Cracking the shared_buffers: How /dev/shm Impacts PostgreSQL Performance on Kubernetes

Hola!! 👋

Recently, while working on a PostgreSQL deployment in Kubernetes, I ran into some interesting challenges that taught me about shared memory and buffer tuning.

As you know, PostgreSQL relies heavily on its shared_buffers parameter for caching database pages in memory. This setting is critical for performance, especially for read-heavy workloads. However, what isn’t always obvious is how tightly it’s linked to the shared memory size exposed by the container’s /dev/shm.

Here’s what happened:

I had deployed PostgreSQL in a StatefulSet and did not configure shared_buffers to optimize performance. But, as the database started up, I was greeted with this error:

FATAL: could not create shared memory segment: No space left on device

At first, I was puzzled. The node had plenty of memory available! So, what was going wrong? 🤔

It turned out that the default shared memory allocation for containers in Kubernetes comes from /dev/shm, which is only 64 MB by default. For comparison, my shared_buffers was set to 256 MB! PostgreSQL was simply unable to allocate enough shared memory to match its buffer size.

🛠️ The Fix: Configuring /dev/shm in Kubernetes

To resolve this, I configured a memory-backed volume (emptyDir) specifically for /dev/shm in my StatefulSet. Here’s how it looks in YAML:

volumes:
- name: shmem
  emptyDir:
    medium: Memory
    sizeLimit: 2Gi # Set this based on your shared_buffers value

Then I mounted this volume to /dev/shm in the container:

volumeMounts:
- mountPath: /dev/shm
  name: shmem

By setting the sizeLimit to 2 GiB, I ensured that PostgreSQL had more than enough shared memory to align with its shared_buffers configuration. Once this was in place, PostgreSQL started seamlessly—and the performance boost was immediately noticeable! 🚀

💡 Key Takeaways

1. Understand the link between shared_buffers and /dev/shm: If your shared_buffers exceeds the size of /dev/shm, PostgreSQL will fail to start. Always ensure /dev/shm has enough room to accommodate your buffer configuration.
2. Kubernetes doesn’t configure shared memory by default: You need to explicitly define a memory-backed volume (emptyDir) for /dev/shm.
3. Plan for your workload: For production workloads, monitor and tune shared memory carefully. A general rule of thumb is to set /dev/shm to 1.5x - 2x the size of shared_buffers.
   Also you use this tool for tuning — https://pgtune.leopard.in.ua/

🤔 Why Does This Matter?

Shared memory is crucial for PostgreSQL’s performance. When configured correctly, it allows for faster query execution by caching more data in memory and reducing disk I/O. But when overlooked (as I initially did), it can cause frustrating errors and degrade performance.

Have you faced similar challenges while running PostgreSQL (or other databases) on Kubernetes? I’d love to hear about your experiences or tips in the comments! 🎉

Thanks for reading my blog. Feel free to hit me up for any AWS/DevOps/Open Source-related discussions. 🙂

Manoj Kumar — LinkedIn.