Quote Originally Posted by tamilselvan
For example, You update millions of rows in table and then before commit/rollback, the system crashed even though several log switches occurred during the update process.
Tamil, I think you are confusing transaction recover with instance recovery.

Redo logs are not required for transaction recovery. At instance recovery, the undo tablespace is recovered, like any other and any active transactions are rolled back. This has nothing to do with checkpointing.

On line redo logs are the only thing that is needed to perform instance recovery or what some are referring to as crash recovery.

Checkpoints may be either incremental or full. In either case oracle records the low cache RBA (redo block address) on disk in the control file. This is the location that is needed inside the redo log to perform instance recovery. See x$kcccp

An active log is needed for instance recovery, You can not switch into an active log.