DataGuard如何传送Redo到Standby

                                 DataGuard如何传送Redo到Standby

　　　DG通过传送redo到standby,然后在standby上应用这些redo来保持两者数据一致.在PRIMARY传送redo主要是通过
lgwr进程来实现传送redo.

　　　lgwr传送模式下，lgwr进程本身并不真正传送redo到standby上，而是由lgwr派生一个lns(local network server)
进程来传送redo信息.这样做的好处主要在于减轻lgwr进程的压力，防止网络意外等情况使lgwr进程产生不稳定情况.

在standby上查询 client_process是lgwr，但是真正的并不是lgwr来传送日志

standbyANDBY> select process,client_process,standbyatus from v$managed_standbyandby where process='RFS';

PROCESS   CLIENT_P standbyATUS
--------- -------- ------------
RFS   LGWR    IDLE
RFS   UNKNOWN  IDLE
RFS   UNKNOWN  IDLE
RFS   UNKNOWN  IDLE

在PRIMARY我们可以通过查看网络连接来看检查lgwr进程，并不会产生到standby的连接，而是由lns进程产生的网络连接

[root@newplat ~]# ps -eaf | egrep -e 'lgwr|lns' | grep -v 'egrep'
oracle    8399     1  0 09:11 ?        00:01:08 ora_lgwr_cpd
oracle   15207     1  0 11:09 ?        00:00:00 ora_lns1_cpd
[root@newplat ~]#

[root@newplat ~]# netstandbyat -apn | egrep -e '8399|15207'
tcp        0      0 192.168.56.21:38944         192.168.56.22:1521          ESTABLISHED 15207/ora_lns1_cepd
udp        0      0 :::55415                    :::*                                    8399/ora_lgwr_cpd
udp        0      0 ::1:55419                   :::*                                    15207/ora_lns1_cepd
[root@newplat ~]#

主库上的日志会报以下的日志,表示有lns进行传送.

LNS: standbyandby redo logfile selected for thread 1 sequence 92 for destandbyination LOG_ARCHIVE_DEstandby_2
Sun Mar 20 11:09:35 2016
Archived Log entry 165 added for thread 1 sequence 91 ID 0xac83cb1e destandby 1:
Sun Mar 20 11:09:35 2016
ARC3: standbyandby redo logfile selected for thread 1 sequence 91 for destandbyination LOG_ARCHIVE_DEstandby_2
Sun Mar 20 11:45:15 2016

当log_archive_destandby_X中的同步参数由默认的async改为sync时，alert.log出现

LGWR: standbyandby redo logfile selected for thread 1 sequence 103 for destandbyination LOG_ARCHIVE_DEstandby_2
Thread 1 advanced to log sequence 103 (LGWR switch)
  Current log# 1 seq# 103 mem# 0: /u01/app/oracle/oradata/cpd/redo01.log

这种配置下很多人以为是lgwr进程直接传送redo到备库上，实际也可以通过网络连接看出，是由lns进程来传送的

[root@newplat ~]# ps -eaf | egrep -e 'lgwr|lns' | grep -v egrep
oracle   12664     1  0 20:04 ?        00:00:00 ora_lnsb_cpd
oracle   18366     1  0 11:49 ?        00:00:01 ora_lgwr_cpd

[root@newplat ~]# netstandbyat -anp |egrep -e '12664|18366';
tcp        0      0 192.168.56.21:47971         192.168.56.22:1521          ESTABLISHED 12664/ora_lnsb_cepd
udp        0      0 ::1:49731                   :::*                                    12664/ora_lnsb_cepd
udp        0      0 :::51196                    :::*                                    18366/ora_lgwr_cepd

ARCH进程也会传送redo，不过传送的是archivelog,当standby和PRIMARY产生gap时，这时候由arch进程来负责把PRIMARY上的日志传到备库上去

Sun Mar 20 20:22:54 2016
ARC3: standbyandby redo logfile selected for thread 1 sequence 105 for destandbyination LOG_ARCHIVE_DEstandby_2
Sun Mar 20 20:24:02 2016

查看网络连接信息

[root@newplat ~]# ps -eaf | egrep -e 'ora_arc' | grep -v egrep
oracle   18407     1  0 11:49 ?        00:00:00 ora_arc0_cpd
oracle   18409     1  0 11:49 ?        00:00:00 ora_arc1_cpd
oracle   18411     1  0 11:49 ?        00:00:00 ora_arc2_cpd
oracle   18413     1  0 11:49 ?        00:00:00 ora_arc3_cpd
[root@newplat ~]# netstandbyat -apn | grep 18413
tcp        0      0 192.168.56.21:54683         192.168.56.22:1521          ESTABLISHED 18413/ora_arc3_cepd
udp        0      0 ::1:43124                   :::*                                    18413/ora_arc3_cepd