Databases Reference
In-Depth Information
The log writer writes
redo records
to the online redo log files. A redo record describes how a block of data was
changed (and does not normally contain the data block itself ), and consists of a group of
change vectors
. Change
vectors are generated for modifications to the data segment block, the undo segment block, and the transaction table
of the undo segment.
When the current online redo log group fills up, a
log switch
occurs, and the log writer starts writing to the next
online redo log group. The log writer writes to the online redo log groups in a round-robin fashion. Since you have
a finite number of online redo log groups, eventually the contents of each online redo log group will be overwritten.
If you want to save a history of the transaction information, then you must place your database in
archivelog mode
.
When your database is in archivelog mode, after every log switch, the archiver background process will copy the
contents of the online redo log file to an
archived redo log
file. In the event of a failure, the archived redo log files allow
you to recover the complete history of transactions that have occurred since your last database backup.
■
in an Oracle real application Cluster (raC) database, each instance has its own set of online redo logs. this is
known as a
thread
of redo. each raC instance writes to its own online redo logs and generates its own thread of archive
redo log files. additionally, each instance must be able to read any other instance's online redo logs. this is important
because if one instance crashes, then the other surviving instances can initiate instance recovery via reading the crashed
instance's online redo logs.
Note
Figure
14-1
displays a typical setup for the online redo log files. This figure shows three online redo log groups,
with each group containing two members (physical files). The database is in archivelog mode. In this figure, group 2
has recently filled with transactions, a log switch has occurred, and the log writer is now writing to group 3. The
archiver process is copying the contents of group 2 to an archived redo log file. When group 3 fills up, another log
switch will occur, and the log writer will begin writing to group 1. At the same time, the archiver will copy the contents
of group 3 to archive log sequence 3 (and so forth).
