Mirroring Online Redo Log Files 

Use

If you lose one or more online redo log files, you can no longer recover the database changes recorded in them. This means that you can only recover the database up to the first gap in the online redo log records. For this reason, SAP highly recommends that you mirror the online redo log files (at least once).

If an online redo log file of a group is lost, the database remains in operation. Oracle then uses the remaining member (or members) of this group to log the database changes. In such a case, you must recover the original mirroring of the online redo log files as quickly as possible.
 Recovery with SVRMGR: Overview
 Recovery: One Inactive Redo Log Missing

Prerequisites

When an R/3 System is installed using Oracle resources, an online redo log group normally consists of the original online redo log file and a mirror copy of this file (two members).

Procedure

You can check the mirroring with SAPDBA.
To do this, select Archive mode ® Show all archive information: ONLINE REDO LOGS
See SAPDBA: Archive Mode

During SAPDBA startup, any missing online redo log files will be displayed if the profile parameters check_initial_settings or check_redolog_mirror are configured properly.
See check_initial_settings

When the database is open, you can also check the mirroring with the ORACLE program SVRMGR.

SVRMGR> connect internal
SVRMGR> select * from V$logfile;

You can also make additional mirror copies. For more information, see the appropriate Oracle documentation. In addition, many systems support hardware-based file mirroring. For more details, see your operating system documentation.

For more information on the security of offline redo log files, see Archiving the Offline Redo Log Files.