| Oracle Recovery Concept [message #369382] |
Thu, 07 September 2000 22:20 |
Nyoman Jendrya
Messages: 1
Registered: September 2000
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Junior Member |
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Hi all,
I have read the Oracle8i Backup and Recovery Guide, 8.1.5 and
I need some more explanation regarding the Oracle Recovery Concept.
As printed in the documentation, Oracle Recovery process has two phases:
rolling forward and rolling back.
During roll forward phase, Oracle applies the changes recorded in the redo
log records to the datafiles.
After rolling forward, the data blocks contain committed as well as
uncommitted changes.
During roll back phase, Oracle use rollback segments to undo the effects of
uncommitted transactions applied during the rolling forward phase.
My first question is, why Oracle doesn't simplify the roll forward phase where
Oracle only applies the committed changes to the datafiles, the absence of
SCN (commit record) in the redo log records is sufficient to ensure that
changes made by the transaction are rolled back. So the rolling back phase
can be omitted from the recovery process.
Second question, If these two steps really exists then how does Oracle
guarentee that all the rollback transaction are written to the redo log
file, cause rolling back a transaction does not trigger LGWR to write to
disk.
Your reply would be very appreciated.
Thanks.
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