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CHAPTER 9 REDO AND UNDO
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At this point, we have the situation depicted in Figure 9-1. We have generated some modified table and index blocks. These have associated undo segment blocks, and all three types of blocks have generated redo to protect them. If you recall from our discussion of the redo log buffer in 4, it is flushed every three seconds, when it is one-third full or contains 1MB of buffered data, or whenever a commit takes place. It is very possible that at some point during our processing, the redo log buffer will be flushed. In that case, the picture looks like Figure 9-2.
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Figure 9-2. State of the system after a redo log buffer flush
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The UPDATE
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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 9-3.
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Figure 9-3. State of the system after the UPDATE
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CHAPTER 9 REDO AND UNDO
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We have more new undo segment blocks in the block buffer cache. To undo the update, if necessary, we have modified database table and index blocks in the cache. We have also generated more redo log buffer entries. Let s assume that some of our generated redo log from the insert is on disk and some is in cache. Hypothetical Scenario: The System Crashes Right Now Upon startup, Oracle would read the redo logs and find some redo log entries for our transaction. Given the state in which we left the system, with the redo entries for the insert in the redo log files and the redo for the update still in the buffer, Oracle would roll forward the insert. We would end up with a picture much like Figure 9-1, with some undo blocks (to undo the insert), modified table blocks (right after the insert), and modified index blocks (right after the insert). Oracle will discover that our transaction never committed and will roll it back since the system is doing crash recovery and, of course, our session is no longer connected. It will take the undo it just rolled forward in the buffer cache and apply it to the data and index blocks, making them look as they did before the insert took place. Now everything is back the way it was. The blocks that are on disk may or may not reflect the INSERT (it depends on whether or not our blocks got flushed before the crash). If they do, then the insert has been, in effect, undone, and when the blocks are flushed from the buffer cache, the data file will reflect that. If they do not reflect the insert, so be it they will be overwritten later anyway. This scenario covers the rudimentary details of a crash recovery. The system performs this as a two-step process. First it rolls forward, bringing the system right to the point of failure, and then it proceeds to roll back everything that had not yet committed. This action will resynchronize the data files. It replays the work that was in progress and undoes anything that has not yet completed. Hypothetical Scenario: The Application Rolls Back the Transaction At this point, Oracle will find the undo information for this transaction either in the cached undo segment blocks (most likely) or on disk if they have been flushed (more likely for very large transactions). It will apply the undo information to the data and index blocks in the buffer cache, or if they are no longer in the cache request, they are read from disk into the cache to have the undo applied to them. These blocks will later be flushed to the data files with their original row values restored. This scenario is much more common than the system crash. It is useful to note that during the rollback process, the redo logs are never involved. The only time redo logs are read is during recovery and archival. This is a key tuning concept: redo logs are written to. Oracle does not read them during normal processing. As long as you have sufficient devices so that when ARCH is reading a file, LGWR is writing to a different device, then there is no contention for redo logs. Many other databases treat the log files as transaction logs. They do not have this separation of redo and undo. For those systems, the act of rolling back can be disastrous the rollback process must read the logs their log writer is trying to write to. They introduce contention into the part of the system that can least stand it. Oracle s goal is to make it so that logs are written sequentially, and no one ever reads them while they are being written.
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