5 Redo Transport Services

5.2.1 Destination Types

There are several types of destinations supported by redo transport services:

• Oracle Data Guard standby databases

  Standby database destinations can be either physical standby databases or logical standby databases. Section 1.1.2 discusses standby databases.

• Archived redo log repository

  This type of destination allows off-site archiving of redo data. An archive log repository is created by using a physical standby control file, starting the instance, and mounting the database. This database contains no datafiles and cannot be used for switchover or failover. This alternative is useful as a way of holding archived redo log files for a short period of time, perhaps a day, after which the log files can then be deleted. This avoids most of the storage and processing expense of another fully configured standby database.

  Oracle recommends using an archived redo log repository for temporary storage of archived redo log files. This can be accomplished by configuring the repository destination for archiver-based transport (using the ARCH attribute on LOG_ARCHIVE_DEST_n parameter) in a Data Guard configuration running in maximum performance mode. For a no data loss environment, you should use a fully configured standby database using the LGWR, SYNC, and AFFIRM transport settings in a Data Guard configuration and running in either maximum protection mode or maximum availability mode.

• Oracle Streams real-time downstream capture database

  This destination type allows Oracle Streams to configure a capture process on a remote downstream database. The Streams downstream capture process uses redo transport services to transfer redo data to the downstream database where a Streams capture process captures changes in the standby redo log files and archived redo log files on the remote destination. See Oracle Streams Concepts and Administration for more information.

• Oracle Change Data Capture staging database

  This destination type supports a Change Data Capture Asynchronous AutoLog configuration remotely at a staging database. Redo data is copied from the source database to the staging database using redo transport services. The Change Data Capture configuration captures changes from the redo data. See Oracle Database Data Warehousing Guide for more information.

For discussion purposes, this guide refers to the production database as a primary database and to archival destinations as standby databases (as defined in Section 1.1). If you are using Oracle Change Data Capture, substitute the terms source and staging database for primary and standby database, respectively. If you are using Oracle Streams, substitute the terms source and downstream capture database for primary and standby database, respectively.

5.2.2 Configuring Destinations with the LOG_ARCHIVE_DEST_n Parameter

The LOG_ARCHIVE_DEST_n initialization parameter defines up to ten (where n = 1, 2, 3, ... 10) destinations, each of which must specify either the LOCATION or the SERVICE attribute to specify where to archive the redo data.

The LOCATION and SERVICE attributes describe either a local disk location or an Oracle Net service name that represents a standby destination to which redo transport services will transmit redo data. Specifying remote destinations with the SERVICE attribute allows Data Guard to maintain a transactionally consistent remote copy of the primary database for disaster recovery.

For every LOG_ARCHIVE_DEST_n initialization parameter that you define, specify a corresponding LOG_ARCHIVE_DEST_STATE_n parameter. The LOG_ARCHIVE_DEST_STATE_n (where n is an integer from 1 to 10) initialization parameter specifies whether the corresponding destination is currently on (enabled) or off (disabled). Table 5-1 describes the LOG_ARCHIVE_DEST_STATE_n parameter attributes.

Example 5-1 provides an example of one destination with the LOCATION attribute.

LOG_ARCHIVE_DEST_1='LOCATION=/arch1/chicago/'
LOG_ARCHIVE_DEST_STATE_1=ENABLE

Figure 5-2 Primary Database Archiving When There Is No Standby Database

Description of "Figure 5-2 Primary Database Archiving When There Is No Standby Database "

It is important to note that the configuration shown in Figure 5-2 does not include a standby database and thus does not provide disaster-recovery protection. To make this simple configuration into a Data Guard configuration that provides disaster recovery, add a standby database at a remote destination by specifying the SERVICE attribute.

Example 5-2 shows the initialization parameters that enable redo transport services to archive the online redo log on the local destination chicago and transmit redo data to a remote standby database with the Oracle Net service name boston. The example takes the default values for all of the other LOG_ARCHIVE_DEST_n attributes:

LOG_ARCHIVE_DEST_1='LOCATION=/arch1/chicago/'
LOG_ARCHIVE_DEST_STATE_1=ENABLE
LOG_ARCHIVE_DEST_2='SERVICE=boston'
LOG_ARCHIVE_DEST_STATE_2=ENABLE

SQL> ALTER SYSTEM SET LOG_ARCHIVE_DEST_2='SERVICE=boston 
  2> VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE)';
SQL> ALTER SYSTEM SET LOG_ARCHIVE_DEST_STATE_2=DEFER;

5.2.3 Setting Up Flash Recovery Areas

The Oracle database enables you to configure a disk area called a flash recovery area that is a directory or Oracle Storage Manager disk group that serves as the default storage area for files related to recovery.

To configure a flash recovery area, use the DB_RECOVERY_FILE_DEST initialization parameter. LOG_ARCHIVE_DEST_10 is implicitly set to USE_DB_RECOVERY_FILE_DEST (meaning that archived redo log files will be sent to the flash recovery area) if you create a recovery area and do not set any other local archiving destinations. (See Oracle Database Backup and Recovery Basics to configure the flash recovery area and Oracle Database Administrator's Guide for more information about Oracle Storage Manager and Oracle Managed Files.)

