Reverting to the old cluster v15

The method you use to revert to a previous cluster varies with the options specified when invoking pg_upgrade:

• If you specified the --check option when invoking pg_upgrade, an upgrade wasn't performed and no modifications were made to the old cluster. You can reuse the old cluster at any time.
• If you included the --link option when invoking pg_upgrade, the data files are shared between the old and new cluster after the upgrade completes. If you started the server that's servicing the new cluster, the new server wrote to those shared files, and it's unsafe to use the old cluster.
• If you ran pg_upgrade without the --link specification or haven't started the new server, the old cluster is unchanged except that the .old suffix was appended to the $PGDATA/global/pg_control and tablespace directories.
• To reuse the old cluster, delete the tablespace directories created by the new cluster. Remove the .old suffix from $PGDATA/global/pg_control and the old cluster tablespace directory names. Restart the server that services the old cluster.