archived redo log gap tips Oracle Database Tips by Donald BurlesonDecember 3, 2015

Data Guard Archive Redo Gap Sequence problems

An Archive Gap Sequence is a set of archived redo logs that could not be transmitted to the standby site. As a result of an archive gap, the RAC cluster will lag behind the primary database. Usually, an archive gap sequence is created as the result of a network outage and the connection between the primary and the standby site is lost. The archive gap sequence can also occur during the creation of a standby database.

In Oracle 9i r2, Oracle significantly improved Data Guard automatic archive gap detection and resolution which reduces the chances of lag between the primary and the standby site.

Detection and Resolution of Data Guard archived redo log gaps

Starting with Release 2 of Oracle 9i, three methods of Data Guard automatic archive gap detection became available.  The following is a description of their features:

• The archiver process of the primary database polls the standby databases every 60 seconds. This is referred to as "heartbeat." During heartbeat, if an Oracle instance
  reports an archive gap, the archiver of the primary database sends the archived redo log files required to fill the gap. Once the files have been transferred, the Data Guard site is marked as up to date.
  

• Archived redo logs are transferred from the Data Guard primary database to the Data Guard standby database. On the primary database side, the LGWR or ARCH is responsible for sending the archived redo logs. On the standby site, the RFS process takes the archived redo log file and writes to the host machine. An archived redo log file is uniquely identified by its sequence number and thread number.

• When the RFS process receives an archived redo log file, it compares the sequence number with the sequence number of the previously received archived redo log file. If the sequence number of the current archived redo log file is greater than the sequence number of the last received archived redo log file plus one, a request is sent to the archiver process of the Data Guard primary database that is providing a list of missing archived redo log files. The archiver of the primary database then retransmits these archived redo log files, thereby resolving the redo log gap sequence.

• For a Data Guard physical standby database, Oracle 9i Release 2 introduced a request-response system for gap resolution through the FAL background process. The FAL method of gap resolution is initiated by the Managed Recovery Process (MRP). If the MRP detects a gap in archived redo logs during the application of log files, it requests the archiver process of the primary database to resend the archived redo log files which have caused the gap sequence.

• There are two important parameters required for redo log gap sequence resolution through the FAL background process and the Data Guard  parameters: fal_server and fal_client. These two parameters need to be set in the initialization parameter file of the physical standby database. fal_server is the service name of the primary database and fal_client is the service name of the standby database.
  
• The fal_client Data Guard parameter should be one of the remote archival destinations of the primary database pointing to this standby database. When MRP detects a gap sequence, it requests the archiver process of the primary database, identified by the fal_server parameter, and passes the sequence number of log files causing the archive gap. In addition, it passes the service name defined by the fal_client parameter to the archiver of the primary database. The archiver process of the primary database then retransmits the archived log files requested by the MRP of the standby database.