Figure 6-40. A diagram of a single data block with its three areas and three associated undo blocks with their relationships shown. Two transactions are active, changing three rows.

Before the cloning can occur, an unpopular free buffer must be found and then replaced with the 7,678 CU buffer. Our server process will acquire the LRU chain latch associated with its LRU and begin scanning, starting at the LRU end of the LRU chain, looking for an unpopular free buffer. Eventually, it will find an unpopular free buffer and replace it with a copy of CU buffer 7,678. And, of course, the CBC structure will need to be updated to reflect the cloned buffer's location in the buffer cache.

We begin with the first ITL that is associated with active transaction 7.3.8. Our server process needs to retrieve any undo that has occurred after our query started at SCN time 12330. Transaction 7.3.8's most recent undo can be found by following its ITL's undo block address (UBA) link to undo block 2,45. Our server process must then access undo block 2,45, which requires CBC activity and could also require an IO call along with LRU activity. Once we have access to undo buffer 2,45, we check to ensure we are working with the correct transaction by comparing transaction numbers. Both the data block and the undo block transaction numbers do match (7.3.8), so we're okay. Because the transaction is active, the undo information should not have been overwritten.