At this point you have committed
transactions that are lost??”the primary log that recorded the changes made by the
transactions is gone, and the copies of the log are not yet up to date with those
changes. To prevent this from occurring, Oracle always waits until all copies of the
redo log have been updated.
How Oracle uses the redo logs
Once Oracle fills one redo log file, it automatically begins to use the next log file.
When the server cycles through all the available redo log files, it returns to the first
one and reuses it. Oracle keeps track of the different redo logs by using a redo log
sequence number. This sequence number is recorded inside the redo log files as they
are used.
To understand the concepts of redo log filenames and redo log sequence numbers,
consider three redo log files called redolog1.log, redolog2.log, and redolog3.log. The
first time Oracle uses them the redo log sequence numbers for each will be 1, 2, and
3, respectively. When Oracle returns to the first redo log??”redolog1.log??”it will reuse
it and assign it a sequence number of 4.
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