I'm in the process of upgrading PG 9.1.3 to 9.4.1 in the near future.
I have several machines which each house unique databases. Each of those are replicated to a standby server with matching configurations. A total of 10 servers, 5 masters, 5 slaves. Everything runs on Ubuntu.
My question, as I can't seem to find any documentation on this part, is once I successfully upgrade the master I will need to upgrade the standby as well. Will I have to rebuild the standby from scratch, or will the standby pick up where it was before the upgrade if I do things correctly?
You upgrade the master, and then you rebuild the standbys from a new basebackup (using pg_basebackup or manually with start/stop backups etc). You can't upgrade the standbys and have them re-join the master, they have to be redone from scratch.