# io_max_concurrency

> io_max_concurrency — Max number of IOs that one process can execute simultaneously. Observed in PG18–19 Beta 3; its last measured boot default is -1 in PG19 Beta 3, with postmaster context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Max number of IOs that one process can execute simultaneously.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `integer` | Upstream pg_settings type |
| Context | `postmaster` | Requires a server restart |
| Unit | — | Raw unit |
| Range | `-1` – `1024` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Resource Usage / I/O | Upstream classification |
| Latest boot value | `-1` | -1 |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG18 |
| Present in | PG18–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`02844012b304`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=02844012b304ba80d1c48d51f6fe10bb622490cc) — aio: Basic subsystem initialization |
| Commit date | 2025-03-17 |
| Discussion | [thread 1](https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG18–19 Beta 3 | `-1` | — | -1 |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

io_max_concurrency is a PostgreSQL 18 server-start ceiling on the number of I/O operations that one process may execute simultaneously. It is per process, not a reservation and not a cluster-wide cap.

The default -1 asks PostgreSQL to derive a value from shared_buffers and configured process maxima, capped at 64. Because many backends and workers can each reach their own ceiling, possible cluster-wide outstanding I/O can be much larger.

effective_io_concurrency and maintenance_io_concurrency are workload targets below this ceiling; io_method chooses the execution mechanism, and combine limits control bytes per request. Changing io_max_concurrency requires a restart.

## Tuning advice {#tuning-advice}

> [!TIP]
> **Advice.** These are workload-specific starting points and must be validated with measurements.

| Workload | Guidance |
| --- | --- |
| OLTP | Leave -1 until measurements show the automatic per-process ceiling constrains eligible AIO. If setting it explicitly, multiply the value across concurrently active processes and test device queueing plus tail latency after the required restart. |
| OLAP | Raise the per-process ceiling only when one scan or maintenance process cannot keep high-IOPS storage busy and the target-concurrency settings are already appropriate. Compare outstanding-operation counts, throughput, and latency; this parameter does not change request size. |
| Small nodes | Prefer -1. An explicit high ceiling is multiplied by concurrent processes and can overwhelm a small device even though no single process exceeds its configured limit. |
{.full-width}

## Pigsty {#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 | — | — |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG18–19 Beta 3 unmodified; OLAP: PG18–19 Beta 3 unmodified; CRIT: PG18–19 Beta 3 unmodified; TINY: PG18–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Treating the per-process ceiling as a cluster-wide I/O limit.
- Reading -1 as unlimited rather than automatic sizing capped at 64.
- Calling the ceiling reserved capacity; it permits concurrency but does not preallocate I/O slots.
- Confusing operation count with io_combine_limit's bytes per request.
- Changing the value without a restart or without multiplying exposure across processes.

## Related parameters {#related-parameters}

[`io_method`](/parameters/io-method/) · [`io_workers`](/parameters/io-workers/) · [`effective_io_concurrency`](/parameters/effective-io-concurrency/) · [`maintenance_io_concurrency`](/parameters/maintenance-io-concurrency/) · [`io_combine_limit`](/parameters/io-combine-limit/) · [`max_connections`](/parameters/max-connections/)

## References {#references}

- [PostgreSQL 19 Beta 3 — io_max_concurrency](https://www.postgresql.org/docs/19/runtime-config-resource.html#GUC-IO-MAX-CONCURRENCY)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)