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Oracle 8 Tips  

by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Sizing Oracle Data Blocks

It is very ironic that the Oracle developer must choose a blocksize when the data warehouse is initially created--a time when knowledge of system performance is very limited. While it is possible to use the Oracle import/export utility to change block sizes, too little attention is given to the proper sizing of database blocks. The physical block size is set with the DB_BLOCK_SIZE parameter in the init.ora file. While the default is to have 4 K blocksizes, many Oracle developers choose at least 8 K blocksizes for large, distributed data warehouses. Some Remote DBAs believe that 16 K is the best blocksize, even for OLTP systems that seldom perform full-table scans. Depending upon the host platform and operating system, Oracle block sizes may be set from 2 K up to 32 K. The Oracle OS manual will provide the acceptable ranges for your operating system, but the generally accepted wisdom is to create your database blocks as large as your operating system will allow. Remember, minimizing disk I/O is one of the most important factors in data warehouse tuning, and the more data that can be read in a single I/O, the faster your warehouse will perform.

Disk I/O is the single most expensive and time-consuming operation within an Oracle database. As such, the more data that can be read in a single I/O, the faster the performance of the Oracle database. This principle is especially true for databases that have many reports that read the entire contents of a table. For systems that read random single rows from the database, blocksize is not as important--especially with database clusters. An Oracle cluster is a mechanism whereby an owner row will reside on the same database block as its subordinate rows in other tables. For example, if we cluster order rows on the same block as their customer owners, Oracle will only need to perform a single I/O to retrieve the customer and all of the order rows. Of course, in a distributed database where joins take place across different Oracle instances, clustering cannot be used. The additional I/O will be required to read the rows individually.

Bear in mind that increasing the block size of an Oracle database will also affect the number of blocks that can be cached in the buffer pool. For example, if we set the DB_BLOCK_BUFFERS init.ora parameter to 8 MB, Oracle will be able to cache 1000 4K blocks, but only 500 8K blocks.

This is an excerpt from "High Performance Data Warehousing", copyright 1997. To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:



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