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About Online Redo Log Files

Every Oracle database has a set of two or morę 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 madę

up of redo entries, which are also called redo reeords.

The online redo log Stores a copy of the changcs madę to data. If a failurc requires a data file to be restored from backup, then the reccnt data changcs 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 failurc. To protect against a failurc involving the online redo log file itself, Oracle Database can multiplex the online redo log file so that two or morę 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 filc 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.

Figurę 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_LOGl and B_LOGl.