Add a new redo log group

Command

ALTER DATABASE ADD LOGFILE GROUP 4
  ('/u01/oradata/ORCL/redo04a.log',
   '/u02/oradata/ORCL/redo04b.log') SIZE 512M;

What it does

Creates a new redo log group with the specified size and members (multiple members = mirror).

When to use it

When redo throughput is insufficient or you want to increase the number of groups.

Caution

Make sure the group number does not collide with existing groups; check with V$LOG.