# dynamic_shared_memory_type

> dynamic_shared_memory_type — Selects the dynamic shared memory implementation used. Observed in PG9.4–19 Beta 3; its last measured boot default is posix 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:** “Selects the dynamic shared memory implementation used.”

## Identity {#identity}

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

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG9.4 |
| Present in | PG9.4–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`0ac5e5a7e152`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=0ac5e5a7e152504c71ce2168acc9cef7fde7893c) — Allow dynamic allocation of shared memory segments. |
| Commit date | 2013-10-09 |
| Discussion | — |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG9.4–19 Beta 3 | `posix` | — | posix |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

dynamic_shared_memory_type selects how PostgreSQL creates dynamic shared memory segments used by parallel query and extensions: POSIX, System V, Windows, or file-backed mmap where supported.

The available enum values and selected boot default are platform dependent. The Docker/Linux catalog reports posix; that is not a portable promise for Windows or systems lacking POSIX shared memory.

The mmap implementation stores mapped files under pg_dynshmem and is generally discouraged because dirty pages may be written repeatedly. min_dynamic_shared_memory can preallocate part of parallel-query memory in the main shared region. Its postmaster context fixes the value at server start; changing it requires a restart.

## Tuning advice {#tuning-advice}

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

| Workload | Guidance |
| --- | --- |
| OLTP | Keep the platform's first supported default, normally posix on the measured Linux images. Change only to solve an API availability or diagnostic requirement, and restart-test parallel query plus extension workers on the target OS. |
| OLAP | Parallel queries use dynamic segments heavily, but choose the implementation by OS support and allocation behavior, not scan throughput alone. Avoid file-backed mmap on ordinary disk because repeated writeback can add I/O; a RAM disk is a special diagnostic case. |
| Small nodes | Keep the platform default. sysv may need kernel tuning, and file-backed mmap can turn memory traffic into disk I/O; neither is a free way to reduce memory use. |
{.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: PG9.4–19 Beta 3 unmodified; OLAP: PG9.4–19 Beta 3 unmodified; CRIT: PG9.4–19 Beta 3 unmodified; TINY: PG9.4–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Treating the Linux-Docker posix boot value as portable to platforms without POSIX shared memory.
- Using file-backed mmap on ordinary storage and creating repeated dirty-page writeback.
- Selecting sysv without checking System V segment limits.
- Confusing dynamic segments with the main region selected by shared_memory_type.
- Changing the startup setting without testing parallel queries and extensions that allocate DSM.

## Related parameters {#related-parameters}

[`shared_memory_type`](/parameters/shared-memory-type/) · [`min_dynamic_shared_memory`](/parameters/min-dynamic-shared-memory/) · [`max_parallel_workers`](/parameters/max-parallel-workers/) · [`max_worker_processes`](/parameters/max-worker-processes/) · [`huge_pages`](/parameters/huge-pages/)

## References {#references}

- [PostgreSQL 19 Beta 3 — dynamic_shared_memory_type](https://www.postgresql.org/docs/19/runtime-config-resource.html#GUC-DYNAMIC-SHARED-MEMORY-TYPE)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)