Redo log for each database instance referred as redo thread - Oracle Redo Log

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• Nirja Shah   -Posted on 24 Sep 15
  - The most crucial structure for recovery operations is the redo log, which consists of two or more preallocated files that store all changes made to the database as they occur.
  
  - Every instance of an Oracle Database has an associated redo log to protect the database in case of an instance failure.
  
  - Redo Threads - When speaking in the context of multiple database instances, the redo log for each database instance is also referred to as a redo thread.
  - In typical configurations, only one database instance accesses an Oracle Database, so only one thread is present.
  - In an Oracle Real Application Clusters environment, however, two or more instances concurrently access a single database and each instance has its own thread of redo.
  - A separate redo thread for each instance avoids contention for a single set of redo log files, thereby eliminating a potential performance bottleneck.
  
  - Redo log files are filled with redo records.
  
  - A redo record, also called a redo entry, is made up of a group of change vectors, each of which is a description of a change made to a single block in the database.

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