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