The database buffer cache holds copies of the data blocks read from the data files. The term data block is used to describe a block containing table data, index data, clustered data. Basically it is a block that contains data. All user processes concurrently connected to the instance share access to the database buffer cache. The database buffer cache
is logically segmented into multiple sets which reduces contention on multiprocessor systems.
This area of the SGA contains only the buffers themselves and not their control structures. For each buffer there is a corresponding buffer header in the variable area of the SGA. From Oracle8 release onwards the buffer cache contains three buffer pools for different type of data usage.
- KEEP, and
The database block buffers act as the holding area for data used by the user and DBWR processes. Any data that gets to the user from the database files, or data that goes into the database files from the user or other processes, passes through the database block buffers unless direct insert or direct read is used for data loading, sorting, or hashing operations. The database block buffers in releases prior to Oracle9i had to be of uniform size, 2, 4, 8, 16, or for 64 bit OS, the 32 kilobytes in size. From Oracle9i onwards, the database has a default database cache block size, but other sizes (2K, 4K, 8K, 16K, or 32K) can also be specified. Based on the tablespace size, appropriate Cache is employed to retrieve and manage the buffers in the SGA.
In the RAC database system, the database block buffers from each of the participating instance, through the process of cache fusion
, are merged to form a logical database block buffer area that becomes many times larger than could be supported in a single instance.