However, SGA auto tuning can also be disabled under certain circumstances, such as:
When MEMORY_TARGET or MEMORY_MAX_TARGET is set to zero: These parameters control the total amount of memory allocated to both the SGA and the PGA (Program Global Area), which is a group of memory structures that store information for each individual process in an Oracle database instance. When these parameters are set to zero, it means that Oracle is not managing the memory allocation, and the user has to manually set the sizes of the SGA and PGA components using other parameters, such as SGA_TARGET, PGA_AGGREGATE_TARGET, DB_CACHE_SIZE, SHARED_POOL_SIZE, etc. This disables SGA auto tuning and prevents Oracle from dynamically adjusting the SGA sizes .
When an error occurs during SGA auto tuning: Sometimes, Oracle may encounter an error while trying to resize a component of the SGA, such as ORA-00604 (error occurred at recursive SQL level), ORA-04031 (unable to allocate shared memory), or ORA-4031 (out of process memory). This may happen due to insufficient memory available, memory fragmentation, or memory leaks. When this happens, Oracle may disable SGA auto tuning and log a message in the alert log file, such as "AUTO SGA: Disabling background sga tuning due to err 604" or "AUTO SGA: Disabling background sga auto-tuning". This prevents Oracle from further resizing the SGA components until the error is resolved or the database is restarted.
To enable SGA auto tuning, the user has to either set MEMORY_TARGET or MEMORY_MAX_TARGET to a non-zero value, or fix the error that caused SGA auto tuning to be disabled. Alternatively, the user can also use manual tuning methods to optimize the SGA sizes according to their needs and preferences.
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