5 Switchover and Failover Operations

5.2.1 Choosing a Target Standby Database for Switchover

When performing a switchover in a configuration whose standby databases are all of the same type (all physical or all logical standby databases), choose the standby database that has the least amount of unapplied redo. By choosing the standby database with the least amount of unapplied redo, you can minimize the overall time it takes to complete the switchover operation. For example:

• Using DGMGRL, you can do this by examining the RecvQEntries monitorable database property of each standby database in the configuration. For example, connect to one of the databases in the configuration and issue the SHOW DATABASE database_name RecvQEntries command. (When the standby is keeping up, this property will return no rows.)

• Using Enterprise Manager, you can view the value of the ApplyLag column for each standby database in the Standby Databases section of the Data Guard Overview Page.

If the configuration contains both physical and logical standby databases, consider choosing a physical standby database (that has the least amount of unapplied redo) to be the target standby database. A switchover to a physical standby database is preferable because all databases in the configuration will be available as standby databases to the new primary database after the switchover operation completes. Whereas a switchover to a logical standby database will invalidate and disable all of the physical and snapshot standby databases in the configuration. You will then need to reenable the physical standby databases from a backup of the new primary database before you can reenable them.

You cannot perform a switchover to a snapshot standby database unless you first convert it back to a physical standby database.

5.2.2 Choosing a Target Standby Database for Failover

When performing a failover in a configuration whose standbys are all of the same type, choose the standby database that has the most redo data archived to it. By choosing the standby database that has the most amount of redo data archived to it, you can minimize the amount of data loss and in some cases, incur no data loss at all.

If the configuration contains physical, snapshot, and logical standby databases, consider choosing a physical standby database as the target standby database. A failover to a physical standby database is preferable because it is likely that all standby databases in the configuration will still be available as standby databases to the new primary database after the failover operation completes.

You may failover to a snapshot standby database. However failing over to a snapshot standby database will require more time because the broker must first convert it back to a physical standby database. After the conversion, the broker will start Redo Apply to apply accumulated redo data, before failing the database over to the primary role. Because the broker performs the failover after converting the snapshot standby database to a physical standby database, it is likely that all standby databases in the configuration will still be available as standby databases to the new primary database after the failover operation completes.

A failover to a logical standby database requires that all other standby databases be reenabled from a copy of the new primary database after the failover completes. In addition, a logical standby database may contain only a subset of the data present in the primary database. (For example, if the DBMS_LOGSTDBY.SKIP procedure was used to specify which database operations done on the primary database will not be applied to the logical standby database.)

However, there may be exceptions to the recommendation to choose a physical standby database as the target standby database. For example, if the physical standby database is lagging behind the logical standby database (the physical standby database was running with a log apply delay via the DelayMins configurable database property) and your business requires minimum downtime, consider selecting the logical standby database as the failover target.

5.3.1 Before You Perform a Switchover Operation

Consider the following points before you begin a switchover:

• When you start a switchover, the broker verifies that at least one standby database, including the primary database that is about to be transitioned to the standby role, is configured to support the overall protection mode (maximum protection, maximum availability, or maximum performance) after the switchover is completed.

• Prepare the primary database in advance for its possible future role as a standby database in the context of the overall protection mode (see Section 4.6). Such preparation includes:

  • Ensuring that standby redo log files are configured on the primary database.

  • Presetting configurable database properties related to redo transport services, such as LogXptMode, NetTimeout, StandbyArchiveLocation, and AlternateLocation. For more details about managing redo transport services using configurable database properties, see Section 4.4.

  • Presetting configurable database properties related to log apply services, such as DelayMins. For more details about managing log apply services using configurable properties, see Section 4.5.

  • For each temporary table, verifying that temporary files associated with that table on the primary database also exist on the standby database.

  Note that the broker does not use the properties to set up redo transport services and log apply services until you actually switch over the primary database to the standby role. Thus, the validity of the values of these properties is not verified until after the switchover. Once you set these properties, their values persist through role changes during switchover and failover.

• If fast-start failover is enabled in either maximum availability or maximum performance mode, a switchover can be performed only to the pre-specified target standby database and only if the standby database is synchronized with the primary database. For information about enabling fast-start failover, see Section 5.5.2.

After a switchover completes, the broker preserves the overall Data Guard protection mode as part of the switchover process by keeping the protection mode at the same protection level (maximum protection, maximum availability, or maximum performance) it was at before the switchover. Also, the network transmission mode (SYNC or ASYNC) for transporting redo to other standby databases not involved in the switchover does not change after a switchover. Log apply services on all other standby databases not involved in the switchover automatically begin applying redo data received from the new primary database.

If there are physical or snapshot standby databases in the configuration and the switchover occurs to a logical standby database, you need to re-create those databases from a copy of the new primary database and then reenable those databases, as described in Section 5.4.3.

