Oracle7 Server Concepts
Recovery from Instance Failure
When an instance is aborted, either unexpectedly (for example, an unexpected power outage or a background process failure) or expectedly (for example, when you issue a SHUTDOWN ABORT or STARTUP FORCE statement), instance failure occurs, and instance recovery is required. Instance recovery restores a database to its transaction-consistent state just before instance failure.
If you experience instance failure during online backup, media recovery might be required. In all other cases, Oracle automatically performs instance recovery for a database when the database is restarted (mounted and opened to a new instance). If necessary, the transition from a mounted state to an open state automatically triggers instance recovery, which consists of the following steps:
1. Rolling forward to recover data that has not been recorded in the datafiles, yet has been recorded in the online redo log, including the contents of rollback segments.
2. Opening the database. Instead of waiting for all transactions to be rolled back before making the database available, Oracle enables the database to be opened as soon as cache recovery is complete. Any data that is not locked by unrecovered transactions is immediately available. This feature is called fast warmstart.
3. Marking all transactions system-wide that were active at the time of failure as DEAD and marking the rollback segments containing these transactions as PARTIALLY AVAILABLE.
4. Recovering dead transactions as part of SMON recovery.
5. Resolving any pending distributed transactions undergoing a two-phase commit at the time of the instance failure.
Read-Only Tablespaces and Instance Recovery
No recovery is ever needed on read-only datafiles after instance recovery. Recovery during startup verifies that an online read-only file does not need any media recovery. That is, the file was not restored from a backup taken before it was made read-only. If you restore a read-only tablespace from a backup taken before the tablespace was made read-only, you cannot access the tablespace until you complete media recovery.