Oracle writes uncommitted changes to disk routinely, even in single-instance
Oracle. If the instance were to crash, there is no recovery risk because the
changes will be rolled back during transaction recovery. Oracle guarantees this
recoverability by ensuring that the redo records containing the rollback segment
changes to undo the most recent changes to any data block always have to be
written to the log file before the data block may be written to disk. That is
why DBWn waits for 'log file sync' events from time to time.


-----Original Message-----
Sent: Friday, 25 August 2000 1:42


I have read "OPS Concept and administration, release 8.0", it mentioned
following several times when explain PCM lock that if:
transaction A modified a row c in block d, and transaction B is going to
modify a row d in the same block d, then oracle instance for transaction A
will write this block d back to disk, EVEN before transaction A committed,
so transaction B is able to modify row d in the same block.

How can oracle to recover this if transaction A rollback, because it seems
to me what transaction A modified has already been written to disk.