5.3.2 Starting a Switchover

The act of switching roles should be a well-planned activity. The primary and standby databases involved in the switchover should have as small a redo lag as possible. Oracle Data Guard Concepts and Administration provides information about setting up the databases in preparation of a switchover.

To start a switchover using Enterprise Manager, select the standby database that you want to change to the primary role and click Switchover. When using DGMGRL, you need to issue only one SWITCHOVER command to specify the name of the standby database that you want to change into the primary role.

The broker controls the rest of the switchover, as described in Section 5.3.3.

5.3.3 How the Broker Performs a Switchover

Once you start the switchover, the broker:

1. Verifies that the primary and the target standby databases are in the following states:

   1. The primary database is enabled and is in the TRANSPORT-ON state.

   2. The target standby database is enabled and is in the APPLY-ON state.

   The broker allows the switchover to proceed as long as there are no errors for the primary database and the standby database that you selected to participate in the switchover operation. Errors occurring for any other standby databases not involved in the switchover will not impede the switchover.

2. Shuts down all instances except one.

   If you are switching over to a physical standby database and either it or the primary database is a RAC database, the broker keeps only one instance running on both databases and shuts down all other instances before it continues the switchover. For an Oracle RAC physical standby, the broker ensures that the apply instance is the only instance running. If the other instances on either database cannot be shut down, the switchover fails and you must manually shut down those other instances and issue the switchover command again. It is also important that you do not start any new instances during the switchover. If you must manually shut down the instances on the physical standby database, do not shut down the apply instance.

   No instances will be shut down if switching over to a logical standby database. You cannot switch over to a snapshot standby database.

3. Switches roles between the primary and standby databases.

   The broker first converts the original primary database to run in the standby role. Then, the broker transitions the target standby database to the primary role. If any errors occur during either conversion, the broker stops the switchover. See Section 10.3, "Troubleshooting Problems During a Switchover Operation" for more information.

4. Updates the broker configuration file to record the change in roles.

   Because the configuration file profiles all database objects in the configuration, this ensures that each database will run in the correct role and state should it be restarted later for any reason.

5. Restarts the new standby (former primary) database if the switchover occurs with a physical standby database, and Redo Apply begins applying redo data from the new primary database. If this is an Oracle RAC physical standby database, the broker directs CRS to restart the instances that were shut down prior to the switchover. In a configuration operating in maximum protection mode, the new primary database will also be restarted.

6. The new primary database is opened in read/write mode and redo transport services are started. If this is an Oracle RAC physical standby database, the broker restarts the instances that it shut down prior to the switchover.

The broker verifies the state and status of the databases to ensure that the switchover transitioned the databases to their new role correctly. Standby databases not involved in the switchover and not disabled by the broker after the switchover will continue operating in the state they were in before the switchover. Redo Apply and SQL Apply on all other standby databases not involved in the switchover automatically begin applying redo data from the new primary database.

5.4.1 Complete and Immediate Manual Failovers

Using Enterprise Manager or DGMGRL, you can perform either a complete (recommended) or an immediate failover:

• A complete failover is the recommended and default failover option. It automatically recovers the maximum amount of redo data for the protection mode the configuration is operating in. A complete failover also attempts to avoid disabling any standby databases that were not the target of the failover, so that they may continue serving as standby databases to the new primary database.

  Whether or not standby databases that were not the target of failover (bystander standby databases) are disabled depends upon how much redo data they have applied relative to the failover target and the standby type of the failover target:

  • If the failover target is a physical or snapshot standby database, the original primary database must be reenabled in order to be a standby database for the new primary database. In addition, some standby databases may be disabled by the broker during the failover if the broker detects that they have applied redo beyond where the new primary database had applied. Any database that was disabled by the broker must be reenabled using the steps described in Section 5.4.3.

    Note that if failover was performed on a snapshot standby database, the old primary must be reenabled as a physical standby database.

  • If the failover target is a logical standby database, the original primary database and all standby databases in the configuration (that were not the target of the failover) will be disabled and must be reenabled to act as standby databases for the new primary database. Any database that was disabled by the broker must be reenabled using the steps described in Section 5.4.3.

  If any of the databases that were disabled as part of a failover have flashback enabled, they can be reinstated using the DGMGRL REINSTATE command or the Reinstate button in Enterprise Manager.

  During a complete failover, the broker performs the failover steps described in Section 5.4.2.1.

• An immediate failover is the fastest type of failover. However, no additional data is applied on the standby database once you invoke the failover. Another consequence of immediate failover is that you must also reenable the original primary database and all other standby databases not involved in the failover before they can serve as standby databases to the new primary database. Section 5.4.3 describes how to do this. During an immediate failover, the broker performs the failover steps described in Section 5.4.2.2.

  Caution:

  Always try to perform a complete failover first. Only when a complete failover is unsuccessful should you perform an immediate failover. Depending on the destination attributes of redo transport services, a complete failover can occur without any data loss, while an immediate failover usually results in data loss.

