One of the new features in Oracle 10g is Fast Incremental Backups, which require that databases be run with Block Change Tracking enabled. While the usage of Fast Incremental Backups has been discussed quite thoroughly in recent years, the internals of the change-tracking feature implementation in Oracle 10g are still obscure. This presentation will show how change-tracking works inside an Oracle instance, and what CTWR process does. It will illustrate how other processes are involved and discuss the overhead caused by Block Change Tracking. This information is obtained by the means of experiments and advanced research techniques. WHY THIS PAPER? Oracle RMAN was able to take incremental backups already in 9i. However, prior to introduction of Oracle 10g block change tracking (BCT), RMAN had to scan the whole datafile to and filter out the blocks that were not changed since base incremental backup and overhead or incremental backup was as high as full backup. Oracle 10g new feature, block change tracking, minimizes number of blocks RMAN needs to read to a strict minimum. With block change tracking enabled RMAN accesses on disk only blocks that were changed since the latest base incremental backup. This feature is widely known in the world of Oracle database administrators. However, hardly anything is available on internal implementation of block change tracking. This makes it difficult to evaluate the impact of enabling BCT in Oracle databases and quantify performance overhead. This paper and presentation try to uncover internals of block change tracking and show which areas of Oracle database are involved, how processes work together, what are hidden limitations and impact of enabling block change tracking. DISCLAIMER I cannot provide any guarantee that presented material is absolutely correct. There is no publicly available documentation on change tracking internals (at least, I’m not aware of any) so this paper is based purely on experiments and research plus few hints from more knowledgeable peers that I’m very grateful for. Please take this material carefully and make sure you validate my assumptions before making critical decisions – either with your own tests or request Oracle support to provide more information if your business requires it
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