I have read your replies in this regard, but am unable to satisfy one of my queries. Let me try to put it in a simple way:

Let's say we have started a transaction:
1. During the course of transction, the data blocks in buffer cache get modified - dirty.
2. The transaction has NOT been committed till now.
3.After some time the DBWR writes the modified blocks to disk. So the original data in the respective blocks are no more available.
4.At this instance the database crashes.
5. Please note that we have not committed our transaction at all.

As far as i could understand, the roll-back segments hold undo data, and redo-logs contain data so as to redo a transaction.

But since in this case, the instance has crashed, anything held in the rollback segments in no more available. And since the redo-logs do not store the initial image of the blocks, i.e. before the transaction started, i fail to understand, how will we be able to get the prior image of the block, i.e. before transaction started.

That brings me to my question - What exactly goes into the redo-log files?

From whatever i could search on net, they only talk about change information to redo a transaction.

But if we are to restore the original block image, we also need to capture the image of the block (w.r.t modified data) before the start of the transction to handle this kind of eventuality.

Please throw some light on this. We are thankful to you in making us understand Oracle better.

Regards,

Praveen Kumar