5.4.2 Performing a Manual Failover Operation

After determining that there is no possibility of recovering the primary database in a timely manner, ensure that the primary database is shut down and then begin the failover operation.

The steps in this section describe how to perform a manual failover. Depending on the failover and the types of standby databases involved, some of the databases may need to be restarted or reenabled. The instructions guide you through the appropriate steps for each type of situation.

Step 1   Determine which of the available standby databases is the best target for the failover.

Follow the guidelines described in Section 5.2, "Choosing a Target Standby Database".

Step 2   Start the failover.

Using Enterprise Manager or DGMGRL, perform either a complete (recommended) or an immediate failover.

Manual Failover Using Enterprise Manager:

On the Data Guard Overview Page in Enterprise Manager, select the standby database that you want to change to the primary role and click Failover. Then, on the Failover Confirmation Page, click Yes to invoke the default Complete failover option.

Manual Failover Using DGMGRL:

On the target standby database, issue the FAILOVER command to invoke a complete failover, specifying the name of the standby database that you want to change into the primary role:

DGMGRL> FAILOVER TO database-name [IMMEDIATE];

If the target standby database is an Oracle RAC physical or snapshot standby database, the broker will direct CRS to shut down all instances except the apply instance before it continues the failover. If the other instances cannot be shut down, the failover fails. In this case, you must manually shut down all instances except the apply instance and issue the FAILOVER command again. It is also important that you do not start any new instances during the failover. The broker will direct CRS to restart instances that were shut down prior to the failover.

No instances will be shut down if failing over to a logical standby database.

Step 3   Reset the protection mode.

After a manual failover (complete or immediate), the overall Data Guard protection mode is handled as follows:

• If the protection mode was at maximum protection, it is reset to maximum performance. You can upgrade the protection mode later, if necessary, as described in Section 4.6.1.

• If the protection mode was at maximum availability, it remains at maximum availability.

Step 4   Re-establish a disaster-recovery configuration.

To maintain a viable disaster-recovery solution in the event of another disaster, you may need to perform the additional steps described in Section 5.4.3 to:

• Reinstate the original primary database to act as a standby database in the new configuration.

• Reinstate or re-create standby databases in the configuration that were disabled by the broker.

  After a complete failover finishes, any standby database not involved in the failover that is not viable as a standby for the new primary database will be disabled by the broker. This can happen for either of these reasons:

  • The broker detects that the standby database has applied redo data beyond what has been applied on the new primary database.

    For instance, this could happen if a standby database not involved in the failover has applied more redo data than the new primary database itself has applied. The standby database must be reenabled or reinstated before it can serve as a standby for the new primary database.

  • The failover was to a logical standby database, the broker disables all of the (physical, snapshot, and logical) standby databases in the configuration that were not involved in the failover. They must be reenabled before they can serve as standby to the new primary database.

5.4.3 Reenabling Disabled Databases After Failover or Switchover

To restore your original disaster-recovery solution after switchover to a logical standby database or after failover to any standby database, you may need to perform additional steps.

Databases that have been disabled after a role transition are not removed from the broker configuration, but they are disabled in the sense that the databases are no longer managed by the broker.

To reenable broker management of these databases, you must reinstate or re-create the databases using one of the following procedures:

• If a database can be reinstated, the database will show the following status after a complete failover:

  ORA-16661: the standby database needs to be reinstated
  

  Reinstate the database using the DGMGRL REINSTATE DATABASE command or the reinstate option in Enterprise Manager, as described in Section 5.4.3.1, "How to Reinstate a Database".

• If a database must be re-created from a copy of the new primary database, it will have the following status:

  ORA-16795: the standby database needs to be re-created
  

  Re-create the standby database from a copy of the primary database and then reenable it. The procedures for creating a standby database are documented in Oracle Data Guard Concepts and Administration. See Section 5.4.3.2, "How to Re-create and Reenable a Disabled Database" for more information.

Whether you reinstate or re-create a database depends on if you performed a switchover or failover and on the type of standby database that was the target of the operation.

The following table describes how to reenable disabled databases based on the type of role transition that was performed. The status value associated with databases that are disabled after a switchover or failover will also guide you in choosing which procedure to use. These status values can be viewed in the output from the DGMGRL SHOW DATABASE or on the Data Guard Overview page in Enterprise Manager.

The following sections describe how to reinstate or reenable a database.

ORA-16661: the standby database needs to be reinstated

DGMGRL> REINSTATE DATABASE db_unique_name;

5.5.1 Prerequisites for Enabling Fast-Start Failover

The following prerequisites must be met before the broker allows you to enable fast-start failover:

• Ensure the broker configuration is operating in either maximum availability mode or maximum performance mode.

