oracle study session2

上一篇 / 下一篇  2010-03-23 22:19:31 / 个人分类:Oracle

Program Global Area (PGA)
The Program Global AreaorProcess Global Area (PGA)is a memory region that contains data and control information for a single server process or a single background process. The PGA is allocated when a process is created and deallocated when the process is terminated. In contrast to the SGA, which is shared by several processes, the PGA is an area that is used by only one process.
Oracle takes advantage of various types of processes:
1)User process: Started at the time a database user requests connection to the Oracle server
2)Server process: Connects to the Oracle instance and is started when a user establishes a session
3)Background processes: Started when an Oracle instance is started
User process:use user process(though a tool using UPI) to start a server process.
Server process:communicate with oracle server using OPI
Background processes:
Database Writer: Writes dirty buffers to the data files
Log Writer: Records database changes for recovery purposes
System Monitor: Is responsible for instance recovery
Process Monitor: Cleans up after failed processes
Checkpoint: Assists with writing to the data file headers
Every time an instance is started, a System Global Area (SGA) is allocated and Oracle background processes are started.
Logical hierarchy :Database->tablespace->segment->extend->block
Tablespace consist of some data files, segment can span data files, but extend can’t





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