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Oracle Database 12c Backup and Recovery Survival Guide by Aman Sharma, Francisco Munoz Alvarez

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What is redo?

Let's look briefly at the redo process. When Oracle blocks (the smallest unit of storage in a database) are changed, including UNDO blocks, Oracle records the changes in the form of vector changes, which are referred to as redo entries or redo records. The changes are written by the server process to the redo log buffer in the System Global Area (SGA). The redo log buffer will then be flushed into the online redo logs in near real-time fashion by the log writer (LGWR) process (if the redo log buffer is too small, then you will start seeing log buffer space waits during bursts of redo generation).

The redo entries are written by the LGWR to a disk when:

  • A user issues a commit
  • The log buffer is one third full
  • The amount of unwritten redo ...

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