This section contains the following topics:

• Using the LOG_ARCHIVE_DEST_10 Destination

• Using Other LOG_ARCHIVE_DEST_n Destinations

• Using the STANDBY_ARCHIVE_DEST Destination

• Sharing a Flash Recovery Area Between Primary and Standby Databases

  Note:

  A primary database cannot write redo data to the flash recovery area of a logical standby database.

See Oracle Database Backup and Recovery Basics to configure flash recovery areas and Section 10.3.4 for information about setting up a deletion policy for archived redo log files in flash recovery areas.

SQL> ALTER SYSTEM SET LOG_ARCHIVE_DEST_10='LOCATION=USE_DB_RECOVERY_FILE_DEST LGWR MANDATORY REOPEN=5 VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE)'

STANDBY_ARCHIVE_DEST='LOCATION=USE_DB_RECOVERY_FILE_DEST'

DB_NAME=PAYROLL
LOG_ARCHIVE_DEST_1='LOCATION=USE_DB_RECOVERY_FILE_DEST'
DB_RECOVERY_FILE_DEST='/arch/oradata'
DB_RECOVERY_FILE_DEST_SIZE=20G

DB_NAME=PAYROLL
DB_UNIQUE_NAME=boston
LOG_ARCHIVE_DEST_1='LOCATION=USE_DB_RECOVERY_FILE_DEST'
STANDBY_ARCHIVE_DEST='LOCATION=USE_DB_RECOVERY_FILE_DEST'
DB_RECOVERY_FILE_DEST='/arch/oradata'
DB_RECOVERY_FILE_DEST_SIZE=5G

5.3.1 Using Archiver Processes (ARCn) to Archive Redo Data

By default, redo transport services use ARCn processes to archive the online redo log files on the primary database. ARCn archival processing supports only the maximum performance level of data protection in Data Guard configurations. You must use the LGWR process to transmit redo data to standby locations that operate in other data protection modes. (See Section 5.6 for more information about the Data Guard data protection modes.)

The following sections discuss these topics:

• Initialization Parameters That Control ARCn Archival Behavior

• ARCn Archival Processing

ALTER SYSTEM SET LOG_ARCHIVE_MAX_PROCESSES = 20;

5.3.1.2 ARCn Archival Processing

Figure 5-3 shows an example of archival processing in a Data Guard configuration, with a primary database located in Chicago and one physical standby database located in Boston.

Archiving happens when a log switch occurs on the primary database:

• On the primary database, after the ARC0 process successfully archives the local online redo log to the local destination (LOG_ARCHIVE_DEST_1), the ARC1 process transmits redo from the local archived redo log files (instead of the online redo log files) to the remote standby destination (LOG_ARCHIVE_DEST_2).

• On the remote destination, the remote file server process (RFS) will, in turn, write the redo data to an archived redo log file from a standby redo log file. Log apply services use Redo Apply (MRP process^Foot 1 ) or SQL Apply (LSP process^Foot 2 ) to apply the redo to the standby database.

Because the online redo log files are archived locally first, the LGWR process reuses the online redo log files much earlier than would be possible if the ARCn processes archived to the standby database concurrently with the local destination.

As shown in Figure 5-3, you need to have at least 2 ARCn processes to separate local archival from remote archival. This can be done by setting the LOG_ARCHIVE_MAX_PROCESSES initialization parameter (the default setting is 4, but the maximum value is 30).

Figure 5-3 Archiving to Local Destinations Before Archiving to Remote Destinations

Description of "Figure 5-3 Archiving to Local Destinations Before Archiving to Remote Destinations"

Because the default ARCn archival processing disassociates local archiving from remote archiving, sites that may have policies to delete archived redo log files on the primary database immediately after backing them up must make sure that the standby destinations receive the corresponding redo data before deleting the archived redo log files on the primary database. You can query the V$ARCHIVED_LOG view to verify the redo data was received on standby destinations.

5.3.2 Using the Log Writer Process (LGWR) to Archive Redo Data

You can optionally enable redo transport services to use the LGWR process to transmit redo data to remote destinations.

Using the LGWR process differs from ARCn processing (described in Section 5.3.1), because instead of waiting for the online redo log to switch at the primary database and then writing the entire archived redo log at the remote destination all at once, the LGWR process selects a standby redo log file at the standby site that reflects the log sequence number (and size) of the current online redo log file of the primary database. Then, as redo is generated at the primary database, it is also transmitted to the remote destination. The transmission to the remote destination will either be synchronous or asynchronous, based on whether the SYNC or the ASYNC attribute is set on the LOG_ARCHIVE_DEST_n parameter. Synchronous LGWR processing is required for the maximum protection and maximum availability modes of data protection in Data Guard configurations. (See Section 5.6 for information about the Data Guard data protection modes.)This section contains the following topics:

• LOG_ARCHIVE_DEST_n Attributes for LGWR Archival Processing

• LGWR SYNC Archival Processing

• LGWR ASYNC Archival Processing

5.3.2.2 LGWR SYNC Archival Processing

Example 5-5 shows the primary role LOG_ARCHIVE_DEST_n parameters that configure the LGWR process for synchronous network transmission.

