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  Oracle Tips by Burleson

Oracle physical I/O Myths, exposed

There are several myths of Oracle physical I/O that must be exposed at this point:

  • All Oracle databases are I/O-bound:  Untrue.  Databases with a reasonable data buffer cache size and a small working set will usually be constrained by CPU or network latency.

  • The Data Buffer Hit Ratio (DBHR) will yield caching efficiency:  Untrue, except in cases of a super small cache.  The DBHR only measures the propensity that a data block will be in the buffer on the second I/O request.

  • Only faster disk can remove I/O bottlenecks:  Untrue.  This is a common myth.  There are other non-RAM approaches to reducing disk I/O for Oracle databases:

  • Adjusting optimizer_mode: Oracle will generate widely differing SQL execution plans depending on the optimizer mode.

  • Re-analyze SQL Optimizer statistics:  Using better quality CBO statistics with dbms_stats and adding column histograms can make a huge difference in disk activity.

  • Adjusting Oracle  parameters: Resetting the optimizer_index_cost_adj and optimizer_index_caching parameters can affect physical reads

  • Improve clustering_factor for index range scans: Manually resequence table rows to improve clustering_factor, sometimes using single-table clusters, can reduce disk I/O.

  • Use Materialized Views: Systems with batch only updates may greatly benefit from Materialized Views to pre-join tables.  Of course, the overhead of refresh commit is too great for high update systems.


The above book excerpt is from:

Oracle RAC & Grid Tuning with Solid State Disk
Expert Secrets for High Performance Clustered Grid Computing

ISBN: 0-9761573-5-7
Mike Ault, Donald K. Burleson

http://www.rampant-books.com/book_2005_2_rac_ssd_tuning.htm  

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