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Tuesday, November 15, 2011

Oracle Parameter related to Database Buffer Cache


DB_CACHE_SIZE
This parameter sets the size of the default buffer pool for those buffers that have the primary block size (this is the block size defined by DB_BLOCK_SIZE). For example, we can use a number like 1,024MB.
Default value: If we're using the SGA_TARGET parameter, the default is 0. If we aren't using
SGA_TARGET, it’s the greater of 48MB or 4MB * number of CPUs * granule size.
Parameter type: Dynamic. It can be modified with the ALTER SYSTEM command.

DB_KEEP_CACHE_SIZE
The normal behavior of the buffer pool is to treat all the objects placed in it equally. That is, any object will remain in the pool as long as free memory is available in the buffer cache. Objects are removed (aged out) only when there is no free space. When this happens, the oldest unused objects sitting in memory are removed to make space for new objects.
The use of two specialized buffer pools—the keep pool and the recycle pool—allows us to specify at object-creation time how we want the buffer pool to treat certain objects. For example, if we know that certain objects don’t really need to be in memory for a long time, we can assign them to a recycle pool, which removes the objects right after they’re used. In contrast, the keep pool always retains an object in memory if it’s created with the KEEP option. The DB_KEEP_CACHE_SIZE parameter specifies the size of the keep pool, and it’s set as follows: DB_KEEP_CACHE_SIZE = 500MB
Default value: 0; by default, this parameter is not configured.
Parameter type: Dynamic. It can be changed by using the ALTER SYSTEM command.

DB_RECYCLE_CACHE_SIZE
The DB_RECYCLE_CACHE_SIZE parameter specifies the size of the recycle pool in the buffer cache. Oracle removes objects from this pool as soon as the objects are used. The parameter is set as follows: DB_RECYCLE_CACHE_SIZE = 200MB
Default value: 0; by default, this parameter is not configured.
Parameter type: Dynamic. It can be changed by using the ALTER SYSTEM command.

DB_nK_CACHE_SIZE
If we prefer to use nonstandard-sized buffer caches, we need to specify the DB_nK_CACHE_SIZE parameter for each, as in the following two examples:
DB_4K_CACHE_SIZE=2048MB
DB_8K_CACHE_SIZE=4096MB
The values for n that can be used in this parameter are 2, 4, 8, 16, or 32.
Default value: 0
Parameter type: Dynamic. You can change this parameter’s value with the ALTER SYSTEM command.

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