Data Guard Gap Detection and Resolution

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Problem: Data Guard Gap Detection and Resolution


Symptoms:
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 database.

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.


Solution:



Methods of Gap Resolution:
===========================

Data Guard provides two methods for gap resolution, automatic and FAL (Fetch
Archive Log). The automatic method requires no configuration while FAL requires
configuration via init.ora parameters. Both methods are discussed below.


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 two
parameters are defined as:

FAL_SERVER: An OracleNet service name that exist in the standby tnsnames.ora
file that points to the primary database 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
2> 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 REGISTER LOGFILE
'/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
2> WHERE NEXT_CHANGE# NOT IN
3> (SELECT FIRST_CHANGE# FROM DBA_LOGSTDBY_LOG WHERE L.THREAD# = THREAD#)
4> 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
database.

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.

References:


Data Guard Gap Detection and Resolution [ID 232649.1]

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