Example 5-5 Initialization Parameters for LGWR Synchronous Archival

LOG_ARCHIVE_DEST_1='LOCATION=/arch1/chicago'
LOG_ARCHIVE_DEST_2='SERVICE=boston LGWR SYNC NET_TIMEOUT=30'
LOG_ARCHIVE_DEST_STATE_1=ENABLE
LOG_ARCHIVE_DEST_STATE_2=ENABLE

Specifying the SYNC attribute on the LOG_ARCHIVE_DEST_n parameter is optional, because this is the default for LGWR archival processing. The NET_TIMEOUT attribute is recommended, because it controls the amount of time that the LGWR process waits for status from the network server process before terminating the network connection. If there is no reply within NET_TIMEOUT seconds, then the LGWR process returns an error message.

Figure 5-4 shows a Data Guard configuration that uses the LGWR process to synchronously transmit redo data to the standby system at the same time it is writing redo data to the online redo log file on the primary database:

• On the primary database, the LGWR process submits the redo data to one or more network server (LNSn) processes, which then initiate the network I/O in parallel to multiple remote destinations. Transactions are not committed on the primary database until the redo data necessary to recover the transaction is received by all LGWR SYNC destinations.

• On the standby system, the remote file server (RFS) receives redo data over the network from the LGWR process and writes the redo data to the standby redo log files.

A log switch on the primary database triggers a log switch on the standby database, causing ARCn processes on the standby database to archive the standby redo log files to archived redo log files on the standby database. Then, Redo Apply (MRP process) or SQL Apply (LSP process) applies the redo data to the standby database. If real-time apply is enabled, Data Guard recovers redo data directly from the current standby redo log file as it is being filled up by the RFS process.

Figure 5-4 LGWR SYNC Archival to a Remote Destination with Standby Redo Log Files

Description of "Figure 5-4 LGWR SYNC Archival to a Remote Destination with Standby Redo Log Files"

5.3.2.3 LGWR ASYNC Archival Processing

Example 5-6 shows the primary role LOG_ARCHIVE_DEST_n parameters that configure the LGWR process for asynchronous network transmission.

Example 5-6 Initialization Parameters for LGWR Asynchronous Archiving

LOG_ARCHIVE_DEST_1='LOCATION=/arch1/chicago'
LOG_ARCHIVE_DEST_2='SERVICE=boston LGWR ASYNC'
LOG_ARCHIVE_DEST_STATE_1=ENABLE
LOG_ARCHIVE_DEST_STATE_2=ENABLE

Figure 5-5 shows the LNSn process collecting redo data from the online redo log files and transmitting it over Oracle Net to the RFS process on the standby database.

When the LGWR and ASYNC attributes are specified, the log writer process writes to the local online redo log file, while the network server (LNSn) processes (one for each destination) asynchronously transmit the redo to remote destinations. The LGWR process continues processing the next request without waiting for the LNS network I/O to complete.

If redo transport services transmit redo data to multiple remote destinations, the LNSn processes (one for each destination) initiate the network I/O to all of the destinations in parallel.

When an online redo log file fills up, a log switch occurs and an archiver process archives the log file locally, as usual.

Note:

Beginning with Oracle Database 10g Release 10.2, it is unnecessary to specify the NET_TIMEOUT attribute on the LOG_ARCHIVE_DEST_n destinations configured with both the LGWR and ASYNC attributes. This is because the log writer process never waits for the LNSn for any reason in release 10.2. Thus, specifying the NET_TIMEOUT attribute is not required.

Figure 5-5 LGWR ASYNC Archival with Network Server (LNSn) Processes

Description of "Figure 5-5 LGWR ASYNC Archival with Network Server (LNSn) Processes"

Note:

Do not use the LOG_ARCHIVE_DEST or the LOG_ARCHIVE_DUPLEX_DEST initialization parameters to specify a flash recovery area destination.

5.3.3 Providing for Secure Redo Data Transmission

Data Guard provides a secure environment and prevents the possible tampering of redo data as it is being transferred to the standby database.

Redo transport services use authenticated network sessions to transfer redo data. These sessions are authenticated using the SYS user password contained in the password file. All databases in the Data Guard configuration must use a password file, and the SYS password contained in this password file must be identical on all systems. This authentication can be performed even if Oracle Advanced Security is not installed, and provides some level of security when shipping redo.

Perform the following steps on the primary database and each standby database:

1. Create a password file (using the orapwd utility) on the primary and all standby databases. For example:

   ORAPWD FILE=orapw PASSWORD=mypassword ENTRIES=10
   

   This example creates a password file with 10 entries, where the password for SYS is mypassword. For redo data transmission to succeed, ensure you set the password for the SYS user account identically for every primary and standby database.

2. Set the REMOTE_LOGIN_PASSWORDFILE initialization parameter to EXCLUSIVE or SHARED to enable Oracle to check for a password file and to specify how many databases can use the password file. For example:

   REMOTE_LOGIN_PASSWORDFILE=EXCLUSIVE
   

   See the Oracle Database Reference for more information about this parameter.

5.4.1 Specifying Role-Based Destinations with the VALID_FOR Attribute

The VALID_FOR attribute enables you to configure destination attributes for both the primary and standby database roles in one server parameter file (SPFILE), so that your Data Guard configuration operates properly after a role transition. This simplifies switchovers and failovers by removing the need to enable and disable the role-specific parameter files after a role transition.

