About Online Redo Log Files
Every Oracle database has a set of two or more online redo log files. The set of online
redo log files is collectively known as the redo log for the database. A redo log is made
up of redo entries, which are also called redo records.
The online redo log stores a copy of the changes made to data. If a failure requires a
data file to be restored from backup, then the recent data changes that are missing
from the restored data file can be obtained from the online redo log files, so work is
never lost. The online redo log files are used to recover a database after hardware,
software, or media failure. To protect against a failure involving the online redo log
file itself, Oracle Database can multiplex the online redo log file so that two or more
identical copies of the online redo log file can be maintained on different disks.
The online redo log for a database consists of groups of online redo log files. A group
consists of an online redo log file and its multiplexed copies. Each identical copy is
considered to be a member of that group. Each group is defined by a number, such as
Group 1.
Figure 6-2 on page 6-4 shows the configuration of a database that has three online
redo log groups and two members in each group. For each group, the members are
stored on separate disks for maximum availability. For example, the members of
Group 1 are the redo log files A_LOG1 and B_LOG1.
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