As long as that modified block is in the buffer cache and is not on disk, we need the contents of that online redo log in the event the database fails. If at the instant after we committed, the power was turned off, the database buffer cache would be wiped out.
If this happens, the only record of our change is in that redo log file. Upon restart of the database, Oracle will actually replay our transaction, modifying the block again in the same way we did, and committing it for us. So, as long as that modified block is cached and not written to disk, we cannot reuse that redo log file. |
