SGA (System Global Area)

Each Oracle instance uses a System Global Area or SGA — a shared-memory area — to store its data and control-information. ^[5]

Each Oracle instance allocates itself an SGA when it starts and de-allocates it at shut-down time. The information in the SGA consists of the following elements, each of which has a fixed size, established at instance startup:

                   the database buffer cache: this stores the most recently-used data blocks. These blocks can contain modified data not yet written to disk (sometimes known as "dirty blocks"), unmodified blocks, or blocks written to disk since modification (sometimes known as clean blocks). Because the buffer cache keeps blocks based on a most-recently-used algorithm, the most active buffers stay in memory to reduce I/O and to improve performance.

                   the redo log buffer: this stores redo entries — a log of changes made to the database. The instance writes redo log buffers to the redo log as quickly and efficiently as possible. The redo log aids in instance recovery in the event of a system failure.

                   the shared pool: this area of the SGA stores shared-memory structures such as shared SQL areas in the library cache and internal information in the data dictionary. An insufficient amount of memory allocated to the shared pool can cause performance degradation.