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Buffer reads are much faster than reads from disk The all-important principle in appropriately sizing the buffer cache is summarized in the phrase touch as few blocks as possible, since disk I/Os necessary for reading data from Oracle blocks on disk are more time-consuming than reading the data from the SGA This is why the buffer cache hit ratio, which measures the percentage of time users accessed the data they needed from the buffer cache (rather than requiring a disk read), is such an important indicator of performance of the Oracle instance You derive the buffer cache hit ratio as follows: hit rate = (1 (physical reads)/(logical reads)) * 100 In this calculation, the physical and logical reads (reads from disk and from memory, respectively) are accumulated from the start of the Oracle instance.

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Win32 SEH exceptions can also be caught as .NET exceptions. In the following code, a managed function (main) calls a native function (f), which throws the SEH exception EXCEPTION_INT_DIVIDE_BY_ZERO. In main, this exception is caught in a catch block that handles exceptions of type System::Exception^. // ExceptionHandling2.cpp // compile with "cl /clr ExceptionHandling2.cpp"

So if you calculate the ratio on Monday morning after a restart on Sunday night, it will show a very low hit ratio As the week progresses, the hit ratio could increase dramatically, because as more read requests come in, Oracle satisfies them with the data that is already in memory Unfortunately, Oracle does not give you any reliable rules or guidelines to indicate how much memory you should allocate for your buffer cache ratio or the SGA Some trial and error with data loads should give you a good idea about the right size In 22, I present much more information on the proper tuning of the database buffer cache A high buffer cache hit ratio doesn t always correlate with superior database performance It is entirely possible for your database to have a very high hit ratio say, in the high 90s and still have a performance problem.

For example, even if your total logical reads and hit ratio are high, your SQL queries could still be inefficient..

It is possible to retrieve most of the information in the files located in the /proc directory tree using other methods. Programs such as hostname, netstat, top, ps, uptime, and others get their data from /proc. In some cases those programs are easier to use because they condense information and present it in a more readable form. In other cases getting the data directly from the files residing in /proc is easier or faster. This is the situation with our script: there is no data to format and there is no unneeded functionality that may impact performance. The part of the script that gets the load average provides a good illustration of this; the data in /proc/loadavg is easier to process than data from other sources. Load-average

The shared pool is a very important part of the Oracle SGA, and sizing it appropriately for your instance will help avoid several types of Oracle instance bottlenecks. Unlike the database buffer cache, which holds actual data blocks, the shared pool holds executable PL/SQL code and SQL statements, as well as information regarding the data dictionary tables. The data dictionary is a set of key tables that Oracle maintains, and it contains crucial metadata about the database tables, users, privileges, and so forth. Proper sizing of the shared pool area benefits you in a couple of ways. First, your response times will be better because you re reducing processing time if you don t have to recompile the same Oracle code each time a user executes a query, you save time. Oracle will reuse the previously compiled code if it encounters the same code again. Second, more users can use the system because the reuse of code makes it possible for the database to serve more users with the same resources. Both the I/O rates and the CPU usage will diminish when your database uses its shared pool memory effectively. The following sections discuss the library cache and the data dictionary cache, both of which are components of the shared pool.