Redo and Undo - Oracle Database Transactions and Locking Revealed - page 113

Figure 6-2. State of the system after a redo log buffer flush
That is, we'll have modified blocks representing uncommitted changes in the buffer cache and redo for those
uncommitted changes on disk. This is a very normal scenario that happens frequently.
The UPDATE
The UPDATE will cause much of the same work as the INSERT to take place. This time, the amount of undo will be
larger; we have some “before” images to save as a result of the UPDATE . Now we have the picture shown in Figure 6-3
(the dark rectangle in the redo log file represents the redo generated by the INSERT , the redo for the UPDATE is still in
the SGA and has not yet been written to disk).
Figure 6-3. State of the system after the UPDATE