Troubleshooting Data Guard

In my last section on Data Guard we are going to cover troubleshooting, I cover common issues and resolutions regarding the configuration and maintainance of a Data Guard environment.

You already know that you have the alert log and trace files, database health check which I covered in monitoring, Data Guard Broker checks and configuration. Data Guard could have any of the following problems

As I mentioned above I will tackle only the most common Data Guard problems, the internet should be able to help with most if not all problems, I am sure that someone out there has had the same problem and fixed it.

First we will start with log files, there are anumber of them to keep a check on

To find the location of some of these logs use the command below

identify log files col name for a25
col value for a65;
select name, value from v$diag_info where value like '%oracle%';

If you are using the Fast-Start Failover (FSFO) you can check for additional logging in the observer log file, you can also check the configuration file called FSFO.dat they are normall kept in $ORACLE_HOME/dbs and $ORACLE_HOME/rdbms/log directories. when starting the observer you can state the logfile name, the file will be in the directory where you run the below command

observer logfile name dgmgrl -logfile observer.log / "start observer"

Data Guard trace files can be turned on by the parameter log_archive_trace, you basically set the level that you want to trace at the higher the level the more detail, when set on the standby database it will capture output from RFS and MRP0 processes.

turn on Data Guard tracing sql> alter system set log_archive_trace=16;

The Data Guard has a number of reports that you can run, these I covered in the Broker section.

Finally before we get in specific errors here is a list of views that might be useful

dba_logstdby_events contains last 100 events that occured on the logical standby
dba_logstdby_progress check whether SQL Apply is progressing
dba_logstdby_log checks whether archive logs are being delivered to a logical standby
dba_logstdby_unsupported identifies SQL Apply unsupported data types
v$archive_dest Describes all the destinations in the Data Guard configuration
v$archive_dest_status Displays runtime and configuration information for the redo transport destinations
v$archive_gap Displays information to help you identify a gap in the archive redo log files on a physical database
v$dataguard_config Lists the db_unique_name parameters defined in the data guard configuration in log_archive_config
v$dataguard_status Displays and records events that would typically be triggered by any message to the alert log or server process trace files
v$log online redo log file information
v$logfile online redo log file and standby redo log file information
v$log_history contains archive log history information from the control file
v$archive_log contains more detail archived log information from the control file
v$logstdby_progress show whether logical standby pocess is running; if query returns no rows, its not running
v$managed_standby display current status information for Oracle database processes related to Data Guard
v$standby_log contains standby log file information

Configuration and Management Errors

We start wiht eh password file error, if you see ORA-01034 or ORA16191 errors then check that the sys account password is set correctly and passwords are set the sam across databases, if you are certain these are set correctly then try recreating the password file, also confirm that the parameter remote_login_password is set to either shared or exclusive.

if the SQL Apply fails with the error ORA-01031 tells you that SQL Apply has failed with insufficent privileges, you should see "grant sysdba to prod1_user" or something similar in the alert log.

granting privileges sql> alter session disable guard;
sql> grant sysdba to prod1_user;
sql> alter session enable guard;

There will be a time when you have to resolve a redo gap manually, normally this is automatic but in some cases it must be done manually. First you have to determine where the gap exists on you physical standby

determining the redo gap ## run this command on the physical standby
select * from v$archive_gap;

thread#    low_sequence#    high_sequence#
2              6233                   6233
3              4531                   4531
4              4938                   4939

Using the example above we can see thread 2 is missing 6233 and thread 3 is missing 4531 and thread 4 is missing between 4938 and 4939, to identify the missing archive use the below command

identify missing archive logs ## run the below on the primary once you have the archive gaps that are missing
select name from v$archived_log where thread# = 2 and dest_id = 2 and sequence# = 6233;


Copy the missing archives from the primary to the standby and register them, then check that they have been applied

register the missing archive logs ## On the standby after you have copied the missing archives across from the primary
sql> alter database resiter logfile '<log file name>';

## determine whether other archive log files are missing
select max(r.sequence#) last_seq_recd, max(l.sequence#) last_seq_sent from v$archived_log r, v$log l where r.dest_id = 2 and l.archived='YES';

## check that if gaps still exist
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#;

Lets say that you have a standby database that has been out of action for a little while and you need to get it back up and running the steps are as follows

get a standby up and rolling again
  1. stop the redo on the standby database
    sql> alter database recover managed standby database cancel;
  2. determine the current SCN for the standby database
    sql> select current_scn from v$database;
  3. use the value for the current SCN to take your incremental backup on the primary
    rman> run
      allocate channel d1 type disk;
      allocate channel d2 type disk;
      backup incremental from scn <scn number from above> database format '/tmp/dba/bkups/PROD1_%U';
      release channel d1;
      release channel d2;
  4. on the primary obtain a new standby control file and copy it to the physical standby
    sql> alter database create standby controlfile as '/tmp/stdby.ctl'
  5. copy the backup set and control from the primary to the standby
    use either scp or sftp, etc
  6. shutdown the standby and replace the controlfile and startup the standby in mount mode
    sql> startup mount
  7. catalog the backup using the target database controlfile
    rman> catalog start with '/tmp/dba/bkups';
  8. recover the database from the increatment backup using noredo
    rman> recover database noredo;
  9. restart the MRP process
    sql> alter database recover managed standby database using current logfile disconnect;

To check that the standby is receiving archive redo logs you run the following command

check that archive logs are being received select dest_id, status, error from v$archive_dest;

Check the following on the listener

also check the archive destination for any problems

Physical Standby Issues

There are a number of common issues regarding a physical standby database

The physical standby cannot be mounted if the controlfile was created with an operating system created backup or backup created using an alter database statement without the standby options. it must be created with ones of the following alter database create standby controlfile statement or rman backup current controlfile with the for standby option.