  See Section 4.6.1 for information about configuring the protection mode, standby redo logs, and the LogXptMode property.

• Ensure that the standby database you choose to be the target of fast-start failover has its LogXptMode property set to SYNC if you wish to enable fast-start failover in maximum availability mode, or to ASYNC if you wish to enable fast-start failover in maximum performance mode. The current primary database must have its LogXptMode property set accordingly and must have standby redo logs configured.

• Enable Flashback Database and set up a flash recovery area on both the primary database and the target standby database.

  See Oracle Database Backup and Recovery User's Guide.

• Install the DGMGRL command-line interface on the observer computer as described in Section 2.1.

• Configure the TNSNAMES.ORA file on the observer system so that the observer is able to connect to the primary database and the pre-selected target standby database.

• Create static service names so that the observer can automatically restart a database as part of reinstatement. See Section 2.2, "Prerequisites" for more information.

5.5.2 Enabling Fast-Start Failover

You can enable fast-start failover from any site while connected to any database in the broker configuration. Enabling fast-start failover does not trigger a failover. Instead, it allows the observer to begin observing the primary and standby databases and initiate a fast-start failover should conditions warrant a failover.

Perform the following steps to enable fast-start failover and start the observer. The steps assume that you are connected as SYS and that a primary and standby database are already set up in a broker configuration.

Step 1   Determine which of the available standby databases is the best target for the failover.

Follow the guidelines described in Section 5.2, "Choosing a Target Standby Database".

Step 2   Specify the target standby database with the FastStartFailoverTarget configuration property.

You can specify only one target standby database when setting the FastStartFailoverTarget configuration property on the current primary database:

• If there is only one standby database in the configuration, you can skip this step and continue with Step 3. The broker will automatically set the FastStartFailoverTarget property on the primary and standby databases to point to each other as their respective target during a failover.

• If there is more than one standby database in the configuration, you must explicitly set the FastStartFailoverTarget property on the primary database and your chosen target standby database to point to each other for the purpose of defining which standby database will be the target of a fast-start failover. For example:

  DGMGRL> EDIT DATABASE 'North_Sales' SET PROPERTY FastStartFailoverTarget = 'DR_Sales';
  DGMGRL> EDIT DATABASE 'DR_Sales' SET PROPERTY FastStartFailoverTarget = 'North_Sales';
  

  In this example, the current primary database, North_Sales, specifies DR_Sales as its failover target, and the target standby database, DR_Sales, specifies the current primary database as its target. When DR_Sales becomes the primary database it must have a standby target to which it can fail over.

  Note:

  To change the FastStartFailoverTarget property to point to a different standby database, disable fast-start failover, set the FastStartFailoverTarget property, and reenable fast-start failover.

See Section 9.2.13, "FastStartFailoverTarget" for more information about this property.

Step 3   Determine the protection mode you want

Fast-start failover can be enabled for either maximum availability mode or maximum performance mode. If you cannot tolerate any loss of data, then ensure that the configuration protection mode is set to maximum availability. To do this, the LogXptMode configurable database property for both the primary and target standby database must be set to SYNC. Then set the configuration protection mode to maximum availability. For example:

DGMGRL> EDIT DATABASE 'North_Sales' SET PROPERTY LogXptMode=SYNC;
DGMGRL> EDIT DATABASE 'DR_Sales' SET PROPERTY LogXptMode=SYNC;
DGMGRL> EDIT CONFIGURATION SET PROTECTION MODE AS MaxAvailability;

If you are more concerned about the performance of the primary database than a minimal loss of data, consider enabling fast-start failover when the configuration protection mode is set to maximum performance. In this mode you will need to consider how much data loss is acceptable in terms of seconds and set the FastStartFailoverLagLimit configuration property accordingly. This property specifies the amount of data, in seconds, that the target standby database can lag behind the primary database in terms of redo applied. If the standby database's redo applied point is within that many seconds of the primary database's redo generation point, a fast-start failover will be allowed. The FastStartFailoverLagLimit configuration property is only used by the broker when enabling fast-start failover for configurations operating in maximum performance mode. The default value is 30 seconds and the lowest possible value is 10 seconds.

In addition to setting the configuration protection mode to maximum performance, you will also need to ensure that the LogXptMode configurable database property for both the primary and target standby database is set to ASYNC. For example:

DGMGRL> EDIT DATABASE 'North_Sales' SET PROPERTY LogXptMode=ASYNC;
DGMGRL> EDIT DATABASE 'DR_Sales' SET PROPERTY LogXptMode=ASYNC;
DGMGRL> EDIT CONFIGURATION SET PROTECTION MODE AS MaxPerformance;
DGMGRL> EDIT CONFIGURATION SET PROPERTY FastStartFailoverLagLimit=45;

Step 4   Set the FastStartFailoverThreshold configuration property.

Fast-start failover will occur if both the observer and the target standby database lose connection to the primary database for the period of time specified by the FastStartFailoverThreshold configuration property.

