Oracle Database Concepts - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 90

Figure 3-8: Redo log files' utilization in action. Redo log files get
utilized in a circular fashion. A database should have at least two Redo
Log groups. Each Redo Log group should have two or more redo log
files called “members”. Redo Log files get utilized by Oracle Instance
in a circular fashion. Redo Log files and Archived Redo Log files play
a great role in Oracle Database recovery in times of any kind of failure.
To avoid overwriting of Redo Log files, you should run the
Oracle Instance in Archive Log Mode. Whenever Oracle is
running in this mode, redo log files gets archived well before
they get overwritten.
This Redo Log mechanism and Archived Redo Log files play a
great role in Oracle database recovery in times of disaster of any
kind. We will learn about Oracle Backup & Recovery later in
this topic in great detail.