The standby redo log files must be the same size as the primary, if the sizes are different and a logswitch occurs, then the physical standby will be shutdown if it is in maximum protection mode. The primary will become unsynchronized if it is in maximum availibility mode you will receive the following error message in the alert log no standby log files of size <#> blocks available.

If you receive an ORA-16066 error message it is because the parameter remote_archive_enable is set to false, the parameter controls whether the archival of the redo logs to remote destinations is permitted. This parameter has been deprecated but make sure that it is set to true.

If you receive an ORA-16204 error message it means that the value in the parameter log_archive_dest_n is incorrect, double check this parameter.

Remote archival to standby database fails with ORA-01031, this is the result of a missing password file on the standby database, copy the password file from the primary to the standby database and restart the standby.

missing password file sql> select status, error from v$archive_dest;

If you receive an ORA-00326 error message, it means that media recovery has found an archive log which was generated after the required archive log, basically it needs the correct log and that the logs are out of order. MRP process will not allow this to happen but it can if you are applying manually. When you plan to manually recover a physical standby for any reason, you must first examine the standby control file to determine what archive logs have been registered for each thread. The you must make sure that you copy all archive logs from the primary that follow that last sequence for each thread, you must not rely on what you see on the disk - you must use the control file on the standby.

Logical Standby Database Failures

Several problems can occur

Log apply cannot apply unsupported DML statements, DDL statements and Oracle-supplied packages to logical standby databases running SQL Apply. You can determine what has failed with the below statement

What has failed

select xidusn, xidslt, xidsqn, status, status_code from dba_logstdby_events where event_time = (select max(event_time) from dba_logstdby_events);

## Logical standby example of an error
XIDUSN     XIDSLT     XIDSQN     STATUS                                         STATUS_CODE
---------- ---------- ---------- ---------------------------------------------  -----------
8          9          900        ORA-16226: DDL skipped due to lack of support  16226

The abaove statement will display what cause the SQL Apply to stop, remeber that you cannot randomly skip errors or transactions in SQL Apply without first fixing the issue that caused the problem in the first place, so never randonly skip transactions at a logical standby never.

ORA-01403 no data found errors is when DML is executed on the logical standby to tables maintained by SQL Apply, the primary and logical standby can get out of sync and the error won't show up until the table on the primary standby is updated. to resolve the issue you would need to skip and re-instantiate th table, if you have a database link created to connect to the primary, you can use the dbms_logstdby.skip and dbms_logstdby.instantiate_table procedure.

no data found sql> exec dbms_logstdby.skip('DML','ACTORS','%');
sql> exec dbms_logstdby.instantiate_table ('PROD_USER', 'ACTORS', 'PROD1_DBLINK');

If it appears that SQL Apply is hanging for the below steps

SQL Apply hanging
  1. check dba_logstdby_progress to see if any activity is coming through
    sql> select applied_scn, applied_time, read_scn, read_time, newest_scn, newest_time from dba_logstdby_progress;
  2. check the high_scn from the v$logstdby view, the scn should change as the SQL Apply progresses
    sql> select type, high_scn, status from v$logstdby;
  3. check v$logstdby_stats for information on the primary

Switchover Issues

If the switchover to the standby from the primary fails you probably have sessions connected you need to use the with session shutdown option, you can check what sessions or jobs are running, but the above option will allow you to switchover

Its is also possible that the standby never received the last redo from the primary, use the below query to determine when the last redo was transmitted, if the last was not transmitted just copy it from the primary and register it, when you restart the log services it will then be applied automatically.

last redo transmitted select thread#, sequence#, archived, applied, status from v$archived_log;

If you get ORA-01102 error message make sure that the db_unique_name has been set on both the primary and the standby, if have not set it then set it and restart the database.

If you active sessions when switching over it may fail with a ORA-01093 error then use the with session shutdown clause as part of the alter database commit to switchover to physical standby. Other processes that prevent switchover are the following

You may end up with physical standbys if the switchover fails in the middle, in this case go to the original primary and run the following

two physical databases after switchover ## On the original primary

sql> alter database commit to switcover to primary;

If you want to back out from an unsuccesssful switchover on a physical standby then try the below

backed out switchover on physical standby
  1. Run the command below on the original primary, if this statement is sucessful stop and restart the database, if not sucessful then continue the next steps
    sql> alter database commit to switchover to primary;
  2. When switchover was started a trace file was written in the log directory, this trace file contains the SQL statements needed to re-create the original primary control file. Capture the statements from this file and execute them from the SQL*Plus on the new standby database, the new standby database will revert back to the primary role.
  3. Shut down the original physical standby and create a new standby control file on the primary database, copy the control file to the standby
  4. Restart the original physical standby instance, hopefully the Fetch Archive Log (FAL) process will start and rearchive any missing archived redo log files to the physical standby.

After a switchover if the archive logs are not being applied that the following actions

archive redo logs are not applied after switchover
  1. verify that the log_archive_dest_n is set correctly
    sql> select dest_id, status, destination from v$archive_dest;
  2. verify the following are set correctly
  3. on the standby check the following
  4. verify the listener.ora file is correct and the tnsnames.ora file is also correct
  5. verify that the listener is up and running on the standby