# io_combine_limit

> io_combine_limit — Limit on the size of data reads and writes. Observed in PG17–19 Beta 3; its last measured boot default is 128 KiB (16 × 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:** “Limit on the size of data reads and writes.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `integer` | Upstream pg_settings type |
| Context | `user` | Settable by an ordinary user |
| Unit | `8kB` | Raw unit |
| Range | `1` – `128` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Resource Usage / I/O | Upstream classification |
| Latest boot value | `16` | 128 KiB (16 × 8kB) |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG17 |
| Present in | PG17–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`210622c60e1a`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=210622c60e1a9db2e2730140b8106ab57d259d15) — Provide vectored variant of ReadBuffer(). |
| Commit date | 2024-04-03 |
| Discussion | [thread 1](https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG17–19 Beta 3 | `16` | `8kB` | 128 KiB (16 × 8kB) |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

io_combine_limit limits the byte size of one I/O request formed by combining adjacent eligible operations. It controls bytes per request, not the number of requests that may execute concurrently.

In PostgreSQL 17 it is a standalone user-settable combine-size limit with a measured Linux-Docker boot value of 128kB; io_max_combine_limit does not exist in that release. In PostgreSQL 18, the effective size is the lower of io_combine_limit and the new server-start io_max_combine_limit.

Actual requests can be smaller when adjacent work is unavailable, and operating-system plus BLCKSZ constraints bound the feasible maximum. Its user context permits session- or transaction-local changes; changing it does not alter io_max_concurrency or worker count.

## Tuning advice {#tuning-advice}

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

| Workload | Guidance |
| --- | --- |
| OLTP | Keep the version-appropriate default until a benchmark shows that combined-request size, rather than queue depth, is a bottleneck. In PG18, verify both io_combine_limit and io_max_combine_limit, and measure tail latency under realistic concurrency. |
| OLAP | Larger combined requests can reduce syscall overhead during eligible sequential work, but may increase service time and reduce fairness. Compare throughput, latency, and actual request sizes; do not infer a useful value from device queue depth alone. |
| Small nodes | Retain 128kB unless measured adjacent I/O shows a benefit from another size. On PG18, raising only io_combine_limit above io_max_combine_limit is ineffective; on PG17 there is no separate clamp GUC. |
{.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: PG17–19 Beta 3 unmodified; OLAP: PG17–19 Beta 3 unmodified; CRIT: PG17–19 Beta 3 unmodified; TINY: PG17–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Applying the PG18 io_max_combine_limit clamp to PostgreSQL 17, where that GUC does not exist.
- Treating a byte-size limit as I/O concurrency or queue depth.
- Raising io_combine_limit above the PG18 server clamp and expecting larger requests.
- Assuming every eligible operation reaches the configured size even when adjacent I/O is unavailable.

## Related parameters {#related-parameters}

[`io_max_combine_limit`](/parameters/io-max-combine-limit/) · [`io_max_concurrency`](/parameters/io-max-concurrency/) · [`io_method`](/parameters/io-method/) · [`effective_io_concurrency`](/parameters/effective-io-concurrency/) · [`maintenance_io_concurrency`](/parameters/maintenance-io-concurrency/)

## References {#references}

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