Beta 3 = 5.0 (dcs). Advice, pending human review — Editorial inference, pending maintainer review: The larger forecast cushion is intended to absorb bursts in buffer demand, accepting possible extra background writes.

Common pitfalls

Changing bgwriter_lru_multiplier 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_lru_maxpages · bgwriter_delay · bgwriter_flush_after · shared_buffers · checkpoint_completion_target