小布老师oracle视频讲座笔记（二）

Oracle Process Structure
Oracle takes advantage of various types of processes:
　—User process: Started at the time a database user requests connection to the Oracle server
  —Server process: Connects to the Oracle instance and is started when a user establishes a session
—Background processes: Started when an Oracle instance is started
User Process: A progrm that requests interaction with the Oracle server, Must first establish a connection, Does not interact directly with the Oracle server.
Server Process: A program that directly interacts with the Oracle Server
  —Fulfills calls generated and returns results
—Can be deficated or shared server
IPC: Inter Process Communication, 包括共享内存、队列、信号量等几种形式。
Background Process
Maintains and enforces relationships between physical and memory structures:
   —Mandatory background processes
      DBWn PMON CKPT LGWR SMON
   —Optional background processes
ARCn  LMDn  QMNn  CJQ0  LMON  RECO  Dnnn   LMS  Snnn
LCKn  Pnnn
DBWn(Database Writer), DBWn writes when:
   —Checkpoint occurs
   —Dirty buffers reach threshold
   —There are no free buffers
   —Timeout occurs
   —RAC ping request is made
   —Tablespace OFFLINE
   —Tablespace READ ONLY
   —Table DROP or TRUNCATE
   —Tablespace BEGIN BACKUP
Log Witer(LGWN), LGWR writes:
   —At commit
   —When one-third full
   —When there is 1MB of redo
   --Every three seconds
   --Before DBWn writes
System Monitor(SMON)
Responsibilities
   —Instance recovery
   —Rolls forward changes in noline redo log files
   —Opens database fo