I. Database Structure
Ii. Oracle Memory Structure
3. Process Structure
1. User process: started when the database user requests to connect to the Oracle server
2. server process: you can connect to an Oracle instance, which starts when a user establishes a session.
3. background process: started when Oracle instance is restarted
Iv. Oracle instance Management
1. System Monitor (SMON): Execute the crash recovery task when the instance is started after a fault occurs.
2. Process Monitor (PMON): executes the process cleaning task when the user's process fails.
3. Database write process (DBWn): writes the modified block from the database buffer cache to the data file on the disk.
4. Check Point (CKPT): indicates the latest check point by updating all data files and control files of the database.
5. Log writing process (LGWR): writes redo log entries to the disk.
6. archiving process (ARCn): Copies the redo log files to the archive storage when a log switch occurs.
V. High-speed cache of server processes and database Buffers
When processing a query, the Oracle server process searches for all data blocks required by the database buffer cache. If no data block is found in the cache, the server process reads the data block from the data file and adds a copy to the cache. Because subsequent requests to the same data block may find this data block in the memory, these requests may not require physical read operations. The Oracle server uses recently used algorithms to free up recently inaccessible buffers, so as to provide space for new data blocks in the database buffer cache.
The buffer in the buffer cache may be in one of the following four States:
① Connected: prevents multiple sessions from simultaneously writing to the same data block. At this time, other sessions are waiting to access the block.
② Clean: the buffer zone is now disconnected. If the current content (data block) is not referenced again, the buffer zone is a candidate item that can be released immediately. At this time, either the buffer content has been synchronized with the data block content stored in the disk, or the buffer contains the consistent read (CR) snapshot of the block.
③ Idle or unused: The instance has just started and the buffer zone is empty. This status is very similar to the "clean" status, except that the buffer is not used yet.
④ Gray data: the buffer is no longer in the connected status, but the content (data block) has been changed. DBWn must first refresh the content to the disk to free up the buffer.