Major upgrades in-place

PostgreSQL provides an additional supplied program, called pg_upgrade, which allows you to migrate between major releases, such as from 9.6 to 11, or from 11 to 15; alternatively, you can upgrade straight to the latest server version. These upgrades are performed in-place, meaning that we upgrade our database without moving to a new system. That does sound good, but pg_upgrade has a few things that you may wish to consider as potential negatives, which are outlined here:

• The database server must be shut down while the upgrade takes place.
• Your system must be large enough to hold two copies of the database server: old and new copies. If it’s not, then you have to use the link option of pg_upgrade, or use the Major upgrades online recipe, coming next in this chapter. If you use the link option on pg_upgrade, then there is no pg_downgrade utility. The only option in that case is a restore from backup, and that means extended unavailability...