Postgresql Shared Memory Segment Docker Compose

Legacy context

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Key point 1. Readers exploring containerized PostgreSQL deployments may find the archived discussions on shared memory segments and configuration useful as background context. The preserved excerpts do not cover current software versions or provide operational instructions. Instead, they offer conceptual groundwork for evaluating data flexibility and query design.

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PostgreSQL Shared Memory Segments in Docker Compose: A Practical Guide. PostgreSQL relies on shared memory for inter-process communication, caching, and WAL (write-ahead log) buffering. When you run PostgreSQL inside a Docker container, the default shared memory size of 64 MB (the kernel default for `/dev/shm`) is often insufficient, leading to errors like `could not resize shared memory segment` or `out of shared memory`. This guide explains how to configure shared memory for PostgreSQL in Docker Compose, why it matters, and how to avoid common pitfalls.

PostgreSQL uses two types of shared memory:. 1. POSIX shared memory (mounted at `/dev/shm`) – used for parallel query execution, logical replication, and some extensions.

1. System V shared memory – used for the main shared buffer pool (`shared_buffers`), WAL buffers, and lock tables.

Key point 5. In Docker, the default `/dev/shm` size is 64 MB. For a PostgreSQL instance with `shared_buffers` set to 128 MB or higher, this is almost guaranteed to cause failures during startup or under load. The error typically appears in the PostgreSQL logs as:

Key point 6. ```

FATAL: could not create shared memory segment: Invalid argument.

DETAIL: Failed system call: shmget(key=..., size=...)

```.

The Core Fix: Setting `shm_size` in Docker Compose. Docker Compose allows you to override the `/dev/shm` size using the `shm_size` directive. This is the most direct and reliable way to give PostgreSQL the shared memory it needs.

Minimal example (docker-compose.yml):. ```yaml.

services:

db:

image: postgres:16

environment:

POSTGRES_PASSWORD: example

shm_size: '1gb'

volumes:

• pgdata:/var/lib/postgresql/data

volumes:

pgdata:

```.

Decision criteria for choosing `shm_size`:

• Start with 1 GB for typical workloads (web apps, CRUD operations).
• Use 2 GB or more if you run complex analytical queries, use parallel query execution, or have `shared_buffers` above 512 MB.
• Match `shm_size` to `shared_buffers` + 20% overhead as a rough rule. For example, if `shared_buffers = 512 MB`, set `shm_size` to at least 768 MB.
• Monitor actual usage with `docker exec <container> df -h /dev/shm` to see how much is consumed. If it's near 100%, increase the size.

Alternative: Mounting a tmpfs Volume

If you need more granular control or want to avoid the `shm_size` syntax (which varies slightly across Docker Compose versions), you can mount a `tmpfs` volume directly:

Key point 11

```yaml

services:

db:

image: postgres:16

environment:

POSTGRES_PASSWORD: example

tmpfs:

• /dev/shm:rw,size=1g

```.

Key point 12

This achieves the same result as `shm_size` but uses the `tmpfs` mount syntax. The `rw` flag ensures read-write access, and `size=1g` sets the limit. Note that `tmpfs` is memory-backed, so it consumes RAM. If you set it too high, you risk OOM (out-of-memory) kills on the host.

Common Mistake #1: Confusing `shm_size` with `shared_buffers`.

`shm_size` is a Docker-level setting that controls the size of `/dev/shm` inside the container. `shared_buffers` is a PostgreSQL configuration parameter that controls how much memory PostgreSQL uses for caching data pages. They are related but not identical.

Key point 14

• If `shared_buffers` is 1 GB but `shm_size` is 64 MB, PostgreSQL will fail.
• If `shm_size` is 1 GB but `shared_buffers` is 128 MB, you are wasting RAM but not causing errors.

Key point 15

Best practice: Set `shm_size` to at least `shared_buffers` + 256 MB to accommodate WAL buffers and other shared structures.

Common Mistake #2: Using `ulimits` Instead of `shm_size`.

Some older guides recommend setting `ulimits` with `memlock` or `nofile`. While `ulimits` can help with file descriptors and locked memory, they do not change the size of `/dev/shm`. The only reliable way to increase shared memory in Docker is via `shm_size` or `tmpfs`.

Example of what does not fix shared memory issues:

```yaml

This does NOT increase /dev/shm

services:

db:

image: postgres:16

ulimits:

memlock: -1

```.