We are PG 11.5 with RHEL 7.9 as it has EOL, we planned to upgrade to PG16.1. When the server was ready with RHEL 9.3, installed 11.15 and set the replica with Master. As the DB is more than 20TB, unmounted the storage while installing RHEL 9 and reattached.
DB started and once it sync with master, made this replica as master, once this done we have encountered several issues like.
What replica?
Physical replication from 11.5 to 16.1? I don't think that was possible. Or do you mean logical replication from the local 11.5 instance to 16.1?
Anyway, if you have PG 11.5 and 16.1 installed on the same RHEL9 server, I think that, after mounting the old disk, I'd have done a pg_upgrade instead of replication. Copying 20TB doesn't take that long.