When you specify the VALID_FOR attribute of the LOG_ARCHIVE_DEST_n parameter, it identifies when redo transport services can transmit redo data to destinations based on the following factors:

• Whether the database is currently running in the primary or the standby role

• Whether archival of the online redo log file, standby redo log file, or both is required depending on the current role of the database

To configure these factors for each LOG_ARCHIVE_DEST_n destination, you specify this attribute with a pair of keywords: VALID_FOR=(redo_log_type,database_role). The redo_log_type keyword identifies the destination as valid for archiving the following: ONLINE_LOGFILE, STANDBY_LOGFILE, or ALL_LOGFILES. The database_role keyword identifies the role in which the current database must be in for the destination to be valid: PRIMARY_ROLE, STANDBY_ROLE, or ALL_ROLES.

If you do not specify the VALID_FOR attribute for a destination, by default, archiving the online redo log and standby redo log is enabled to the destination, regardless of the database role. This default behavior is equivalent to setting the (ALL_LOGFILES,ALL_ROLES) keyword pair on the VALID_FOR attribute. For example:

LOG_ARCHIVE_DEST_1='LOCATION=/ARCH1/CHICAGO/ VALID_FOR=(ALL_LOGFILES,ALL_ROLES)'

Although the (ALL_LOGFILES,ALL_ROLES) keyword pair is the default, it is not recommended for every destination. For example, logical standby databases, unlike physical standby databases, are open databases that generate redo data and have multiple log files (online redo log files, archived redo log files, and standby redo log files). In most cases, the online redo log files generated by the logical standby database are located in the same directory as the standby redo logs files that are receiving redo from the primary database.

Therefore, it is recommended that you define a VALID_FOR attribute for each destination so that your Data Guard configuration operates properly, including after a role transition. See the scenarios in Section 12.1 for examples of the VALID_FOR attribute settings for various Data Guard configurations,

If you choose not to use the VALID_FOR attribute to configure destinations, you must maintain two database server parameter files (SPFILEs) for each database: one for when the database is in the primary role and the other for the standby role. See Chapter 12 for more configuration examples.

5.4.2 Specify Unique Names for Primary and Standby Databases

The DB_UNIQUE_NAME attribute enables you to specify unique database names when you configure destinations. This makes it possible to dynamically add a standby database to a Data Guard configuration that contains a Real Applications Clusters primary database, when that primary database is operating in either the maximum protection or the maximum availability level of protection. The DB_UNIQUE_NAME initialization parameter is required if the LOG_ARCHIVE_CONFIG parameter has been defined.

Together, the DB_UNIQUE_NAME attribute of the LOG_ARCHIVE_DEST_n parameter and the DG_CONFIG attribute of the LOG_ARCHIVE_CONFIG parameter specify the unique name of each database of the Data Guard configuration. The names you supply must match what was defined for each database with the DB_UNIQUE_NAME initialization parameter.

For example, the following initialization parameters show the DB_UNIQUE_NAME and LOG_ARCHIVE_CONFIG definitions for the primary database (chicago) in the Data Guard configuration described in Chapter 3:

DB_NAME=chicago
DB_UNIQUE_NAME=chicago
LOG_ARCHIVE_CONFIG='DG_CONFIG=(chicago, boston)'
LOG_ARCHIVE_DEST_1='LOCATION=/arch1/chicago/ VALID_FOR=(ALL_LOGFILES,ALL_ROLES) 
LOG_ARCHIVE_DEST_2=
 'SERVICE=boston LGWR ASYNC
  VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) 
  DB_UNIQUE_NAME=boston'

The DB_UNIQUE_NAME attribute is required for remote destinations specified with the SERVICE attribute. In the example, the LOG_ARCHIVE_DEST_2 parameter specifies the DB_UNIQUE_NAME=boston for the remote destination; redo transport services validate this information at the remote destination. If the names do not match, the connection to that destination is refused.

The LOG_ARCHIVE_CONFIG parameter also has SEND, NOSEND, RECEIVE, and NORECEIVE attributes:

• SEND enables a database to send redo data to remote destinations

• RECEIVE enables the standby database to receive redo from another database

To disable these settings, use the NOSEND and NORECEIVE keywords.

For example, to ensure the primary database never accidentally receives any archived redo data, set the LOG_ARCHIVE_CONFIG initialization parameter to NORECEIVE on the primary database, as follows:

LOG_ARCHIVE_CONFIG='NORECEIVE,DG_CONFIG=(chicago,boston)'

However, keep in mind that specifying either the NOSEND or the NORECEIVE attributes may limit the database instance's capabilities after a role transition. For example, if a standby database with the NOSEND attribute set is transitioned to the primary role, it would not be able to transmit redo data to other standby databases until you reset the parameter value to SEND. Similarly, a database that has the NORECEIVE attribute specified cannot receive redo from the primary database.

By default, the LOG_ARCHIVE_CONFIG parameter allows the primary database to send redo data to the standby database and allows the standby database to receive redo from the primary database for archiving. This is equivalent to setting both SEND and RECEIVE attributes on the LOG_ARCHIVE_CONFIG parameter.

5.5.1 Retrying the Archival Operation

Use the REOPEN attribute to determine if and when the ARCn process or the LGWR process attempts to transmit redo data again to a failed destination following an error.

