The Oracle Instance
The memory structures and server processes that do the work in the database.
The System Global Area is a shared block of memory for Oracle’s use. At a minimum contains:
– Redo Log buffer: short term storage for redo information so it can be written to the redo logs.
– Shared Pool: further broken down into the library cache, which holds recently parsed and executed code, and the data dictionary cache, which store recently used object definitions.
– Database buffer cache: Oracle’s work area for executing SQL.
The instance also houses the processes
– System Monitor: Opening database, maintain connection between instance and database
– Database Writer: writes to the database files (writes as little as possible. Minimizing disk I/O for performance)
– Process Monitor: Monitors user sessions
– Log Writer: writes to the redo logs (writes as close to real time as possible. Ideally save all changes.)
– Checkpoint: ensure instance is synchronized with the database from time to time.
– Archiver: writes archived redo logs
The Oracle Database
The database refers to the physical files on the os that contain the data and data dictionary. At the minimum the database requires datafiles, control files, and redo logs.
Parameter File: Holds the parameters to start the instance
Password File: Encrypted file that holds the sysdba password. Allows sys to log on regardless of the state of the database.
Datafiles: Core of the database, the files that hold the data.
Control Files: Holds all the parameters used to connect the instance and database. For Example, pointers to the rest of the database (redo logs, datafiles…) and various data to maintain database integrity (scn and timestamp). Often multiplexed to allow recovery from file corruption.
Redo Log: maintains all changes made to the data over a given period of time or until the log is full. Often multiplexed to allow recovery from file corruption.
Archived Redo Logs: Copies of filled redo logs kept for recovery purposes.
(Special thanks to “Josh” for this information)