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CHAPTER 9 REDO AND UNDO
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blocks we modify exceeds 10 percent of the block buffer cache size, Oracle will stop allocating new lists for us. For example, if our buffer cache is set to cache 3,000 blocks, Oracle will maintain a list of up to 300 blocks (10 percent of 3,000) for us. Upon COMMIT, Oracle will process each of these lists of 20 block pointers, and if the block is still available, it will perform a fast cleanout. So, as long as the number of blocks we modify does not exceed 10 percent of the number of blocks in the cache and our blocks are still in the cache and available to us, Oracle will clean them out upon COMMIT. Otherwise, it just skips them (i.e., does not clean them out). Given this understanding, we can set up artificial conditions to see how the cleanout works. I set my DB_CACHE_SIZE to a low value of 4MB, which is sufficient to hold 512 8KB blocks (my blocksize is 8KB). Then, I created a table such that a row fits on exactly one block I ll never have two rows per block. Then, I fill this table up with 500 rows and COMMIT. I ll measure the amount of redo I have generated so far, run a SELECT that will visit each block, and then measure the amount of redo that SELECT generated. Surprising to many people, the SELECT will have generated redo. Not only that, but it will also have dirtied these modified blocks, causing DBWR to write them again. This is due to the block cleanout. Next, I ll run the SELECT once again and see that no redo is generated. This is expected, as the blocks are all clean at this point. ops$tkyte@ORA10G> create table t 2 ( x char(2000), 3 y char(2000), 4 z char(2000) 5 ) 6 / Table created. ops$tkyte@ORA10G> set autotrace traceonly statistics ops$tkyte@ORA10G> insert into t 2 select 'x', 'y', 'z' 3 from all_objects 4 where rownum <= 500; 500 rows created. Statistics ---------------------------------------------------------... 3297580 redo size ... 500 rows processed ops$tkyte@ORA10G> commit; Commit complete. So, this is my table with one row per block (in my 8KB blocksize database). Now I will measure the amount of redo generated during the read of the data: ops$tkyte@ORA10G> select * 2 from t; 500 rows selected. Statistics ----------------------------------------------------------
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CHAPTER 9 REDO AND UNDO
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... 36484 ... 500 rows processed So, this SELECT generated about 35KB of redo during its processing. This represents the block headers it modified during the full scan of T. DBWR will be writing these modified blocks back out to disk at some point in the future. Now, if I run the query again ops$tkyte@ORA10G> select * 2 from t; 500 rows selected. Statistics ---------------------------------------------------------... 0 redo size ... 500 rows processed ops$tkyte@ORA10G> set autotrace off I see that no redo is generated the blocks are all clean. If we were to rerun the preceding example with the buffer cache set to hold at least 5,000 blocks, we ll find that we generate little to no redo on any of the SELECTs we will not have to clean dirty blocks during either of our SELECT statements. This is because the 500 blocks we modified fit comfortably into 10 percent of our buffer cache, and we are the only users. There is no one else mucking around with the data, and no one else is causing our data to be flushed to disk or accessing those blocks. In a live system, it will be normal for at least some of the blocks to not be cleaned out sometimes. This behavior will most affect you after a large INSERT (as just demonstrated), UPDATE, or DELETE one that affects many blocks in the database (anything more than 10 percent of the size of the cache will definitely do it). You will notice that the first query to touch the block after this will generate a little redo and dirty the block, possibly causing it to be rewritten if DBWR had already flushed it or the instance had been shut down, clearing out the buffer cache altogether. There is not too much you can do about it. It is normal and to be expected. If Oracle did not do this deferred cleanout of a block, a COMMIT could take as long to process as the transaction itself. The COMMIT would have to revisit each and every block, possibly reading them in from disk again (they could have been flushed). If you are not aware of block cleanouts and how they work, it will be one of those mysterious things that just seems to happen for no reason. For example, say you UPDATE a lot of data and COMMIT. Now you run a query against that data to verify the results. The query appears to generate tons of write I/O and redo. It seems impossible if you are unaware of block cleanouts; it was to me the first time I saw it. You go and get someone to observe this behavior with you, but it is not reproducible, as the blocks are now clean on the second query. You simply write it off as one of those database mysteries. redo size
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