io_workers

io_workers — Number of IO worker processes, for io_method=worker. Observed in PG18; its last measured boot default is 3 in PG18, with sighup context. It was removed in PG19 Beta 3.

Note

Fact — official short description: “Number of IO worker processes, for io_method=worker.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valuesighup: Takes effect after configuration reload
Unit , Value—: Raw unit
Range , Value1 – 32: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueResource Usage / I/O: Upstream classification
Latest boot value , Value3: 3

Lifecycle

Fact	Value
First observed	PG18
Present in	PG18
Removed in	PG19 Beta 3
Introduction commit	`55b454d0e140` — aio: Infrastructure for io_method=worker
Commit date	2025-03-18
Discussion	thread 1 · thread 2 · thread 3

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG18	`3`	—	3

How it works

io_workers sets the number of dedicated I/O worker processes used by PostgreSQL 18 when io_method=worker. It has no effect for io_uring or sync.

I/O workers execute asynchronous requests on behalf of database processes; this is an execution pool, distinct from parallel query workers and background worker slots.

Changing the count is reloadable, but useful capacity still depends on io_max_concurrency, workload queue depth, storage latency, and CPU. More workers do not guarantee more device throughput. 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 io_workers only when io_method=worker. Start from the PG18 default 3, measure worker saturation, CPU use, and tail latency, and increase the pool only when queued eligible I/O waits for worker execution rather than for the device.
OLAP	Sustained eligible scans can benefit from enough I/O workers to feed storage, but extra processes add scheduling overhead and cannot exceed the per-process or device concurrency bottleneck. Compare throughput and worker utilization; request size is controlled elsewhere.
Small nodes	Keep 3 or a measured smaller value when process and CPU budgets are tight. io_workers has no effect under io_uring or sync, so never spend tuning effort on it until io_method is confirmed as worker.

Pigsty

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG18; this does not assert current Pigsty support for that historical or beta release.

Template	Effective value	Versus upstream boot	Source expression
OLTP	`4`	different	`{{ pg_io_workers }}`
OLAP	`4`	different	`{{ pg_io_workers }}`
CRIT	`4`	different	`{{ pg_io_workers }}`
TINY	`3`	same as boot	`{{ pg_io_workers }}`

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG18 = 4 (dcs); OLAP: PG18 = 4 (dcs); CRIT: PG18 = 4 (dcs); TINY: PG18 = 3 (dcs). Advice, pending human review — Editorial inference, pending maintainer review: The resolved worker pool gives PG18’s worker AIO method more executors on non-tiny profiles while keeping the tiny profile at the upstream boot count.

Common pitfalls

Changing io_workers while io_method is io_uring or sync, where it has no effect.
Treating worker-process count as bytes per request or as the per-process I/O ceiling.
Adding workers when the storage device, not the worker pool, is already saturated.
Forgetting that the reloadable pool still consumes process slots, CPU, and scheduling capacity.

io_method · io_max_concurrency · effective_io_concurrency · maintenance_io_concurrency · max_worker_processes