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I tested this using Longhorn in a Fedora VM, but any storage enforcing the PV capacity will do the trick.

To test the patch, you need to finish your WAL storage. To keep things easy, I used:

apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
  name: cluster-example
spec:
  instances: 1

  storage:
    size: 256Mi

And then:

CREATE TABLE storage_area (t text);

-- repeat the following query 20-30 times (you need to be fast!)
INSERT INTO storage_area (t) (select repeat('Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do', 5*1024*1024));

With the predefined WAL settings, you'll finish your WAL disk space before you finish the space for PGDATA.

/test limit=local

@armru, here's the link to the E2E on CNPG workflow run: https://github.com/cloudnative-pg/cloudnative-pg/actions/runs/9110497781

We removed the WALSpaceAvailable because we're worried that it could be misunderstood and used as a metric instead of proper monitoring infrastructure.

The operator sets this value from the exit code of the instance Pods. The instance Pods check whether they have enough WAL disk space before starting PG and after PostgreSQL exits with an error condition.
This is not a way to monitor disk space.

Full E2e run: https://github.com/EnterpriseDB/cloudnative-pg/actions/runs/9363810554