Oracle has an exciting new v$ view called v$bh that shows the contents of the data buffers as well as the number of blocks for each type of segment in the buffer.
The v$bh view is especially useful for showing the amount of table and index caching in databases with multiple data buffer pools. For example, researchers have shown that creating a separate index buffer with a very large blocksize can improve Oracle index structures. The v$bh view also provides important information for the proper setting of the optimizer_index_caching parameter, an important cost-based optimizer parameter that tells the CBO how much of the databases index reside in the buffer cache.
Today, many Oracle professionals are segmenting their db_cache_size, using the new db_2k_cache_size, db_keep_cache_size, db_4k_cache_size, db_8k_cache_size, db_16k_cache_size and db_32k_cache_size to create separate data buffers to isolate the caching of data blocks to improve manageability and caching.
Combining the v$bh view with dba_objects and dba_segments provides a block-by-block listing of the data buffer contents and indicates how well the buffers are caching tables and indexes. Of course, this is very important in Oracle9i, since the data buffer sizes can be altered dynamically.
There are several data dictionary tricks required when writing a script for mapping data objects to RAM buffers:
This is the most important script in this text because it provides a detailed analysis of those objects in the data buffers. This information is critical when considering an alteration to the data buffer sizes.
A sample listing from this exciting report is shown below. We can see that the report lists the tables and indexes that reside inside the data buffer. This is important information for the Oracle professional who needs to know how many blocks for each object reside in the RAM buffer. To effectively manage the limited RAM resources, the Oracle DBA
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must be able to know the ramifications of decreasing the size of the data buffer caches.
Here is the report from this script when run against a large Oracle data warehouse.
This is a very important report because we see three object types (tables, indexes, and partitions), and we also see the sub-sets of the DEFAULT pool for KEEP and RECYCLE. Also, note that all indexes are defined in the largest supported block size (db_32k_cache_size), and multiple buffer pools of 4K, 8K, 16K and 32K sizes are defined.
The output of this script is somewhat confusing because of the repeated DEFAULT buffer pool name. This is misleading because the KEEP and RECYCLE buffer pools are sub-sets of db_cache_size and can ONLY accommodate objects with the DEFAULT db_block_size.
Conversely, any block sizes that are NOT the default db_block_size, go into the buffer pool named DEFAULT. As you can see from the output listing, there are really 6 mutually exclusive and independently-sized buffer pools, and four of them are called "DEFAULT."
It is valuable to run this report repeatedly because the Oracle data buffers are dynamic and constantly changing. Running this script frequently allows us to view the blocks entering and leaving the data buffer. We can see the midpoint insertion method in action and the hot and cold regions as they update. Each time a block is re-referenced it moves to the head of the MRU chain on the hot side of the data buffer. Blocks that are accessed less frequently will age-out, first moving into the cold region and eventually being paged-out to make room for new incoming blocks.
This approach is even more important when considering a decrease to a cache size. When you issue an alter system command to decrease the cache size, Oracle will grab pages from the least recently used (LRU) end of the buffer. Depending on the amount of RAM removed, an alter system command will un-cache data blocks that may be needed by upcoming SQL statements.
For more information on SGA internals and data buffer management, see my book, Creating a Self-Tuning Database. If you want details on the new Oracle10g automation features, see Mike Ault's latest book Oracle Database 10g New Features.
About the author
Don Burleson is one of the world's top Oracle Database experts with more than 20 years of full-time DBA experience. He specializes in creating database architectures for very large online databases and he has worked with some of the world's most powerful and complex systems. A former Adjunct Professor, Don Burleson has written 32 books, published more than 100 articles in National Magazines, and serves as Editor-in-Chief of Oracle Internals. Don is a popular lecturer and teacher and is a frequent speaker at Oracle Openworld and other international database conferences.
