12C开始oracle实现了SCALABLE LGWR多进程并行写redo log

jiawang

在oracle 12.1版本之前,oracle日志写进程LGWR为单进程,oracle 12.1开始实现了SCALABLE LGWR多进程，采用多个LGWR协同对redo log日志进行并行写入,这是Oracle数据库内部最重要的改进，解决了最后的并发瓶颈点



SCALABLE LGWR主要受到隐藏参数_use_single_log_writer和_max_outstanding_log_writes的控制

_use_single_log_writer参数主要有三个可选值 true,false,adaptive,默认值为adaptive
false,adaptive:如果CPU个数大于一个则会有多个lg0n进程
true:不生成多个lg0n进程，如同12.1之前那样仅有单个LGWR

_max_outstanding_log_writes参数默认值为2

[oracle@12C ~]$ ps -ef |grep ora_lg |grep -v grep oracle     6305      1  0 14:53 ?        00:00:00 ora_lgwr_OCP oracle     6311      1  0 14:53 ?        00:00:00 ora_lg00_OCP oracle     6315      1  0 14:53 ?        00:00:00 ora_lg01_OCP SQL> select VERSION from v$instance; VERSION ----------------- 12.1.0.2.0 SQL>select a.ksppinm name, b.ksppstvl value, a.ksppdesc description from x$ksppi a, x$ksppcv b where a.indx = b.indx and a.ksppinm in ('_max_outstanding_log_writes','_use_single_log_writer'); NAME                           VALUE      DESCRIPTION ------------------------------ ---------- -------------------------------------------------- _use_single_log_writer         ADAPTIVE   Use a single process for redo log writing _max_outstanding_log_writes    2          Maximum number of outstanding redo log writes

修改_max_outstanding_log_writes=3

SQL> alter system set "_max_outstanding_log_writes"=3 scope=spfile; System altered. SQL> startup force SQL>select a.ksppinm name, b.ksppstvl value, a.ksppdesc description from x$ksppi a, x$ksppcv b where a.indx = b.indx and a.ksppinm in ('_max_outstanding_log_writes','_use_single_log_writer'); NAME                           VALUE      DESCRIPTION ------------------------------ ---------- -------------------------------------------------- _use_single_log_writer         ADAPTIVE   Use a single process for redo log writing _max_outstanding_log_writes    3          Maximum number of outstanding redo log writes [oracle@12C ~]$ ps -ef|grep lg|grep -v grep oracle     8589      1  0 15:13 ?        00:00:00 ora_lgwr_OCP oracle     8593      1  0 15:13 ?        00:00:00 ora_lg00_OCP oracle     8597      1  0 15:13 ?        00:00:00 ora_lg01_OCP oracle     8601      1  0 15:13 ?        00:00:00 ora_lg02_OCP

修改_use_single_log_writer='true'

SQL>  alter system set "_use_single_log_writer"='true' scope=spfile;
System altered.
SQL> startup force
SQL>select a.ksppinm name, b.ksppstvl value, a.ksppdesc description from x$ksppi a,
x$ksppcv b where a.indx = b.indx and a.ksppinm in
('_max_outstanding_log_writes','_use_single_log_writer');
NAME                           VALUE      DESCRIPTION
------------------------------ ---------- --------------------------------------------------
_use_single_log_writer         true       Use a single process for redo log writing
_max_outstanding_log_writes    1          Maximum number of outstanding redo log writes