QUOTE=tamilselvan---one doubt--urgent
Quote:
Originally Posted by tamilselvan
Yes, you are partially right. In general, after a checkpoint, the redo in the redo log files is no longer needed for crash/instance recovery. However, the redo logs may be needed for instance recovery if ta single transaction's redo size is greater than the redo log file size.
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.
Another example is when you clone the database, the recovery process may ask you to enter redo log files which are older than the current redo log.
The only way I can see is, do manual checkpoint, and switch log files.
You can experiment with a test case.
Update millions of rows.
Do log switch.
Shutdown abort
Start up the instance. Note down the recovery time.
Do the same exercise with "alter sytem checkpoint", and you see the vast difference in recovery time.
Does this mean that there is no situation when DBWn writes uncommitted data to disk..coz if during checkpoint, if only bufferes related to redo log files are return to disk...then since redo log files always hold committed data(LGWR writes data from redo log buffer to redo log file on commit)-- so there is no case of writing uncommitted data to disk or what?????????