Muhammad Nadeem Chaudhry's Blog

Instance Recovery

Posted in Uncategorized by Muhammad Nadeem Chaudhry on November 11, 2010

Oracle performs instance recovery when the database is restarted due to instance failure or shutdown the database with ABORT option(shutdown abort) or startup the database with FORCE option(startup force). Instance recovery is taken care by SMON oracle background process automatically. Instance recovery consists of two steps. One is Roll forward, next is Roll backward.

Roll forward:
Changes being made to the database are recorded in the database buffer cache and buffer cache will be written into data files. At the same time, the changes are recorded in redo log buffer and redo log buffer will be written to redo log file. Oracle writes the data from database buffer cache to data file only when there is enough data in data buffer cache. It is not necessary that, every commit, oracle writes the data buffer cache into data file. When instance fails before committed data are written into data files, Oracle uses the data recorded in the online redo log files to recover the lost data when the associated database is re-started. This process is called Roll forward or cache recovery.

Roll backward:
When we make any changes in the database, the old image will be written into undo segment. Later undo segment will be used to rollback the data when we rollback the transaction. DBWR writes the buffer cache content into data file under different circumstances. It might be possible to write the uncommitted data from database buffer cache into data file. When instance fails and associated database is restarted, it rollback the uncommitted transaction in data file by using undo segment to maintain the read consistency. This is called roll forward or transaction recovery.

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