Donald K. Burleson
Oracle Data Buffer Caches
Oracle allows the SGA to include up to seven RAM regions for caching
incoming disk data blocks. They are the KEEP pool, RECYCLE pool, and
DEFAULT pool, plus instantiated pools for 2K, 4K, 8K, 16K, and 32K data
blocks (Figure 1.2).
Please note that the KEEP and RECYCLE regions were subsets of the DEFAULT
pool prior to Oracle, but are separate RAM regions in Oracle and
beyond. Remember, once one of the sized pools (2K-32K) is assigned to the
DEFAULT pool, its size cannot be assigned to another buffer. The sized data
buffers correspond to tablespaces, and a 32K block tablespace will load
into the 32K data buffer.
Prior to Oracle, the KEEP and RECYCLE pools were a sub-set of the DEFAULT
pool. The DEFAULT pool is created by the db_cache_size parameter in
Oracle, while former versions use the db_block_buffers parameter.
Starting with Oracle, the KEEP and RECYCLE pools are allocated, in
addition to the db_cache_size. Also, note that Oracle allows the use of
multiple block regions, ranging in size from 2K-32K, each with its own
distinct buffer cache.
The buffer caches are in memory areas of the SGA where incoming Oracle data
blocks are kept. These data buffer sizes can have a tremendous impact on
overall system performance. There is a greater probability that data from
past transactions will still be in a large buffer, minimizing expensive
physical disk I/O in Oracle. RAM access is at least two thousand times
faster than disk access.
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