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ARCH vs LGWR log transmission in data guard

Redo log information can be transmitted in one of two ways from the primary database to the standby database: either by ARCH or LGWR.

LGWR transmission

All writings into the online redo log are synchronously or asynchronously transmitted to the standby database. If there is a standby redo log on the standby site, it is used. If there is no standby redo log on the standby site, an archive log will be gradually filled. However, the uncompletely filled archive log cannot be used in case of a disaster. Therefor, it is recommended to use standby redo logs.
LGWR transmission is specified through the log_archive_dest_n option LGWR:
log_archive_dest_2='service=to_standby lgwr'
If LGWR transmission is used, there should be a row having client_process=LGWR in v$managed_standby.

ARCH transmission

Data is transmitted as an archived redo logs.
ARCH transmission is specified through the log_archive_dest_n option ARCH:
log_archive_dest_2='service=to_standby arch'
If ARCH transmission is used, there should be a row having client_process=ARCH in v$managed_standby.