Use the REOPEN=seconds attribute to specify the minimum number of seconds that must elapse following an error before the archiving process will try again to access a failed destination. The default value is 300 seconds. The value set for the REOPEN attribute applies to all errors, not just connection failures. You can turn off the option by specifying REOPEN=0, which prevents the destination from being retried after a failure occurs.

If transmission to the alternate destination fails and the REOPEN attribute is set to zero (0), redo transport services will attempt to send redo data to the alternate destination the next time redo data is archived.

5.5.2 Using an Alternate Destination

The ALTERNATE attribute defines an alternate archiving destination that can be used when the original archiving destination fails. If no alternate destination is specified, the destination does not automatically change to another destination upon failure.

Figure 5-6 shows a scenario where redo data is archived to a local disk device. If the original destination device becomes full or unavailable, the archival operation is automatically redirected to the alternate destination device.

Figure 5-6 Archival Operation to an Alternate Destination Device

Description of "Figure 5-6 Archival Operation to an Alternate Destination Device"

The REOPEN attribute takes precedence over the ALTERNATE attribute. The alternate destination is used only if one of the following conditions is true:

• A value of zero (0) is specified for the REOPEN attribute.

• A nonzero REOPEN attribute and a nonzero MAX_FAILURE count have been exceeded.

The ALTERNATE attribute takes precedence over the MANDATORY attribute. This means that a destination fails over to a valid alternate destination even if the current destination is mandatory.

5.5.3 Controlling the Number of Retry Attempts

Use the MAX_FAILURE attribute to specify the maximum number of consecutive times that redo transport services attempt to transmit redo data to a failed destination. To limit the number of consecutive attempts that will be made to reestablish communication with a failed destination, use the REOPEN attribute in conjunction with the MAX_FAILURE attribute. Once the specified number of consecutive attempts is exceeded, the destination is treated as if the REOPEN attribute was set to zero.

The REOPEN attribute is required when you use the MAX_FAILURE attribute. Example 5-7 shows how to set a retry time of 60 seconds and limit retries to 3 attempts.

LOG_ARCHIVE_DEST_1='LOCATION=/arc_dest REOPEN=60 MAX_FAILURE=3' 

5.6.1 Choosing a Data Protection Mode

To determine the appropriate data protection mode to use, review the following descriptions of the data protection modes to help assess your business requirements for data availability against user demands for response time and performance. Also, see Section 5.6.2 for information about setting up the data protection mode.

5.6.2 Setting the Data Protection Mode of a Data Guard Configuration

To set up redo transport services and specify a level of data protection for the Data Guard configuration, perform the following steps.

Step 1   Configure the LOG_ARCHIVE_DEST_n parameters on the primary database.

On the primary database, configure the LOG_ARCHIVE_DEST_n parameter attributes appropriately. Each of the Data Guard data protection modes requires that at least one standby database in the configuration meet the minimum set of requirements listed in Table 5-2.

The following example shows how to configure the maximum availability mode:

SQL> ALTER SYSTEM SET LOG_ARCHIVE_DEST_2='SERVICE=chicago
  2> OPTIONAL LGWR SYNC AFFIRM 
  3> VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE)
  4> DB_UNIQUE_NAME=chicago';

If they are not already specified in the SPFILE, you should also specify unique names with the DB_UNIQUE_NAME initialization parameter and list all databases on the LOG_ARCHIVE_CONFIG parameter with the DG_CONFIG attribute. For example:

SQL> ALTER SYSTEM SET LOG_ARCHIVE_CONFIG='DG_CONFIG=(chicago,boston)'

This will enable the dynamic addition of a standby database to a Data Guard configuration that has a Real Application Clusters primary database running in either maximum protection or maximum availability mode.

Step 1   If you are upgrading the protection mode, perform this step.

Perform this step only if you are upgrading the protection mode (for example, from maximum performance to maximum availability mode). Otherwise, go to Step 3.

Assume this example is upgrading the Data Guard configuration from the maximum performance mode to the maximum availability mode. Shut down the primary database and restart it in mounted mode:

SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP MOUNT;

For a Real Application Clusters database, shut down all of the primary instances but start and mount only one primary instance.

Step 2   Set the data protection mode.

To specify a data protection mode, issue the SQL ALTER DATABASE SET STANDBY DATABASE TO MAXIMIZE {PROTECTION | AVAILABILITY | PERFORMANCE} statement on the primary database. For example, the following statement specifies the maximum availability mode:

SQL> ALTER DATABASE SET STANDBY DATABASE TO MAXIMIZE AVAILABILITY;

Step 3   Open the primary database.

If you performed Step 1 to upgrade the protection mode, open the database:

SQL> ALTER DATABASE OPEN;

If you are downgrading the protection mode, the database will already be open.

Step 4   Configure the LOG_ARCHIVE_DEST_n parameters on standby databases.

On the standby databases, configure the LOG_ARCHIVE_DEST_n parameter attributes so the configuration can continue to operate in the new protection mode after a switchover.

For example:

SQL> ALTER SYSTEM SET LOG_ARCHIVE_DEST_2='SERVICE=boston
  2> OPTIONAL LGWR SYNC AFFIRM
  3> VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE)
  4> DB_UNIQUE_NAME=boston';

Step 5   Confirm the configuration is operating in the new protection mode.

