ORACLE后台进程

135.Note the functionalities of various background processes:
1: Perform recovery at instance startup.
2: Free the resources used by a user process when it fails.
3: Cleanup the database buffer cache when a process fails.
4: Dynamically register database services with listeners.
5: Monitor sessions for idle session timeout.
6: Cleanup unused temporary segments.
7: Record the checkpoint information in control file.
Which option has the correct functionalities listed for a background process?
A. Checkpoint (CKPT): 1, 2, 5
B. System Monitor (SMON): 1, 6
C. Process Monitor (PMON): 4, 6, 7
D. Database Writer (DBWR): 1, 3, 4

 

SMON一般都是系统级的，PMON用户级，结合oracle文档可以看出B才是正确选项。

System monitor (SMON)：

The system monitor performs recovery when a failed instance starts up again. In an Oracle Real Application Clusters database, the SMON process of one instance can perform instance recovery for other instances that have failed. SMON also cleans up temporary segments that are no longer in use and recovers terminated transactions skipped during system failure and instance recovery because of file-read or offline errors. These transactions are eventually recovered by SMON when the tablespace or file is brought back online

Process monitor (PMON)：

The process monitor performs process recovery when a user process fails. PMON is responsible for cleaning up the cache and freeing resources that the process was using. PMON also checks on the dispatcher processes (described later in this table) and server processes and restarts them if they have failed.

Database writer (DBWn or BWnn)：

The database writer writes modified blocks from the database buffer cache to the data files. Oracle Database allows a maximum of 100 database writer processes. The names of the first 36 database writer processes are DBW0-DBW9 and DBWa-DBWz. The names of the 37th through 100th database writer processes are BW36-BW99.

The DB_WRITER_PROCESSES initialization parameter specifies the number of database writer processes. The database selects an appropriate default setting for this initialization parameter or adjusts a user-specified setting based on the number of CPUs and the number of processor groups.

Log writer (LGWR)：

The log writer process writes redo log entries to disk. Redo log entries are generated in the redo log buffer of the system global area (SGA). LGWR writes the redo log entries sequentially into a redo log file. If the database has a multiplexed redo log, then LGWR writes the redo log entries to a group of redo log files. See Managing the Redo Log for information about the log writer process.

Checkpoint (CKPT)：

At specific times, all modified database buffers in the system global area are written to the data files by DBWn. This event is called a checkpoint. The checkpoint process is responsible for signalling DBWn at checkpoints and updating all the data files and control files of the database to indicate the most recent checkpoint.