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When an Oracle instance fails,Oracle performs an instance recovery when the associated database is re-started.
Instance recovery occurs in two steps:
Cache recovery: Changes being made to a database are recorded in
the database buffer cache. These changes are also recorded in online redo log
files simultaneously. When there are enough data in the database buffer cache,
they are written to data files. If an Oracle instance fails before the data in
the database buffer cache are written to 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 cache recovery.
Transaction recovery: When a transaction modifies data in a
database, the before image of the modified data is stored in an undo segment.
The data stored in the undo segment is used to restore the original values in
case a transaction is rolled back. At the time of an instance failure, the
database may have uncommitted transactions. It is possible that changes made by
these uncommitted transactions have gotten saved in data files. To maintain
read consistency, Oracle rolls back all uncommitted transactions when the
associated database is re-started. Oracle uses the undo data stored in undo
segments to accomplish this. This process is called transaction recovery.
Instance recovery occurs in two steps:
Cache recovery: Changes being made to a database are recorded in
the database buffer cache. These changes are also recorded in online redo log
files simultaneously. When there are enough data in the database buffer cache,
they are written to data files. If an Oracle instance fails before the data in
the database buffer cache are written to 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 cache recovery.
Transaction recovery: When a transaction modifies data in a
database, the before image of the modified data is stored in an undo segment.
The data stored in the undo segment is used to restore the original values in
case a transaction is rolled back. At the time of an instance failure, the
database may have uncommitted transactions. It is possible that changes made by
these uncommitted transactions have gotten saved in data files. To maintain
read consistency, Oracle rolls back all uncommitted transactions when the
associated database is re-started. Oracle uses the undo data stored in undo
segments to accomplish this. This process is called transaction recovery.
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Cache Recovery (Rolling Forward)During the cache recovery step,
Oracle applies all committed and uncommitted changes in the redo log files to
the affected data blocks. The work required for cache recovery processing is
proportional to the rate of change to the database (update transactions each
second) and the time between checkpoints.
Oracle applies all committed and uncommitted changes in the redo log files to
the affected data blocks. The work required for cache recovery processing is
proportional to the rate of change to the database (update transactions each
second) and the time between checkpoints.
Transaction Recovery (Rolling Back)To make the database consistent,
the changes that were not committed at the time of the crash must be undone (in
other words, rolled back). During the transaction recovery step, Oracle applies
the rollback segments to undo the uncommitted changes.
the changes that were not committed at the time of the crash must be undone (in
other words, rolled back). During the transaction recovery step, Oracle applies
the rollback segments to undo the uncommitted changes.
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