Query the V$DATABASE view to confirm the Data Guard configuration is operating in the new protection mode. For example:

SQL> SELECT PROTECTION_MODE, PROTECTION_LEVEL FROM V$DATABASE;

PROTECTION_MODE                   PROTECTION_LEVEL
---------------------             ---------------------
MAXIMUM AVAILABILITY              MAXIMUM AVAILABILITY

See Chapter 15 and Oracle Database SQL Reference for information about SQL statements.

5.7.1 Specifying Alternate Directory Locations for Archived Redo Log Files

Typically, when redo data is received from the primary database, the redo data is written to archived redo log files that are stored in the directory you specify with the LOCATION attribute of the LOG_ARCHIVE_DEST_n parameter. Alternatively, you can specify the STANDBY_ARCHIVE_DEST initialization parameter on the standby database to indicate an alternate directory where the archived redo log files are to be stored when received from the primary database.

If both parameters are specified, the STANDBY_ARCHIVE_DEST initialization parameter overrides the directory location specified with the LOG_ARCHIVE_DEST_n parameter.

The location where archived redo log files are stored on the standby database is determined according to the following list of rules. When the database instance is started, the archived redo log files are evaluated in the list order:

1. If the STANDBY_ARCHIVE_DEST initialization parameter is specified on the standby database, that location is used.

2. If the LOG_ARCHIVE_DEST_n parameter contains the VALID_FOR=(STANDBY_LOGFILE,*) attribute, then the location specified for this destination is used.

3. If the COMPATIBLE parameter is set to 10.0 or greater and none of the LOG_ARCHIVE_DEST_n parameters contain the VALID_FOR=(STANDBY_LOGFILE,*)attribute, then an arbitrary LOG_ARCHIVE_DEST_n parameter that is valid for the destination is used.

4. If none of the initialization parameters have been specified, then archived redo log files are stored in the default location for the STANDBY_ARCHIVE_DEST initialization parameter.

   To see the implicit default value of the STANDBY_ARCHIVE_DEST initialization parameter, query the V$ARCHIVE_DEST view:

   SQL> SELECT DEST_NAME, DESTINATION FROM V$ARCHIVE_DEST 
     2> WHERE DEST_NAME='STANDBY_ARCHIVE_DEST';
   
   DEST_NAME
   ------------------------------------------------------------------------------------------------------------------------------------
   DESTINATION
   ------------------------------------------------------------------------------------------------------------------------------------
   STANDBY_ARCHIVE_DEST
   /oracle/dbs/arch
   

Redo transport services use the value specified with the STANDBY_ARCHIVE_DEST initialization parameter in conjunction with the LOG_ARCHIVE_FORMAT parameter to generate the filenames for the archived redo log files on the standby site. For example:

STANDBY_ARCHIVE_DEST='/arc_dest/arls' 
LOG_ARCHIVE_FORMAT=log%t_%s_%r.arc

In the example, %s corresponds to the sequence number, and %r corresponds to the resetlogs ID. Together, these ensure unique names are constructed for the archived redo log files across multiple incarnations of the database. The %t, which is required for Real Application Clusters configurations, corresponds to the thread number.

For a physical standby database, redo transport services store the fully qualified filenames in the standby database control file, and Redo Apply uses this information to perform recovery on the standby database.

To display the list of archived redo log files that are on the standby system, query the V$ARCHIVED_LOG view on the standby database:

SQL> SELECT NAME FROM V$ARCHIVED_LOG;
NAME                                                                            
--------------------------------------------------------------------------------
/arc_dest/log_1_771.arc                                       
/arc_dest/log_1_772.arc                                       
/arc_dest/log_1_773.arc                                       
/arc_dest/log_1_774.arc                                       
/arc_dest/log_1_775.arc

5.7.2 Reusing Online Redo Log Files

You can specify a policy for reusing the online redo log file by setting the OPTIONAL or MANDATORY attribute of the LOG_ARCHIVE_DEST_n parameter. By default, remote destinations are set to OPTIONAL. The archival operation of an optional destination can fail, and the online redo log file can be reused even though transmitting the redo data and writing the log contents was not successful. If the archival operation of a mandatory destination fails, online redo log files cannot be overwritten until the failed archive is completed to the mandatory destination.

By default, one local destination is mandatory even if you designate all destinations to be optional.

Example 5-8 shows how to set a mandatory local archiving destination and enable that destination. When specifying the MANDATORY attribute, also consider specifying the REOPEN and MAX_FAILURE attributes as described in Section 5.5 to handle failure conditions.

LOG_ARCHIVE_DEST_3 = 'LOCATION=/arc_dest MANDATORY'

5.7.3 Managing Standby Redo Log Files

This section contains the following topics:

• Determining If a Standby Redo Log File Group Configuration Is Adequate

• Adding Standby Redo Log Members to an Existing Group

SQL> ALTER DATABASE ADD STANDBY LOGFILE MEMBER '/disk1/oracle/dbs/log2b.rdo' 
  2> TO GROUP 2;

5.7.4 Planning for Growth and Reuse of the Control Files

This section describes:

• Sizing the Disk Volumes that Contain the Control Files

• Specifying the Reuse of Records in the Control File

krcpwnc: following controlfile record written over:

5.7.5 Sharing a Log File Destination Among Multiple Standby Databases

Use the DEPENDENCY attribute of the LOG_ARCHIVE_DEST_n initialization parameter to define one archival destination to receive redo data on behalf of several destinations, rather than transmitting redo data to each individual destination.

