Meaning of Undocumented parameters in Oracle
These parameters allow you to ignore corrupt data blocks when your database is corrupted. These should only be used in emergencies.
_allow_resetlogs_corruption – This parameter may be the only way to start a db backed-up open without setting backup on tablespace, it will result in an unsupported system.
_corrupted_rollback_segments – The only way to start up with corrupted public rollback segments. This undocumented parameter can be used without fear of invalidating support.
_allow_read_only_corruption – This parameter allows the database to be opened even if it has corruption. This should only be used to export as much data from a corrupted database as is possible before re-creating a database. A database that has been opened in this manner should not be used in a normal manner, as it will not be supported.
_corrupt_blocks_on_stuck_recovery – This parameter can sometimes be useful for getting a corrupted database started. However, it probably won’t be supported if done without Oracle’s blessing. Immediately export the tables needed and rebuild the database if used.