When
the database is in noarchivelog mode, the circular reuse of the redo log files (also known as the
online redo log files) means that redo entries (the contents of previous transactions) are no longer
available in case of a failure to a disk drive or another media-related failure. Operating in
noarchivelog mode does protect the integrity of the database in the event of an instance failure
or system crash, because all transactions that are committed but not yet written to the datafiles
are available in the online redo log files.
In contrast, archivelog mode sends a filled redo log file to one or more specified destinations
and can be available to reconstruct the database at any given point in time in the event that a
database media failure occurs. For example, if the disk drive containing the datafiles crashes,
the contents of the database can be recovered to a point in time before the crash, given a recent
backup of the datafiles and the redo log files that were generated since the backup occurred.
The use of multiple archived log destinations for filled redo log files is critical for one of
Oracle??™s high-availability features known as Oracle Data Guard, formerly known as Oracle
Standby Database. Oracle Data Guard is covered in detail in Chapter 13.
Initialization Parameter Files
When a database instance starts, the memory for the Oracle instance is allocated, and one of
two types of initialization parameter files is opened: either a text-based file called init
.
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