Figure 5-7 shows a Data Guard configuration in which the primary database transports redo data to one archiving destination that shares its archived redo log files with both a logical standby database and a physical standby database. These destinations are dependent on the successful completion of archival operations to the parent destination.

Figure 5-7 Data Guard Configuration with Dependent Destinations

Description of "Figure 5-7 Data Guard Configuration with Dependent Destinations"

Specifying a destination dependency can be useful in the following situations:

• When you configure a physical standby database and a logical standby database on the same system.

• When you configure the standby database and the primary database on the same system. Therefore, the archived redo log files are implicitly accessible to the standby database.

• When clustered file systems are used to provide remote standby databases with access to the primary database archived redo log files.

• When operating system-specific network file systems are used, providing remote standby databases with access to the primary database archived redo log files.

For example, assume there are two standby databases stdby1 and stdby2 that reside on the same piece of hardware. There is no need to use network bandwidth and disk space to send the same redo data to both destinations. The databases can share the same archived redo log files if you use the DEPENDENCY attribute to designate one of the destinations as being a dependent destination. That is, the primary database sends redo to be archived on the destination that is not defined as the dependent destination. If the redo data successfully arrives at that destination, the primary database considers it archived to both destinations. For example:

LOG_ARCHIVE_DEST_1='LOCATION=DISK1 MANDATORY'
LOG_ARCHIVE_DEST_2='SERVICE=stdby1 OPTIONAL'
LOG_ARCHIVE_DEST_3='SERVICE=stdby2 OPTIONAL DEPENDENCY=LOG_ARCHIVE_DEST_2'

With these parameter definitions, the primary database transmits redo data to stdby1 but not to stdby2. The stdby2 database instead recovers redo from the archived redo log files that are shipped to stdby1.

5.8.1 When Is an Archive Gap Discovered?

An archive gap can occur whenever the primary database archives a log locally, but the log is not received at the standby site. Every minute, the primary database polls its standby databases to see if there are gaps in the sequence of archived redo log files.

5.8.2 How Is a Gap Resolved?

Gap recovery is handled through the polling mechanism. For physical and logical standby databases, Oracle Change Data Capture, and Oracle Streams, Data Guard performs gap detection and resolution by automatically retrieving missing archived redo log files from the primary database. No extra configuration settings are required to poll the standby databases, to detect any gaps, or to resolve the gaps.

The important consideration here is that automatic gap recovery is contingent on the availability of the primary database. If the primary database is not available and you have a configuration with multiple physical standby databases, you can set up additional initialization parameters so that the Redo Apply can resolve archive gaps from another standby database, as described in Section 5.8.3.

See Section 12.11 for a scenario that shows how to resolve a gap manually.

5.8.3 Using the Fetch Archive Log (FAL) to Resolve Archive Gaps

The fetch archive log (FAL) client and server resolve gaps detected in the range of archived redo log files generated at the primary database and received at the physical standby database.

• The FAL client requests the transfer of archived redo log files automatically.

• The FAL server services the FAL requests coming from the FAL client.

The FAL mechanism handles the following types of archive gaps and problems:

• When creating a physical or logical standby database, the FAL mechanism can automatically retrieve any archived redo log files generated during a hot backup of the primary database.

• When there are problems with archived redo log files that have already been received on the standby database, the FAL mechanism can automatically retrieve archived redo log files to resolve any of the following situations:

  • When the archived redo log file is deleted from disk before it is applied to the standby database.

  • When the archived redo log file cannot be applied because of a disk corruption.

  • When the archived redo log file is accidentally replaced by another file (for example, a text file) that is not an archived redo log file before the redo data has been applied to the standby database.

• When you have multiple physical standby databases, the FAL mechanism can automatically retrieve missing archived redo log files from another physical standby database.

The FAL client and server are configured using the FAL_CLIENT and FAL_SERVER initialization parameters that are set on the standby database. Define the FAL_CLIENT and FAL_SERVER initialization parameters only for physical standby databases in the initialization parameter file as shown in the following table:

5.8.4 Manually Determining and Resolving Archive Gaps

In some situations, automatic gap recovery may not take place and you will need to perform gap recovery manually. For example, you will need to perform gap recovery manually if you are using logical standby databases and the primary database is not available.

The following sections describe how to query the appropriate views to determine which log files are missing and perform manual recovery.

