Oracle Database Architecture
Scope
This domain tests the instance/database split, SGA and PGA, background processes, and the logical-to-physical storage map in Oracle 19c. Questions often name a symptom and ask which component owns it.
Core rules
- The instance is memory plus background processes; the database is the physical files. In a non-RAC database one instance mounts one database; in RAC one database is served by multiple instances.
- SGA includes the buffer cache, shared pool (library cache and row cache), redo log buffer, large pool, Java pool, and fixed SGA. PGA is private server-process memory for sorts, hash joins, and session variables.
- DBWn writes dirty blocks; LGWR writes redo and commits; CKPT coordinates checkpoints and updates datafile headers; SMON performs instance recovery and segment cleanup; PMON registers services and cleans up failed sessions; ARCn archives redo when ARCHIVELOG is enabled.
- Logical hierarchy: tablespace, segment, extent, Oracle block. Physical files: datafiles, online redo log files, control file, server parameter file, password file, archived redo logs.
Worked example
A session updates a row. The server process reads the block into the buffer cache, creates undo in the undo tablespace, changes the block, and writes redo change vectors to the log buffer. LGWR writes redo at commit; DBWn writes the dirty block later. If the instance crashes before DBWn writes, SMON uses redo and undo to roll forward and roll back.
Traps
A checkpoint does not write every dirty block; CKPT coordinates and updates headers. PGA is not part of the SGA. The control file is not the parameter file. A commit is not a datafile write.