Meaning of undocumented parameters in oracle

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.