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  Oracle Tips by Burleson

Oracle blocksize and performance

Small block size

Tables containing small rows that are accessed randomly should be placed into tablespaces with smaller block sizes. This way, more of the buffer RAM remains available to store rows from other tables that are referenced frequently.

Larger block size

Larger block sizes are suitable for indexes, row-ordered tables, single-table clusters, and tables with frequent full-table scans. In this way, a single I/O will retrieve many related rows, and future requests for related rows will already be available in the data buffer.

Some objects that may benefit from a larger blocksize (16K or 32K) include:

  • Most indexes (because of the serial nature of index range scans)

  • Large tables that are the target of full table scans

  • Tables with large object (BLOB, CLOB, etc.) data

  • Tables with large row sizes that might blossom into chained/migrated rows

  • Temporary tablespaces used for sorting

The simple goal is to maximize the amount of RAM available to the data buffers by setting the block size according to the amount of I/O the table or index sees. Smaller block sizes are appropriate for randomly accessed small rows, while larger blocks are more suitable for rows sequentially accessed.

To illustrate, suppose a query retrieves 100 random 80 byte rows from Oracle. Since the rows are randomly accessed, we can safely assume that no two rows exist on the same block, implying that it is necessary to read 100 blocks to fulfill the task.

If the blocks are sized 16K, the db_16k_cache_size buffer will need 16 MB (16K * 100) of RAM. If the blocks are instead 2K, we only need 2 MB of RAM in the buffer for the 100 I/Os. Using the smaller block size would save 14 MB of RAM for this query alone, RAM that will be available elsewhere to hold other data.


The above text is an excerpt from "Creating a Self Tuning Oracle Database", by Rampant TechPress.  It is only $9.95 and all scripts in this tips can be immediately downloaded.

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