Data guard GAP Detection and Resolution

Overview:

An archive gap is a range of missing archived redo logs created whenever the standby system is unable to receive the next archived redo log generated by the primary  db.

For example, an archive gap occurs when the network becomes unavailable and automatic archiving from the primary database to the standby database stops. When the network is available again, automatic transmission of the redo data from the primary database to the failed standby database resumes.

Methods of Gap Resolution:

Data Guard provides 2 methods for gap resolution, Automatic and FAL (Fetch Archive Log). The automatic method requires no configuration while FAL requires configuration via init.ora parameters.

Automatic Gap Resolution:

In both 9.0.1 and 9.2.0 Automatic Gap Resolution is implemented during log transport processing. As the LGWR or ARCH process begins to send redo over to the standby, the sequence number of the log being archived is compared to the last sequence received by the RFS process on the standby. If the RFS process detects that the archive log being received is greater than the last sequence received plus one, then the RFS will piggyback a request to the primary to send the missing archive logs. Since the standby destination requesting the gap resolution is already defined by the LOG_ARCHIVE_DEST_n parameter on the primary, the ARCH process on the primary sends the logs to the standby and notifies the LGWR that the gaps have been resolved.

            Starting in 9.2.0, automatic gap resolution has been enhanced. In addition to the above, the ARCH process on the primary database polls all standby databases every minute to see if there is a gap in the sequence of archived redo logs. If a gap is detected then the ARCH process sends the missing archived redo log files to the standby databases that reported the gap. Once the gap is resolved, the LGWR process is notified that the site is up to date.

FAL Gap Resolution:

As the RFS process on the standby receives an archived log, it updates the standby controlfile with the name and location of the file. Once the MRP (Media Recovery Process) sees the update to the controlfile, it attempts to recover that file. If the MRP process finds that the archived log is missing or is corrupt, FAL is called to resolve the gap or obtain a new copy. Since MRP has no direct communications link with the primary, it must use the FAL_SERVER and FAL_CLIENT initialization parameters to resolve the gap.Both of these parameters must be set in the standby init.ora. The 2 parameters are defined as:

FAL_SERVER:  An OracleNet service name that exist in the standby tnsnames.ora file that points to the primary db listener. The FAL_SERVER parameter can contain a comma delimited list of locations that should be attempted during gap resolution.

FAL_CLIENT:   An OracleNet service name that exist in the primary tnsnames.ora file that points to the standby database listener. The value of FAL_CLIENT should also be listed as the service in a remote archive destination pointing to the standby.

Once MRP needs to resolve a gap it uses the value from FAL_SERVER to call the primary database. Once communication with the primary has been established, MRP passes the FAL_CLIENT value to the primary ARCH process. The primary ARCH process locates the remote archive destination with the corresponding service name and ships the missing archived redo logs. If the first destination listed in FAL_SERVER is unable to resolve the gap then the next destination is attempted until either the gap is resolved or all FAL_SERVER destination have been tried.

As of 9.2.0 FAL Gap Resolution only works with Physical Standby databases as the process is tied to MRP. Gap recovery on a logical standby database is  handled through the heartbeat mechanism.

Simulating Gap Recovery

The follow steps can be used to illustrate and verify both automatic and FAL gap recovery. As the steps involve shutting down the standby database, which can impact disaster recovery, it is recommended to perform these procedures in a test environment.

Automatic Gap Resolution:

1. Shutdown the physical standby database.

2. Determine the current sequence on the primary database.

3. Perform at least three log switches on the primary database.

4. Verify that the logs did not get transferred to the standby archive dest.

5. Start the standby database.

6. Perform a log switch on the primary and verify that the gap gets resolved on the standby.

FAL Gap Resolution:

1. In the standby init.ora define the fal_server and fal_client parameters.

2. Bounce the standby database so that the parameters are put into effect.

3. Perform three log switches on the primary database.

4. In the standby_archive_dest directory delete the middle archive log on the standby.

5. Start managed recovery and verify that the gap is resolved by FAL_SERVER and FAL_CLIENT.

Manually Resolving a Gap:

In some rare cases it might be necessary to manually resolve gaps. The following section describes how to query the appropriate views to determine if a gap exists.

On your physical standby database:

Query the V$ARCHIVE_GAP view:

SQL> SELECT * FROM V$ARCHIVE_GAP;

THREAD#   LOW_SEQUENCE#  HIGH_SEQUENCE#

-----------             -------------       --------------

1                        443                    446

The query results show that your physical standby database is currently missing logs from sequence 443 to sequence 446 for thread 1. After you identify the gap, issue the following SQL statement on the primary database to locate the archived redo logs on your primary database:

SQL> SELECT NAME FROM V$ARCHIVED_LOG WHERE THREAD#=1 AND DEST_ID=1  AND  SEQUENCE# BETWEEN 443 AND 446;

NAME

-------------------------------------------------------

/u01/oradata/arch/arch_1_443.arc

/u01/oradata/arch/arch_1_444.arc

/u01/oradata/arch/arch_1_445.arc

Copy the logs returned by the query to your physical standby database and

register using the ALTER DATABASE REGISTER LOGFILE command.

SQL> ALTER DATABASE REGISTERLOGFILE    '/u01/oradata/stby/arch/arch_1_443.arc';

SQL> ALTER DATABASE REGISTER LOGFILE    '/u01/oradata/stby/arch/arch_1_444.arc';

SQL> ALTER DATABASE REGISTER LOGFILE    '/u01/oradata/stby/arch/arch_1_445.arc';

Once the log files have been registered in the standby controlfile, you can restart the MRP process.

On a logical standby database:

Query the DBA_LOGSTDBY_LOG view.

SQL> SELECT THREAD#, SEQUENCE#, FILE_NAME FROM DBA_LOGSTDBY_LOG L    WHERE NEXT_CHANGE# NOT IN  (SELECT FIRST_CHANGE# FROM DBA_LOGSTDBY_LOG WHERE L.THREAD# =THREAD#)  ORDER BY THREAD#,SEQUENCE#;

THREAD#          SEQUENCE#             FILE_NAME

----------                    ----------             -----------------------------------------------

1                         451                   /u01/oradata/logical_stby/arch/arch_1_451.arc

1                         453                   /u01/oradata/logical_stby/arch/arch_1_453.arc

Copy the missing logs to the logical standby system and register them using the ALTER DATABASE REGISTER LOGICAL LOGFILE statement on your logical standby db.

SQL> ALTER DATABASE REGISTER LOGICAL LOGFILE /u01/oradata/logical_stby/arch/arch_1_452.arc;

After you register these logs on the logical standby database, you can restart log apply services.