Expected downtime for in-place Cloud SQL PostgreSQL upgrade from 11 to 15 for a 130 GB database

Google Cloud Database
1

We are planning an in-place major version upgrade of a Cloud SQL for PostgreSQL instance from PostgreSQL 11 to PostgreSQL 15.

The database is approximately 130 GB in size, and we would like to estimate the expected application downtime during the upgrade.

Could you please help clarify:

  1. What is the typical or expected downtime for an in-place PostgreSQL 11 → 15 major version upgrade for a database of approximately 130 GB?
  2. Is the downtime primarily dependent on the database size, or are there other factors that significantly affect the upgrade duration?
  3. Does Cloud SQL use a process similar to pg_upgrade, and is the database unavailable for the entire duration of the upgrade?
  4. Is there any way to estimate or benchmark the expected downtime before performing the production upgrade?