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Database hit ratios are the most commonly used measures of performance. These include the buffer cache hit ratio, the library cache and dictionary cache hit ratios, the latch hit ratio, and the disk sort ratios. These hit ratios don t indicate how well your system is performing. They re broad indicators of proper SGA allocation, and they may be high even when the system as a whole is performing poorly. The thing to remember is that the hit ratios only measure such things as how physical reads compare with logical reads, and how much of the time a parsed version of a statement is found in memory. As to whether the statements themselves are efficient or not, the hit ratios can t tell you anything. When your system is slow due to bottlenecks, the hit ratios are of little help, and you should turn to a careful study of wait statistics instead.
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Even if you have a 99.99 percent buffer cache hit ratio, you may still have major inefficiencies in your application. What if you have an extremely high number of unnecessary logical reads This makes your buffer cache hit ratio look good, as that hit ratio is defined as physical reads over the sum of logical reads. Although you may think your application should run faster because you re doing most of your reads from memory instead of disk, this may well not happen. The reason is that even if you re doing logical reads, you re still burning up the CPU units to do the unnecessary logical reads. In essence, by focusing zealously on the buffer cache hit ratio to relieve the I/O subsystem, you could be an unwitting party to a CPU usage problem. Please read Cary Millsap s interesting article, Why You Should Focus on LIOs Instead of PIOs (http://www.hotsos.com/e-library/abstract.php id=7), which explains why a high logical I/O level could be a major problem.
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When faced with a slow-performing database or a demand for shorter response times, Oracle DBAs have traditionally looked to increase their database hit ratios and tune the database by adjusting
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CHAPTER 20 PERFOR MAN CE TUNING: TUNING THE INSTA NCE
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a host of initialization parameters (such as SGA allocations). More recently, there s been awareness that the key area to focus on is clearing up database bottlenecks that contribute to a higher response time. The total response time for a query is the time Oracle takes to execute it, plus the time the process spends waiting for resources such as latches, data buffers, and so on. For a database instance to perform well, ideally your application should spend little time waiting for access to critical resources. Let s now turn to examining the critical wait events in your database, which can be real showstoppers on a busy day in a production instance.
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When your users complain that the database is crawling and they can t get their queries returned fast enough, there s no use in your protesting that your database is showing high hit ratios for the shared pool and the buffer cache (and the large pool and redo log buffer as well). If the users are waiting for long periods of time to complete their tasks, then the response time will be slow, and you can t say that the database is performing well, the high hit ratios notwithstanding.
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Note For an interesting review of the Oracle wait analysis (the wait interface), please read one of the early papers in this area, titled Yet Another Performance Profiling Method (or YAPP-Method), by Anjo Kolk, Shari Yamaguchi, and Jim Viscusi. It s available at the OraPerf web site at http://www.oraperf.com (a free registration is required).
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Once it starts executing a SQL statement, an Oracle process doesn t always get to work on the execution of the statement without any interruptions. Often, the process has to pause or wait for some resource to be released before it can continue its execution. Thus, an active Oracle process is doing one of the following at any given time: The process is executing the SQL statement. The process is waiting for something (for example, a resource such as a database buffer or a latch). It could be waiting for an action such as a write to the buffer cache to complete. That s why the response time the total time taken by Oracle to finish work is correctly defined as follows: response time = service time + wait time When you track the total time taken by a transaction to complete, you may find that only part of that time was taken up by the Oracle server to actually do something. The rest of the time, the server may have been waiting for some resource to be freed up or waiting for a request to do something. This busy resource may be a slow log writer or a database writer process. The wait event may also be due to unavailable buffers or latches. The wait events in the V$SYSTEM_EVENT view (instancelevel waits) and the V$SESSION_EVENT view (session-level waits) tell you what the wait time is due to (full table scans, high number of library cache latches, and so on). Not only do the wait events tell you what the wait time in the database instance is due to, but they also tell you a lot about bottlenecks in the network and the application.
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