DataGuard how to transmit Redo to Standby

DataGuard how to transmit Redo to Standby

DataGuard transfers redo to standby and then applies the redo on standby to ensure data consistency between the two.
The lgwr process is used to transmit redo logs.

In the lgwr transfer mode, the lgwr process itself does not actually send redo to standby, but derives an lns (local network server) from lgwr)
Process to transmit redo information. The main advantage of this is to reduce the pressure on the lgwr process and prevent the lgwr process from being unstable due to network exceptions.

Client_process on standby is lgwr, but it is not lgwr to transmit logs.

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

In PRIMARY, we can check the lgwr process by looking at the network connection. The connection to standby is not generated, but the network connection generated by the lns process.

[Root @ newplat ~] # Ps-eaf | egrep-e 'lgwr | lns' | grep-v 'egrep'
Oracle 8399 1 0? 00:01:08 ora_lgwr_cpd
Oracle 15207 1 0? 00:00:00 ora_lns1_cpd
[Root @ newplat ~] #

[Root @ newplat ~] # Netstandbyat-apn | egrep-e '2017 | 100'
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 ~] #

Logs in the master database report the following logs, indicating that lns is available for transmission.

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

When the synchronization parameter in log_archive_destandby_X is changed from the default async to sync, alert. log appears

LGWR: standbyandby redo logfile selectedfor 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

In this configuration, many people think that the lgwr process directly transmits redo to the slave database. In fact, it can be seen through the network connection that the lns process sends the redo to the slave database.

[Root @ newplat ~] # Ps-eaf | egrep-e 'lgwr | lns' | grep-v egrep
Oracle 12664 1 0? 00:00:00 ora_lnsb_cpd
Oracle 18366 1 0? 00:00:01 ora_lgwr_cpd

[Root @ newplat ~] # Netstandbyat-anp | egrep-e '2017 | 100 ';
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

The ARCH process also transmits redo logs, but archivelog is transmitted. When standby and PRIMARY generate a gap, the arch process is responsible for uploading logs on the PRIMARY to the standby database.

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

View network connection information

[Root @ newplat ~] # Ps-eaf | egrep-e 'ora _ arc' | grep-v egrep
Oracle 18407 1 0? 00:00:00 ora_arc0_cpd
Oracle 18409 1 0? 00:00:00 ora_arcw.cpd
Oracle 18411 1 0? 00:00:00 ora_arc2_cpd
Oracle 18413 1 0? 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

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