Problem affecting rman and redolog

 
I have worked on a db  were a rman configured to make a backup every 2 hour that means every two hours we found in the alert.log ALTER SYSTEM ARCHIVE LOG and the db suddenly slows.
The problem was the db has only 3 redolog.
When the backup started rman forces the ALTER SYSTEM ARCHIVE LOG and oracle needs to find a free redolog. it switched the redolog and as soon it fill the new redolog the archive must finish. if not oracle does not have a free redolog to switch, forced it to wait ( the db seems frozen ) the end of the backup.
the solution is to add more redolog, at least 8 to be sure one is always free to oracle’s disposal.