Oracle - SPLessons

Oracle Online Redo Log

Chapter 26

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Oracle Online Redo Log

Oracle Online Redo Log

shape Description

The most essential structure for recuperation operations is the Oracle Online Redo Log file, which comprises of two or more preallocated documents that store all progressions made to the database. Each occurrence of an Oracle Database has an related redo log to ensure the database in the case of an instance failure.

Oracle Online Redo Log File Basics

shape Description

Oracle Online Redo Log Files empower the Oracle Server or DBA to redo exchanges, when the database break down occurs. It is the one of  reason to empower recuperation.

  • Exchanges are composed synchronously to the Oracle Online Redo Log Buffer in the System Global Area.
  • All database changes are composed to Oracle Online Redo Log to empower recuperation.
  • As the Redo Log Buffer fills, the substance are composed to Redo Log Files.
  • This incorporates uncommitted exchanges, fix section information,and outline/object administration data.
  • During database recuperation,data in Redo Log Files empower information that has not yet been composed to data files to be recouped.

shape Example

By viewing the below example the concept of redo log file can be easily understood.

  ADD LOGFILE GROUP 10 ('/oracle/dbs/log1c.rdo', '/oracle/dbs/log2c.rdo')
      SIZE 4M;
    TO ('/oracle/dbs/log2a.rdo', '/oracle/dbs/log2b.rdo'); 

Redo Thread

shape Description

If a database is accessed by multiple instances then redo log is called as redo thread.

  • Having a different individual  thread for each occuranace avoids controversy when composing a single set of redo log files and this eliminates the execution congestions.
  • This implies that, for the certain part in an Oracle Real Application Clusters environment.

Redo Log File Organization – Multiplexing

shape Description

Redo Log Files are created, during the design stage of database articulations, Rather in a class to contribute for multiplexing. Further groups of files can be increased as they needs get emerged.

shape Conceptual

Each Redo Log Group has indistinguishable Redo Log Files.

  • The LGWR simultaneously composes indistinguishable data to each Redo Log File in a Group.
  • If you have Redo Log Files in groups, then it should contain more than two groups. The Oracle Server needs at least two on-line Redo Log Groups for ordinary database operation.
  • Redo Log Files in a Group are called Members.
  • If Disk one get crashed then it will not effect the remaining log files, and they are really lost in the matter of the fact that there are copied.
  • Every Group Member has an indistinguishable log arrangement number and is having the same size as the one contain for individuals inside the group.
  • If the set has more number of individuals, then it require more number of plate drives all together for the utilization of multiplexed Redo Log Files.
  • The log arrangement number is relegated by the Oracle Server as it keeps in touch with a log bunch and the present log grouping number is put away in the control records within the header data  and contain all the  data files, this empowers synchronization in the middle of data files and Redo Log Files.

Active and inactive redo log files

shape Description

Oracle Database utilizes single redo log documents at once to store redo log records composed from the redo log support. The redo log document that LGWR is continently composed and  is called as the current redo log record.

Redo log documents that are required for instance recuperation are called dynamic redo log records. Redo log records are not necessary required for occasion recuperation and an be called as idle redo log documents. If archive is empowered, then the database can’t overwrite an active on-line log file until one of the archiver backdrop developments has archived its essences. If archiving is paralysed, and when the last redo log file is full, LGWR continues by overwriting the first available active file.


shape Key Points

  • Oracle Online Redo Log Files – Consists of redo log records in the database server.
  • Redo thread – Are private and public threads used in the database instance.
  • Active redo log files – Instance recovery of files.
  • Inactive redo log files – Instance recovery is not updated.