Set the FastStartFailoverThreshold property to specify the number of seconds you want the observer and target standby database to wait (after detecting the primary database is unavailable) before initiating a failover. For example:

DGMGRL> EDIT CONFIGURATION SET PROPERTY FastStartFailoverThreshold = 45;

This is a configuration-level property. The default value for the FastStartFailoverThreshold property is 30 seconds and the lowest possible value is 6 seconds. If you have an Oracle RAC primary database, consider specifying a higher value to minimize the possibility of a false failover in the event of an instance failure.

The time interval starts when the observer first loses its connection to the primary database. If the observer is unable to regain a connection to the primary database within the specified time, then the observer begins a fast-start failover provided the standby database is ready to fail over. Although the default value of 30 seconds is typically adequate for detecting outages and failures on most configurations, you can adjust failover sensitivity with this property to decrease the probability of false failovers in a temporarily unstable environment.

If the FastStartFailoverPmyShutdown configuration property is set to TRUE, the primary database will shut down after FastStartFailoverThreshold seconds has elapsed if redo generation has been stalled and the primary database is unable to reestablish connectivity with either the observer or target standby database.

Note that the FastStartFailoverThreshold property can be changed even when fast-start failover is enabled.

Step 5   Set other database properties (optional).

You can optionally set the database properties described in the following table:

Step 6   Enable additional fast-start failover conditions (optional)

By default, a fast-start failover is done when both the observer and the standby cannot reach the primary after the configured time threshold (FastStartFailoverThreshold) has passed.

You can optionally indicate the database health conditions that should cause fast-start failover to occur. These conditions are described in the following table:

In Oracle RAC configurations, the Inaccessible Logfile and Stuck Archiver health conditions may only be applicable to a single instance. Careful consideration should be given before enabling fast-start failover for either of these conditions because doing so will supersede availability options provide by CRS.

You can also specify errors raised by the Oracle server (ORA errors) as conditions for which fast-start failover should occur.

You can specify particular conditions for which a fast-start failover should occur using either Oracle Enterprise Manager or the DGMGRL ENABLE FAST_START FAILOVER CONDITION and DISABLE FAST_START FAILOVER CONDITION commands.

Step 7   Enable fast-start failover.

Use the Enterprise Manager Fast-Start Failover wizard or the DGMGRL ENABLE FAST_START FAILOVER command to enable fast-start failover. To enable fast-start failover, both the primary and target standby databases must be running and have connectivity, and satisfy all of the prerequisite conditions listed in Section 5.5.1.

Enable Fast-Start Failover Using Enterprise Manager

To enable fast-start failover in Enterprise Manager, use the Fast-Start Failover wizard. On the Data Guard Overview Page next to the "Fast-Start Failover" status field, click Disabled to invoke the Fast-Start Failover Page. Then, on the Fast-Start Failover Change Mode Page, click Enabled. Enterprise Manager will start the observer. Then, on the Fast-Start Failover Configure Page, select the standby database that should be the target of a failover. See Section 5.2, "Choosing a Target Standby Database" for helpful advice. This page will not allow you to alter the protection mode. Rather, fast-start failover will be enabled in accordance with the current protection mode. If the currently configured mode is maximum protection, Enterprise Manager will downgrade the mode to maximum availability.

Enable Fast-Start Failover Using DGMGRL

To enable fast-start failover with DGMGRL, issue the ENABLE FAST_START FAILOVER command while connected to any database in the broker configuration, including on the observer computer. For example:

DGMGRL> ENABLE FAST_START FAILOVER;
Enabled.

Step 8   Start the Observer.

The primary database must be running in order to start the observer.

You can start the observer before or after you enable fast-start failover. However, it is recommended that you have the observer running whenever you have fast-start failover enabled. If fast-start failover is enabled, the observer immediately begins monitoring the status and connections to the primary and target standby databases.

Starting the Observer Using Enterprise Manager

If the Enterprise Manager agent is installed on the observer computer, it automatically starts the observer when you enable fast-start failover through Enterprise Manager. If the agent is not present, you must start the observer manually using the following instructions for the DGMGRL command-line interface.

Starting the Observer Using DGMGRL

To start the observer with DGMGRL, issue the following command on the observer computer:

DGMGRL> START OBSERVER;

The observer is a continuously executing process that is created when the START OBSERVER command is issued. Thus, the command-line prompt on the observer computer does not return until you issue the STOP OBSERVER command from another DGMGRL session. To issue commands and interact with the broker configuration, you must connect through another DGMGRL client session.

See the START OBSERVER command for more information.

Step 9   Verify the fast-start failover environment.

