Oracle体系结构及备份（十四）——bg-dbwr

傷城~ 2022-01-22 10:53 290阅读 0赞

### 一 什么是DBWR进程 ###

DBWR，数据库写进程。

The database writer writes modified blocksfrom the database buffer cache to the datafiles. Oracle Database allows amaximum of 20 database writer processes (DBW0-DBW9 and DBWa-DBWj). TheDB\_WRITER\_PROCESSES initialization parameter specifies the number of DBWnprocesses. The database selects an appropriate default setting for thisinitialization parameter or adjusts a user-specified setting based on thenumber of CPUs and the number of processor groups.

![20130615164949875][]

DBWn写的条件：

发生检查点

脏缓存到达限制

没有自由的缓存

超时发生

RAC Ping请求

表空间离线

表空间只读

表被删除或者截断

开始备份表空间

### 二 操作示例 ###

[oracle@localhost 桌面]$ sqlplus / as sysdba;
    
    SQL*Plus: Release 10.2.0.1.0 - Production on Thu Jun 6 11:05:47 2013
    
    Copyright (c) 1982, 2005, Oracle.  All rights reserved.
    
    Connected to an idle instance.
    
    SQL> startup
    ORACLE instance started.
    
    Total System Global Area  838860800 bytes
    Fixed Size		    1222192 bytes
    Variable Size		  775948752 bytes
    Database Buffers	   58720256 bytes
    Redo Buffers		    2969600 bytes
    Database mounted.
    
    SQL> SELECT name FROM v$bgprocess WHERE paddr<>'00'; 
    
    NAME
    -----
    PMON
    PSP0
    MMAN
    DBW0
    LGWR
    CKPT
    SMON
    RECO
    CJQ0
    QMNC
    MMON
    
    NAME
    -----
    MMNL
    
    12 rows selected.
    
    SQL> alter system set db_writer_processes=2 scope=spfile;
    
    System altered.
    
    SQL> startup
    ORA-01081: cannot start already-running ORACLE - shut it down first
    SQL> startup force
    ORACLE instance started.
    
    Total System Global Area  838860800 bytes
    Fixed Size		    1222192 bytes
    Variable Size		  775948752 bytes
    Database Buffers	   58720256 bytes
    Redo Buffers		    2969600 bytes
    Database mounted.
    
    SQL> SELECT name FROM v$bgprocess where paddr<>'00';
    
    NAME
    -----
    PMON
    PSP0
    MMAN
    DBW0
    DBW1
    LGWR
    CKPT
    SMON
    RECO
    CJQ0
    QMNC
    
    NAME
    -----
    MMON
    MMNL
    
    13 rows selected.
    SQL> alter system set db_writer_processes=12 scope=spfile;
    
    System altered.
    
    SQL> startup force
    ORACLE instance started.
    
    Total System Global Area  838860800 bytes
    Fixed Size		    1222192 bytes
    Variable Size		  784337360 bytes
    Database Buffers	   50331648 bytes
    Redo Buffers		    2969600 bytes
    Database mounted.
    Database opened.
    SQL> select name from v$bgprocess where paddr<>'00';
    
    NAME
    -----
    PMON
    PSP0
    MMAN
    DBW0
    DBW1
    DBW2
    DBW3
    DBW4
    DBW5
    DBW6
    DBW7
    
    NAME
    -----
    DBW8
    DBW9
    DBWa
    DBWb
    LGWR
    CKPT
    SMON
    RECO
    CJQ0
    QMNC
    MMON
    
    NAME
    -----
    MMNL
    
    23 rows selected.
    
    SQL> alter system set db_writer_processes=24 scope=spfile;
    
    System altered.
    
    SQL> startup force
    ORACLE instance started.
    
    Total System Global Area  838860800 bytes
    Fixed Size		    1222192 bytes
    Variable Size		  788531664 bytes
    Database Buffers	   46137344 bytes
    Redo Buffers		    2969600 bytes
    Database mounted.
    Database opened.
    SQL> select name from v$bgprocess where paddr<>'00';
    
    NAME
    -----
    PMON
    PSP0
    MMAN
    DBW0
    DBW1
    DBW2
    DBW3
    DBW4
    DBW5
    DBW6
    DBW7
    
    NAME
    -----
    DBW8
    DBW9
    DBWa
    DBWb
    DBWc
    DBWd
    DBWe
    DBWf
    DBWg
    DBWh
    DBWi
    
    NAME
    -----
    DBWj
    LGWR
    CKPT
    SMON
    RECO
    CJQ0
    QMNC
    MMON
    MMNL
    
    31 rows selected.
    SQL> select name from v$bgprocess where paddr<>'00' and name like 'DB%';
    
    NAME
    -----
    DBW0
    DBW1
    DBW2
    DBW3
    DBW4
    DBW5
    DBW6
    DBW7
    DBW8
    DBW9
    DBWa
    
    NAME
    -----
    DBWb
    DBWc
    DBWd
    DBWe
    DBWf
    DBWg
    DBWh
    DBWi
    DBWj
    
    20 rows selected.
    --32bit系统：dbw0----dbw9(10个)
    --64bit系统：dbwa----dbwj(20个)
    SQL> select count(name) from v$bgprocess where paddr<>'00' and name like 'DB%';
    
    COUNT(NAME)
    -----------
    	 20
    
    SQL> alter system set db_writer_processes=1 scope=spfile;
    
    System altered.
    
    SQL> startup force
    ORACLE instance started.
    
    Total System Global Area  838860800 bytes
    Fixed Size		    1222192 bytes
    Variable Size		  788531664 bytes
    Database Buffers	   46137344 bytes
    Redo Buffers		    2969600 bytes
    Database mounted.
    Database opened.

### 三 总结 ###

1.DBWR，数据库写进程。

2.DBWn写的条件：发生检查点、脏缓存到达限制、没有自由的缓存、超时发生、RAC Ping请求、表空间离线、表空间只读、表被删除或者截断、开始备份表空间。

3. db\_writer\_processes控制DBWn进程的最大数目，可以修改，但不能超过最大值。

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[20130615164949875]: /images/20220122/f14d664c68aa46b680fa13c99262f9d6.png