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Oracle Tips by Burleson 

Reducing disk I/O in UNIX

When using multiple blocksizes, the DBA is given additional control over the UNIX disk I/O sub-system.

The Oracle DBA understands the basic truths about UNIX disk I/O:

* Disk I/O is the largest component of Oracle response time. A reduction in disk I/O will always result in faster performance for that task.
* Creating larger data blocks allows Oracle to access more row data in a single I/O. Oracle9i supports multiple block sizes, and the Oracle9i DBA can move tables easily from one blocksize to a tablespace with another blocksize, thereby load-balancing disk I/O.
* The Oracle DBA has tools (in-place table reorgs, CTAS with order by) to allow easy table reorganization, and the DBA can use these tools to re-sequence table rows in the same order as the primary index to reduce disk I/O on index range scans. For more information, see Turning the Tables on Disk I/O, January 2000, at Oracle Magazine online (

So, how do we reduce disk I/O in UNIX? There are three generally accepted techniques for the DBA to reduce disk I/O:

* Tune SQL statements to retrieve data with a minimum of disk I/O - This is generally performed by finding large-table full-table scans and replacing the full-table scan with an index scan.
* Change the Oracle SGA - When we increase the shared_pool, large_pool, or db_cache_size, the resulting performance improvement is related to the reduction in disk I/O.
* Reorganize tables to reduce disk I/O – This is done by selectively moving tables to tablespaces with a different blocksize, and re-sequencing table rows into the primary key order.

The above is an excerpt from the "Oracle9i UNIX Administration Handbook" by Oracle press, authored by Donald K. Burleson.


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