To verify the readiness of the fast-start failover configuration, issue the DGMGRL SHOW CONFIGURATION VERBOSE command or the SHOW FAST_START FAILOVER command on the primary database. For example:

DGMGRL> SHOW FAST_START FAILOVER;
 
Fast-Start Failover: ENABLED
 Threshold:           60 seconds
 Target:              DR_Sales
 Observer:            observer.foo.com
 Lag Limit:           30 seconds (not in use)
 Shutdown Primary:    TRUE
 Auto-reinstate:      TRUE
 
Configurable Failover Conditions
 Health Conditions:
   Corrupted Controlfile          YES
   Corrupted Dictionary           YES
   Inaccessible Logfile            NO
   Stuck Archiver                  NO
   Datafile Offline               YES
 
 Oracle Error Conditions:
   (none)

The following sections provide more information about the fast-start failover environment:

• When Fast-Start Failover Is Enabled and the Observer Is Running

• Restrictions When Fast-Start Failover is Enabled

• Shutting Down the Primary Database When Fast-Start Failover Is Enabled

• Performing Manual Role Changes When Fast-Start Failover Is Enabled

5.5.3 Directing a Fast-Start Failover From an Application

You can use the DBMS_DG PL/SQL package to allow an application to direct a fast-start failover when it encounters specific conditions. When a serious condition uniquely known to the application is detected, it may call the DBMS_DG.INITIATE_FS_FAILOVER function, thus alerting the primary database that it wants a fast-start failover to occur immediately. The primary database will notify the observer of this and the observer will immediately initiate a fast-start failover, assuming the standby is ready for failover. Once the observer has initiated a fast-start failover, the primary database will shut down automatically. The observer will not attempt to reinstate the former primary database.

If the configuration is not failable, the DBMS_DG.INITIATE_FS_FAILOVER function will return an ORA error number (not signal an exception) informing the caller that a fast-start failover could not be performed.

5.5.4 Viewing Fast-Start Failover Configuration Statistics and Status

To verify the observer is started and the configuration is ready for fast-start failover, you can issue the DGMGRL SHOW DATABASE <target-standby-database> StatusReport command or query the V$DATABASE view on the target standby database.

You can also query the V$FS_FAILOVER_STATS view to display statistics about fast-start failover occurring on the system.

The rest of this section provides examples of using DGMGRL SHOW commands to display fast-start failover information and includes sections describing the following views:

• V$DATABASE View

• V$FS_FAILOVER_STATS View

Example 1   SHOW FAST-START FAILOVER

The DGMGRL SHOW FAST-START FAILOVER command displays all the fast-start failover related information. For example:

DGMGRL> SHOW FAST_START FAILOVER;
 
Fast-Start Failover: ENABLED
 Threshold:           60 seconds
 Target:              DR_Sales
 Observer:            observer.foo.com
 Lag Limit:           30 seconds (not in use)
 Shutdown Primary:    TRUE
 Auto-reinstate:      TRUE
 
Configurable Failover Conditions
 Health Conditions:
   Corrupted Controlfile          YES
   Corrupted Dictionary           YES
   Inaccessible Logfile            NO
   Stuck Archiver                  NO
   Datafile Offline               YES
 
 Oracle Error Conditions:
   (none)

Example 2   SHOW CONFIGURATION VERBOSE

The following example shows the fast-start failover information for the DRSolution configuration:

DGMGRL> SHOW CONFIGURATION VERBOSE;
 
Configuration
 Name:                DRSolution
 Enabled:             YES
 Protection Mode:     MaxAvailability
 Databases:
   North_Sales - Primary database
   DR_Sales    - Physical standby database
               - Fast-Start Failover target
 
Fast-Start Failover: ENABLED
 Threshold:           60 seconds
 Target:              DR_Sales
 Observer:            observer.foo.com
 Lag Limit:           30 seconds (not in use)
 Shutdown Primary:    TRUE
 Auto-reinstate:      TRUE
 
Current status for "DRSolution":
SUCCESS

SQL> SELECT LAST_FAILOVER_TIME, LAST_FAILOVER_REASON FROM V$FS_FAILOVER_STATS;
 
LAST_FAILOVER_TIME
--------------------
LAST_FAILOVER_REASON
------------------------------------------------------------------------------------------------------------------------------------
02/13/2007 16:53:10
Primary Disconnected

5.5.5 Disabling Fast-Start Failover

Disabling fast-start failover prevents the observer from initiating a failover to the target standby database. In this case, manual failover may still be possible. See Section 5.4 for information about manual failover.

To disable fast-start failover, use the Fast-Start Failover wizard in Enterprise Manager or the DGMGRL DISABLE FAST_START FAILOVER [FORCE] command. The FORCE option disables fast-start failover on the database to which you are connected even when errors occur. Whether or not you need the FORCE option depends mostly on if the primary and target standby database have network connectivity:

• If the primary and target standby database have network connectivity, and the database to which you are connected has network connectivity with the primary database, the FORCE option has no effect. Simply use DISABLE FAST_START FAILOVER. This method will disable fast-start failover on all databases in the broker configuration.

