commit_siblings

commit_siblings — Sets the minimum number of concurrent open transactions required before performing “commit_delay”. Observed in PG9.0–19 Beta 3; its last measured boot default is 5 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: “Sets the minimum number of concurrent open transactions required before performing “commit_delay”.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valueuser: Settable by an ordinary user
Unit , Value—: Raw unit
Range , Value0 – 1000: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueWrite-Ahead Log / Settings: Upstream classification
Latest boot value , Value5: 5

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	`5`	—	5

How it works

Sets the minimum number of concurrent open transactions required before performing “commit_delay”. It can be changed at session scope, so different sessions may observe different behavior.

This is the concurrency gate for commit_delay, counted from other active transactions when a commit flush is needed. It is not a count of commits already waiting and has no useful effect while commit_delay is zero.

Monitor and change commit_siblings together with commit_delay, synchronous_commit, wal_writer_delay. Validate on the relevant server role and real workload, then use its user 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	Benchmark commit_siblings only under high commit concurrency with material WALSync waits and an acceptable p95/p99 latency budget. Tune commit_delay and commit_siblings as a pair and retain a zero-delay baseline.
OLAP	Bulk jobs should usually reduce commit frequency with sensible transaction batches; do not use group-commit delay to compensate for row-at-a-time ETL commits.
Small nodes	Low concurrency provides little opportunity, so commit_delay=0 is simplest. Even a microsecond value needs end-to-end latency evidence.

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	`10`	different	`10`
OLAP	`10`	different	`10`
CRIT	`10`	different	`10`
TINY	`10`	different	`10`

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG9.0–19 Beta 3 = 10 (dcs); OLAP: PG9.0–19 Beta 3 = 10 (dcs); CRIT: PG9.0–19 Beta 3 = 10 (dcs); TINY: PG9.0–19 Beta 3 = 10 (dcs). Advice, pending human review — Editorial interpretation, pending human maintainer review: the override appears intended to apply commit_delay only after the server has substantial concurrent transaction activity; confirm it against the current Pigsty templates, hardware fixture, and operational guarantees before publication.

Common pitfalls

Tuning it while commit_delay is zero, in which case the threshold has no effect.
Treating it as a count of commits already waiting rather than other active transactions.
Setting the gate too low and adding latency during ordinary moderate concurrency.
Applying a session-level experiment globally without comparing commit latency and WALSync behavior.

commit_delay · synchronous_commit · wal_writer_delay · wal_sync_method · fsync · full_page_writes