I hate being wrong. But it does keep happening. Last week I sat through a RAC installation (12c GI with 11gR2 database) with a client and one of the steps involved enabling archivelog mode for one of the databases. They shut down all instances, then started one of them in mount mode only.
srvctl stop database -d RAC srvctl start instance -d RAC -i RAC1 -o mount
Just as they logged on to sqlplus and started typing “ALTER DATABASE ARCHIVELOG;” I proclaimed that this was not going to work unless they first changed CLUSTER_DATABASE in the spfile on restarted that instance. Blank looks. An enter key was pressed. To my surprise the database came back with:
SQL> ALTER DATABASE ARCHIVELOG; Database altered.
Apparently this has changed after 10gR2 and it is not longer neccessary to fiddle with CLUSTER_DATABASE while switching archivelog mode. For years and dozens of installations I have done this extra step believing this was the way it has to be done. But not any more. I re-learned something.