  If errors occur during the disable operation, the broker returns an error message and stops the disable operation.

• If the primary and target standby databases do not have network connectivity or if the database to which you are connected does not have network connectivity with the primary database, consider using DISABLE FAST_START FAILOVER with the FORCE option.

  The broker may not be able to disable fast-start failover on all databases in the broker configuration when you issue the DISABLE FAST_START FAILOVER FORCE command. As a result, there is no guarantee that the observer will not perform a fast-start failover to the target standby database if the observer determines that conditions warrant a failover. The following list indicates the extent to which fast-start failover is disabled in the broker configuration when the DISABLE FAST_START FAILOVER FORCE command is issued on the primary database, target standby database, and a standby database that is not the fast-start failover target.

  If you issue this command on:

  • The target standby database when it does not have connectivity with the primary database, fast-start failover is disabled only on the target standby database. In this case, the observer cannot perform a fast-start failover even if conditions warrant a failover. Disabling fast-start failover with the FORCE option when connected to the target standby database guarantees that fast-start failover will not occur.

    When the primary database and the target standby database regain network connectivity, the broker will disable fast-start failover for the entire broker configuration.

  • The primary database, the primary database attempts to disable fast-start failover on as many databases in the configuration with which it has a network connection. If the primary database does not have connectivity with the target standby database, fast-start failover remains enabled on the target standby database and the observer may still attempt a fast-start failover if conditions warrant a failover.

    Caution:

    This action may result in two databases in the configuration simultaneously assuming the primary database role should fast-start failover occur. For this reason, you should first issue this command on the target standby database.

  • Another standby database that does not have connectivity with the primary database, fast-start failover is disabled for this database. Because fast-start failover was not disabled on the target standby database, the observer may still attempt a fast-start failover to the target standby database should conditions warrant a failover.

    When the primary database and the (non-target) standby database regain network connectivity, the broker will propagate its current fast-start failover setting (ENABLED or DISABLED) to the non-target standby.

    Caution:

    When you are experiencing network disconnections and you issue the DISABLE FAST_START FAILOVER FORCE command on the primary database or a standby database that does not have connectivity with the primary database, fast-start failover may not be disabled for all databases in the broker configuration. As a result the observer may still initiate fast-start failover to the target standby database, if conditions warrant a failover. This may result in two databases in the configuration simultaneously assuming the primary database role.

Conditions Requiring the FORCE Option

Disabling fast-start failover without the FORCE option can succeed only if the database on which the command is issued has a network connection with the primary database and if the primary database and target standby database have a network connection. This is the recommended method for disabling fast-start failover.

However, there may be situations in which you must disable fast-start failover when the primary database and the target standby database do not have a network connection, or the database on which you issued the disable fast-start failover command does not have a network connection to the primary database. In cases where there is a lost network connection, be aware that the observer may attempt a fast-start failover to the target standby database if conditions warrant a failover.The FORCE option may be the preferred method for disabling fast-start failover when:

• A network outage isolates the primary database from the observer and the target standby database, while the databases are ready to failover.

  In this case, the primary database stalls and prevents any further transactions from committing because a fast-start failover may have occurred while it was isolated. If you expect the network to be disconnected for a long time and you need to make the primary database available, first confirm that a fast-start failover has not occurred to the target standby database. Then, disable fast-start failover with the FORCE option on the primary database.If possible, confirm that fast-start failover has not occurred to the target standby database prior to disabling fast-start failover with the FORCE option on the primary database.

  Caution:

  This action may result in two databases in the configuration simultaneously assuming the primary database role. This can be avoided by first disabling fast-start failover with the FORCE option on the target standby.

• You want to conduct a manual failover to any standby database in the configuration (for example, because a failure occurred on the primary database at a time when the primary and target standby database were not ready to failover).

  In this case fast-start failover cannot occur because the databases are not ready to failover. You cannot perform a manual failover to the target standby database for the same reason. To proceed, you must first disable fast-start failover using the FORCE option, and then perform a manual failover.

  Caution:

  This action will result in loss of data and the possibility of two databases in the configuration simultaneously assuming the primary database role. This can be avoided by first disabling fast-start failover with the FORCE option on the target standby.

• A fast-start failover to the target standby database fails.

  If the failover fails for any reason, it could leave the target standby database inoperable, regardless of whether the target standby database is ready to failover. If there is another standby database that is available for failover, you can perform a manual failover to that standby database after you first disable fast-start failover using the FORCE option on that standby database.

• You want to prevent fast-start failover from occurring because the primary database will resume service soon.

  In this case, disable fast-start failover using the FORCE option on the target standby database. Once the primary database regains connectivity with the target standby database, fast-start failover will be disabled for all the databases in the configuration.

