Read the Google Cloud SQL documentation on shrinking storage capacity.

The documentation refers to checking max_wal_size — at the time of writing, it must be below 5 128 MB for the operation to proceed:

SHOW max_wal_size;

The documentation refers to checking installed extensions - you can use this command:

SELECT * FROM pg_extension;

Find the minimum target size and estimated duration:

gcloud sql instances get-storage-shrink-config <instance-name>

Example output:

The estimated operation time is 124 minutes.
minimalTargetSizeGb: '722'

Choose a target size slightly above the minimum (e.g. 750 GB if the minimum is 722 GB).

Stop the application:

nais app stop <app-name>

Create a manual backup in the Cloud SQL Console (https://console.cloud.google.com/sql/instances/<instance-name>/backups) before proceeding.

Start the shrink operation:

gcloud sql instances perform-storage-shrink <instance-name> \
  --storage-size=<target-size-gb> \
  --no-async

--no-async keeps the command running and shows a spinner for the duration of the operation. Completely optional.

Optional monitoring of progress (in a separate terminal):

There is (currently) no way to monitor the progress of the operation. You can, however, check if the operation is ongoing:

List recent operations and find the one without an end time:

gcloud sql operations list --instance <instance-name> --limit 10

Inspect the running operation, using the UUID from the previous command:

gcloud sql operations describe <operation-uuid>

Optionally, watch for the application to scale back up (it should not during the operation):

watch -n5 nais app status <app-name>

Start the application once the operation completes:

Either:

nais app start <app-name>

or:

nais app set replicas <app-name> --min 4 --max 4

Verify that the application is running and that disk usage is as expected.

Optionally delete the manual backup after the application has been verified stable — typically the next business day.