Understanding the ARCHIVE_LAG_TARGET Parameter in Oracle Database

The ARCHIVE_LAG_TARGET parameter is an important configuration setting in Oracle databases, particularly in environments where data consistency and recovery point objectives (RPO) are critical, such as Data Guard setups. This parameter helps manage and minimize the lag between the primary and standby databases by controlling the frequency of log switches.

What is ARCHIVE_LAG_TARGET?

ARCHIVE_LAG_TARGET is an Oracle initialization parameter that specifies the target time (in seconds) for which the redo data can lag behind on a standby database relative to the primary database. By setting this parameter, you can ensure that the primary database frequently archives its redo logs, thereby reducing the potential data loss in case of a failover to the standby database.

How ARCHIVE_LAG_TARGET Works

When the ARCHIVE_LAG_TARGET parameter is set, Oracle ensures that the primary database archives its current redo log at least once every specified number of seconds. This helps in reducing the maximum amount of data that can be lost in a disaster recovery scenario.

For example, if ARCHIVE_LAG_TARGET is set to 1800 seconds (30 minutes), Oracle will perform a log switch and archive the current redo log if no log switch has occurred in the last 30 minutes.

Benefits of ARCHIVE_LAG_TARGET

Improved Data Protection:

Ensures that redo logs are archived regularly, which helps in maintaining a more up-to-date standby database.

Controlled Data Lag:

Helps control the maximum potential data loss by limiting how far behind the standby database can fall.

Enhanced RPO:

By reducing the lag, the recovery point objective (RPO) is improved, meaning less data is lost in the event of a failover.

Setting ARCHIVE_LAG_TARGET

The ARCHIVE_LAG_TARGET parameter can be set dynamically using the ALTER SYSTEM command or by adding it to the database's initialization parameter file (init.ora or spfile).

To set this parameter dynamically:

ALTER SYSTEM SET ARCHIVE_LAG_TARGET = 1800 SCOPE=BOTH;

To set this parameter in the initialization parameter file:

Add below value

ARCHIVE_LAG_TARGET=1800

Monitoring Archive Lag

You can monitor the archive lag by querying the V$DATAGUARD_STATS and V$ARCHIVE_DEST_STATUS views. These views provide information on the current lag between the primary and standby databases.

Example query to check the archive lag:

On Standby Database

SELECT NAME, VALUE, TIME_COMPUTED

FROM V$DATAGUARD_STATS

WHERE NAME = 'apply lag' OR NAME = 'transport lag';

Example output:

NAME VALUE TIME_COMPUTED

apply lag +00 00:00:30 27-JUL-24 10:30:00

transport lag +00 00:00:20 27-JUL-24 10:30:00

Best Practices for ARCHIVE_LAG_TARGET

Assess Business Requirements:

Set the ARCHIVE_LAG_TARGET based on your organization's recovery objectives and the criticality of the data.

Balance Performance and Protection:

While frequent log switches can enhance data protection, they can also impact system performance. Find a balance that meets your RPO without causing unnecessary overhead.

Regular Monitoring:

Continuously monitor the archive lag and adjust the parameter as needed to ensure optimal performance and data protection.

Testing:

Regularly test your failover and switchover procedures to ensure that the standby database can take over with minimal data loss, adhering to the RPO defined by your ARCHIVE_LAG_TARGET.