Disabling Fast-Start Failover Using Enterprise Manager

Click Disable in the Fast-Start Failover wizard. Then, click Continue to proceed to the next page. See the Enterprise Manager online Help system for more information.

Disabling Fast-Start Failover Using DGMGRL

Issue the DISABLE FAST_START FAILOVER command or the DISABLE FAST_START FAILOVER FORCE command. See the "DISABLE FAST_START FAILOVER" command in Chapter 8 for more information.

5.5.6 Performance Considerations for Fast-Start Failover

Consider the following recommendations to obtain better performance when using fast-start failover:

• The failover time is dependent upon whether the target standby database (physical or logical standby database) has applied all of the redo data it has received from the primary database.

• Enabling fast-start failover in a configuration operating in maximum performance mode provides better overall performance on the primary database because redo data is sent asynchronously to the target standby database. Note that this does not guarantee no data will be lost.

• Fast-start failover is faster when you take steps to optimize recovery so that the application of redo data to the standby database is kept up to date with the primary database's rate of redo application. To optimize the log apply rate:

  • Do not configure the DelayMins configurable database property to delay applying archived redo log files to the standby database (see Section 4.5 for more information).

  • See Oracle Data Guard Concepts and Administration for information about tuning the log apply rate for a physical standby database.

  • See the Oracle Maximum Availability Architecture white papers at:

    http://otn.oracle.com/deploy/availability/
    

• When setting the FastStartFailoverLagLimit configuration property, consider these tradeoffs between performance and potential data-loss:

  • A low lag limit will minimize data loss but may impact the performance of the primary database.

  • A high lag limit may lead to more data loss but may lessen the performance impact of the primary database.

5.5.7 Managing the Observer

The observer is integrated in the DGMGRL client-side component of the broker and typically runs on a different computer from the primary or standby databases and from the computer where you manage the broker configuration. The observer continuously monitors the fast-start failover environment to ensure the primary database is available (described in Section 5.5.2.1). The observer's main purpose is to enhance high availability and lights out computing by reducing the human intervention required by the manual failover process that can add minutes or hours to downtime.

You can manage the observer through either the Data Guard Overview pages in Oracle Enterprise Manager or using DGMGRL commands. Figure 5-2 shows the observer monitoring a fast-start failover configuration.

Figure 5-2 The Observer in the Fast-Start Failover Environment

Description of "Figure 5-2 The Observer in the Fast-Start Failover Environment"

The following sections provide information about managing the observer:

• Installing and Starting the Observer

• Viewing Information About the Observer

• What Happens if the Observer Fails?

• Stopping the Observer

• Moving the Observer to Another Computer

• How the Observer Maintains Fast-Start Failover Configuration Information

ORA-16647: could not start more than one observer

ORA-16658: unobserved fast-start failover configuration
ORA-16820: fast-start failover observer is no longer observing this database

% dgmgrl -logfile $ORACLE_HOME/rdbms/log/config1.log
DGMGRL> CONNECT /@primary1;
DGMGRL> START OBSERVER FILE=$ORACLE_HOME/dbs/config1.dat;
 
% dgmgrl -logfile $ORACLE_HOME/rdbms/log/config2.log
DGMGRL> CONNECT /@primary2;
DGMGRL> START OBSERVER FILE=$ORACLE_HOME/dbs/config2.dat;

5.5.8 Reinstating the Former Primary Database in the Broker Configuration

If a fast-start failover was initiated because the primary database had crashed or lost connectivity with the observer and target standby database, the observer automatically attempts to reinstate the former primary database as a standby database, if the FastStartFailoverAutoReinstate configuration property is set to TRUE. Reinstatement restores high availability to the broker configuration so that, in the event of a failure of the new primary database, another fast-start failover can occur. The reinstated database acts as the fast-start failover target for the new primary database, making a subsequent fast-start failover possible. The new standby database is a viable target of a failover when it begins receiving redo data received from the new primary database.

To allow the observer to automatically reinstate the former primary database, the database must be started and mounted, but it cannot be opened. The broker reinstates the database as a standby database of the same type as the former standby database of the new primary database.

If the former primary database cannot be reinstated automatically, you can manually reinstate it using either the DGMGRL REINSTATE command or Enterprise Manager. Step-by-step instructions for manual reinstatement are described in Section 5.4.3.

5.5.9 Shutting Down Databases In a Fast-Start Failover Environment

Perform the following steps if you need to shut down the primary or standby databases:

1. Shut down the observer and wait for the FS_FAILOVER_OBSERVER_PRESENT column in the V$DATABASE fixed view to contain the value "NO" for both the primary and target standby databases. This ensures that a fast-start failover will not occur while you are shutting down the primary database.

2. Shut down the primary database and the target standby database using either DGMGRL SHUTDOWN command or the SQL*Plus SHUTDOWN statement.

When restarting the databases, you may restart them in any order. When both databases have been restarted, you may restart the observer.