On a physical standby database

To determine if there is an archive gap on your physical standby database, query the V$ARCHIVE_GAP view as shown in the following example:

SQL> SELECT * FROM V$ARCHIVE_GAP;
    THREAD#  LOW_SEQUENCE#  HIGH_SEQUENCE#
-----------  -------------  --------------
          1              7              10

The output from the previous example indicates your physical standby database is currently missing log files from sequence 7 to sequence 10 for thread 1. After you identify the gap, issue the following SQL statement on the primary database to locate the archived redo log files on your primary database (assuming the local archive destination on the primary database is LOG_ARCHIVE_DEST_1):

SQL> SELECT NAME FROM V$ARCHIVED_LOG WHERE THREAD#=1 AND DEST_ID=1 AND   2> SEQUENCE# BETWEEN 7 AND 10;
NAME
--------------------------------------------------------------------------------
/primary/thread1_dest/arcr_1_7.arc /primary/thread1_dest/arcr_1_8.arc /primary/thread1_dest/arcr_1_9.arc

Copy these log files to your physical standby database and register them using the ALTER DATABASE REGISTER LOGFILE statement on your physical standby database. For example:

SQL> ALTER DATABASE REGISTER LOGFILE
'/physical_standby1/thread1_dest/arcr_1_7.arc';
SQL> ALTER DATABASE REGISTER LOGFILE
'/physical_standby1/thread1_dest/arcr_1_8.arc';

After you register these log files on the physical standby database, you can restart Redo Apply.

On a logical standby database:

To determine if there is an archive gap, query the DBA_LOGSTDBY_LOG view on the logical standby database. For example, the following query indicates there is a gap in the sequence of archived redo log files because it displays two files for THREAD 1 on the logical standby database. (If there are no gaps, the query will show only one file for each thread.) The output shows that the highest registered file is sequence number 10, but there is a gap at the file shown as sequence number 6:

SQL> COLUMN FILE_NAME FORMAT a55
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          6 /disk1/oracle/dbs/log-1292880008_6.arc
         1         10 /disk1/oracle/dbs/log-1292880008_10.arc

Copy the missing log files, with sequence numbers 7, 8, and 9, to the logical standby system and register them using the ALTER DATABASE REGISTER LOGICAL LOGFILE statement on your logical standby database.

For example:

SQL> ALTER DATABASE REGISTER LOGICAL LOGFILE '/disk1/oracle/dbs/log-1292880008_10.arc';

After you register these log files on the logical standby database, you can restart SQL Apply.

5.9.1 Monitoring Log File Archival Information

This section describes using views to monitor redo log archival activity for the primary database. See Oracle Data Guard Broker and Oracle Enterprise Manager online help for more information about the graphical user interface that automates many of the tasks involved in monitoring a Data Guard environment

Step 1   Determine the status of redo log files.

Enter the following query on the primary database to determine the status of all online redo log files:

SQL> SELECT THREAD#, SEQUENCE#, ARCHIVED, STATUS FROM V$LOG;

Step 2   Determine the most recent archived redo log file.

Enter the following query on the primary database to determine recently archived thread and sequence number:

SQL> SELECT MAX(SEQUENCE#), THREAD# FROM V$ARCHIVED_LOG GROUP BY THREAD#;

Step 3   Determine the most recent archived redo log file at each destination.

Enter the following query on the primary database to determine which archived redo log file was most recently transmitted to each of the archiving destinations:

SQL> SELECT DESTINATION, STATUS, ARCHIVED_THREAD#, ARCHIVED_SEQ#
  2> FROM V$ARCHIVE_DEST_STATUS
  3> WHERE STATUS <> 'DEFERRED' AND STATUS <> 'INACTIVE';

DESTINATION         STATUS  ARCHIVED_THREAD#  ARCHIVED_SEQ#
------------------  ------  ----------------  -------------
/private1/prmy/lad   VALID                 1            947
standby1             VALID                 1            947

The most recently written archived redo log file should be the same for each archive destination listed. If it is not, a status other than VALID might identify an error encountered during the archival operation to that destination.

Step 4   Find out if archived redo log files have been received.

You can issue a query at the primary database to find out if an archived redo log file was not received at a particular site. Each destination has an ID number associated with it. You can query the DEST_ID column of the V$ARCHIVE_DEST fixed view on the primary database to identify each destination's ID number.

Assume the current local destination is 1, and one of the remote standby destination IDs is 2. To identify which log files are missing at the standby destination, issue the following query:

SQL> SELECT LOCAL.THREAD#, LOCAL.SEQUENCE# FROM
  2> (SELECT THREAD#, SEQUENCE# FROM V$ARCHIVED_LOG WHERE DEST_ID=1)
  3> LOCAL WHERE
  4> LOCAL.SEQUENCE# NOT IN
  5> (SELECT SEQUENCE# FROM V$ARCHIVED_LOG WHERE DEST_ID=2 AND
  6> THREAD# = LOCAL.THREAD#);

  THREAD#  SEQUENCE#
---------  ---------
  1        12
  1        13
  1        14

See Appendix A for details about monitoring the archiving status of the primary database.

Step 5   Trace the progression of transmitted redo on the standby site.

To see the progression of the transmission of redo data to the standby destination, set the LOG_ARCHIVE_TRACE parameter in the primary and standby initialization parameter files. See Appendix G for complete details and examples.

5.9.2 Monitoring the Performance of Redo Transport Services

This section describes the wait events that monitor the performance of the redo transport services that were specified on the primary database with the ARCH, LGWR, SYNC, and ASYNC attributes on the LOG_ARCHIVE_DEST_n initialization parameter.

The following sections describe the wait events and associated timing information that are displayed by the V$SYSTEM_EVENT view:

• ARCn Process Wait Events

• LGWR SYNC Wait Events

• LGWR ASYNC Wait Events