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Title: Why a 99%+ Database Buffer Cache Hit Ratio is Not Ok
Author: Cary Millsap
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Abstract: Many tuning professionals and textbook authors sell advice encouraging their customers to enjoy the performance virtues of Oracle database buffer cache hit ratios that approach 100%. However, database buffer cache hit ratio is not a reliable system performance metric. Buffer cache hit ratios above 99% usually indicate particularly serious SQL inefficiencies. Repairs of these inefficiencies often yield 100x or greater performance improvements, yet the optimizations result in reduced database buffer cache hit ratios. This paper shows why a 99%+ buffer cache hit ratio almost always signals performance inefficiencies, including real-life examples. It shows how to detect the inefficiencies and how to repair them. It concludes with recommendations about what metrics the performance analyst should use instead of the venerable but unreliable database buffer cache hit ratio.
Document type: article
Updated: Wed 15 Oct 2003 (10.8 years ago)
File size: 238,824 bytes
MD5 checksum: aca9d3856d0eaca2362e9f3edbca2160
File type: .pdf
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